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Formation evaluation in Dezful embayment of Iran using oil-based-mud imaging techniques
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Evaluation in Dezful Embayment of Iran Using Oil-Based -Mud Imaging Techniques Zohreh Movahed, Radzuan Juninb, Zeynalabedin Safarkhanlou, Mahmood Akbar
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PII: DOI: Reference:
S0920-4105(14)00136-3 http://dx.doi.org/10.1016/j.petrol.2014.05.019 PETROL2668
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Journal of Petroleum Science and Engineering
Received date: 16 October 2012 Revised date: 20 May 2014 Accepted date: 24 May 2014 Cite this article as: Zohreh Movahed, Radzuan Juninb, Zeynalabedin Safarkhanlou, Mahmood Akbar, Evaluation in Dezful Embayment of Iran Using Oil-Based -Mud Imaging Techniques, Journal of Petroleum Science and Engineering, http://dx.doi.org/10.1016/j.petrol.2014.05.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Formation Evaluation in Dezful Embayment of Iran Using Oil-Based -Mud Imaging Techniques Zohreh Movaheda, Radzuan Juninbb, Zeynalabedin Safarkhanlouc, Mahmood Akbarc a
UTM University and Schlumberger UTM University c Schlumberger b
Abstract Structural delineation and fracture characterization are two of the main issues for the evaluation of carbonate reservoirs in structurally complex areas. Some of such fields are located in the southwest (Dezful embayment) of Iran. Getting to the Asmari reservoir is not so easy in some cases due to structural complexities where a thick pile of evaporites of Gachsaran Formation overly the reservoir. In such cases structural dip information plays an important role in mapping the structure in the area around the wells. In addition to structural complications, fracture characterization is quite important for Asmari carbonates and in particular for Sarvak carbonates. In most fields of the Dezful embayment, it is not feasible to drill wells with water-based mud, hence getting information on structural dip and fractures is not so easy with conventional borehole imaging tools. A combination of oil-based -mud imaging tools provided a new way of imaging the characteristics of formations drilled with oil-based -mud. Complicated structures were resolved utilizing the dip data gathered with such techniques. Fractures were characterized for their aperture (open or closed), intensity of fracturing, and directional attributes. The information helped to understand reservoir characteristics of tight carbonates of Sarvak formation. In addition to structural details, the OBM imaging provided quantitative Rxo (resistivity of invaded zone) measurement in oilbased -mud environment. It helped to understand invasion profiles, which generally is a function of permeability, in carbonate and sandstone reservoirs of Asmari Formation, and helped reservoir engineers and petrophysicists to understand response of RFT (reservoir formation tester) measurements for formation pressure and reservoir fluid mobility. The paper describes applications of OBM imaging in Asmari and Bangestan reservoirs of Dezful embayment of Iran.
Highlights •
A complete understanding of these reservoirs including structural, fractures.
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Depositional Bedding / Layering and resolve complex structural problems.
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Fracture Occurrence, Orientation & Characteristics and Geomechanics & Stresses.
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Depositional Trend for Sandstone Reservoirs.
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Evaluation of permeable zones for selection of MDT points.
Keywords Structural delineation, fracture characterization, oil-based -mud imaging, carbonate and sandstone, FMI, OBMI, UBI 1. Introduction The structural style of the fields located in the southern basin of Iran is quite complex due to compression along the northern edge of the Arabian plate marked by Zagros Mountain belt of 200300 km width (Figure 1&Figure 2). It stretches almost 1200-1400 km and crosses most of Iran (Stocklin 1968, Szabo & Kheradpir 1989, Tatar et al, 2004). It was caused by Arabian plate’s collision with the continental blocks of Central Iran during the Zagros orogeny, which began in Miocene time and continuing today (Stocklin 1968). It was the strongest tectonic event to affect southwest Iran (Figure 1&Figure 2&Figure 3). So keeping in view the complex nature of structures (like Gachsaran, Agha Jari, Bibi Hakimeh and etc.) comprising the Zagros Mountain belt, precise information on the structural dip and fault pattern in the subsurface is mandatory to plan development / infill wells successfully. In some wells higher than expected thickness of formations is found. In some cases, it is caused by steeper bedding dip and in some it is due to reverse faults. In some cases it is not so easy to determine the exact cause of unexpectedly higher thickness. Apart from structural complexities, it is also desirable to know whether productive fractures ahre present in a well which is penetrating a reservoir of very low matrix permeability, such as, Sarvak carbonates in the Dezful embayment (Figure 4). Since most reservoirs in this basin are comprised of carbonates and have a complex tectonics history, therefore the chances of finding good or bad fractures are quite high in these reservoirs and the main challenge is to find out where they are more concentrated in the reservoir and what orientations they have in relation to the structural axis, prevailing stress regime and gas-oil or oilwater contacts. But, because of certain drilling limitations, it is not possible to always drill through such complex structures and reservoirs with water-based mud. Therefore wells are started to drill with oil-based
mud. Which means getting information on structural dip and fracture occurrence
through conventional ways (water based mud imaging) is not so easy. In some cases it is possible but it requires lots of effort and resources to achieve that. In such cases the oil-based mud has to be completely replaced with water-based mud. But still the chances are not high to get the optimal results. To avoid such procedure, which costs considerable amount of extra money, a new combination of borehole imaging tools is being used to map fractures and measure structural dip in the wells drilled with oil-based -mud in the Dezful embayment of Iran and the paper discusses following different applications in oil based mud imaging:
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Structural dip analysis
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Fracture analysis using OBMI-UBI images and open hole logs
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In-situ stress analysis
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Sedimentology
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Petrophysical Analysis
2. Used Imaging Tools So far borehole imaging is considered to be the best technique to identify and measure orientation of planar geological features in the subsurface. However, the increasing use of oil and synthetic based mud systems to limit drilling risks and improve efficiency poses many challenges for formation imaging. Even a thin film of nonconductive mud is essentially an opaque curtain, preventing conventional micro-resistivity imagers from measuring the formation. The presence of nonconductive mud cake or mud filtrate further complicates the situation. Oil-based mud can be displaced with water-based mud at considerable expense, but there is no guarantee that measurement will be possible. Addressing the need for images in this difficult environment clearly demanded a novel approach, which was implemented in the Iranian fields of the Dezful embayment. The approach used in this part of Iran is comprised of two independent techniques that are used in an integrated way to describe the geological features of formations traversed by the wells. The first technique gives acoustic ((UBI (Ultrasonic Borehole Imager Tool) amplitude and transit-time)) map of the features exposed to the borehole surface and the second technique gives the resistivity map ((OBMI (Oil Based Mud Micro Imager Tool)) of the same features. ultrasonic images tend to be relatively straightforward to interpret since they are usually quite simple and they are best for identification of features with high acoustic contrast and high apparent dip. In comparison micro resistivity images tend to be much more complex and can be quite difficult to interpret and they are best for revealing the low angle sedimentary structure with small conductivity contrasts. They are in most cases rich with information and it is well worth investing the time and effort to extract the details. The combination of acoustic(UBI) and electrical(OBMI) borehole imaging devices provided high-resolution pictures of the geological features exposed to the borehole surfaces in the wells drilled in the Dezful embayment. Main applications of these tools in this part of Iran are discussed in the following:
3. Discussion 3.1. Structural Dip Analysis OBMI (a laterolog type electrical imaging tool) is the best tool to provide an image of depositional bedding / layering in the wells drilled with oil-based -mud (OBM). The use of which provides information on structural dip and sedimentary dips and structures. The OBMI images reveal layering / bedding much clearer than the one shown by ultrasonic borehole images (UBI) from the same intervals (Figure 5). OBMI / UBI images in wells drilled with OBM (oil-based mud) helped to measure structural dip in simple (like Marun) and complex fields (like Gachsaran) of NISOC. Layer / bed boundaries in carbonate sequences are not always sharp and planar, due to digenetic processes, to be used for structural dip determination. Therefore while interpreting OBMI / UBI images, the dips from such boundaries are computed interactively with a geological workstation and categorized into two types based on their sharpness and planarity. The dips corresponding to sharp and well planar bed / layer boundaries are categorized as High Confidence. While the dips corresponding to vague and uneven bed / layer surfaces are categorized as Low Confidence. The bedding dips identified on the OBMI and UBI images from the study well were analyzed with the same approach. See Figure 5 and Figure 6 to see difference between high and low confidence bed / layer boundaries Low to high angle structural dips were measured in wells from different fields to understand the structure around
them. Figure 5and Figure 6 show low angle structural dip in well ‘X’ from Marun field and high angle structural dips in well ‘A’ of Gachsaran field, seen clearly by the UBI amplitude and OBMI electrical images. In a case study where a well ‘A’ was drilled in the Gachsaran field according to a pre-drill plan showing a possibility of a reverse fault in the vicinity of the well at the Cap Rock and Asmari reservoir levels (Figure 7). When drilled, the well penetrated the Cap Rock more than 200m deeper than the expected depth (Figure 8). Further drilling to hit the Asmari reservoir was stopped because of the shallow depth of oil-water contact. A side-tract hole had to be drilled towards northeast to penetrate the oil pool of the reservoir (Figure 8). The well successfully entered the reservoir but much larger thickness of it was encountered. The unexpectedly deeper level of the Asmari reservoir in the original hole and its much larger thickness than expected was related to higher structural dip of the Cap Rock and Asmari reservoir or a reverse fault. To investigate the real cause of the problem, OBM imaging was carried out. No indication of faulting was observed, however, a structural dip of 75 degrees SSW was computed .So the increased thickness of the Asmari Reservoir and more than 200m deeper than the expected depth of it in the original hole was a result of much higher structural dip than originally modeled(Figure 9&Figure 10&Figure 11&Figure 12).
3.2. Fracture analysis using OBMI-UBI images and open hole logs Fractures are planar features with no apparent displacement of blocks along their planes. Generally, they have a steep dip in tensional and wrench regimes. Whereas in compressional regimes, they may have high to low angle dips. Their aperture may be open, tight (closed) or filled with some minerals like clays, calcite, anhydrite, pyrite and etc. On the OBMI and UBI images, fractures tend to occur as linear features that generally have a dip steeper than the structural dip. The criteria to differentiate between open and closed fractures are different for oil-based mud imaging tools (OBMI and UBI) than the water-based mud imaging tools (FMI and FMS). On the OBMI images, open fractures and the fractures with their apertures filled with resistive material, like calcite and anhydrite, have the same resistive appearance. It is because of the fact that the open fractures invaded with the oil-based mud have the same resistive appearance as the one filled with resistive minerals like calcite and anhydrite. Such closed fractures can be differentiated from the open fractures using the amplitude image of UBI. The amplitude of the acoustic pulse sent by the UBI decrease in front of the open fractures filled with oil-based mud, thus open fractures appear as darker linear features on the UBI amplitude image. Whereas the calcite / anhydrite filled fractures do not affect the amplitude image because the rock matrix and the fracture filling material have more or less the same amplitude range. However, in some cases such filled fractures can be seen on the amplitude images when there is some amplitude contrast between the rock matrix and the fracture filling material (Figure 13&Figure 14). Fractures play an important role in reservoir characterization to determine reservoir potential, for production management, injection planning, well planning (vertical, inclined or horizontal), and in the end for reservoir simulation, in particular for carbonate reservoirs of low matrix permeability. Sometimes it is desirable to have fractures in a reservoir and sometime it is not. The choice is a direct function of the objectives (producer or injector) of the well, reservoir quality, proximity of the well to contacts between different reservoirs fluids (gas-oil, gas-water and oil-water)
and so on. In general Sarvak carbonates, Cinomanian to Turonian age, of the Dezful embayment of Iran are tight with very low matrix permeability. Therefore it was desirable to have some knowledge of open fracture occurrences in this reservoir. In pursuant of which, OBM imaging was carried out in some wells of the Gachsaran Field. The technique successfully indicated occurrence of open and closed fractures in some wells (Figure 13&Figure 14). Their orientation and distribution across the Sarvak reservoir in those wells were measured. Large variation in their distribution was observed. Some wells intersected no open fractures while some intersected hundreds of them. The well ‘X’ intersected more than 560 open fractures in Sarvak reservoir (420m) (Figure 15). Fracture distribution in the reservoir was computed to indicate zones of high density of open fractures and zones with no such fractures. Nearly entire logged interval of Sarvak formation is fractured with open fractures. The fractures tend to occur in large clusters of 10-20 m length. These clusters of fractures alternate with zones of no fractures or very small number of fractures. The highest density (i.e., number of fractures per meter) of fractures is present in zones at 2470-2485m, 2500-2520m, 2585-2595m, 26202630m,2640-2650m, 2670-2680m, and 2830-2850m (Figure 15).Cores from the same zones also indicated similar distribution of fractures. In a recent example from the Gachsaran field a core was taken from the Sarvak formation. Subsequently the borehole was logged using OBM imaging services to complete a comprehensive dataset for the analysis of fractures.
The core and log images from
this example well were matched in order to verify the log (Figure 16). 3.3. In-Situ Stress Analysis The subsurface of the continental crust rarely stays at hydrostatic stress condition, the stress state under which all points in the crust are subjected from all directions to equal stresses (σ1 = σ2 = σ3). However, such stress conditions are rarely met in the earth's subsurface as many structural movements keep taking place in it. The large portion of the disturbance in the equilibrium in the stress state is contributed by the plates' movements that ultimately result in the formation of regional stress system for the area bounded by them. However, sometimes the regional stress is completely overprinted due to stresses localized to a certain area (Mount & Suppe 1987, Bell 1990). The source of local stress system may be associated with faults, folding, diapirism and so forth. The orientation of such local stresses may be changed abruptly over short distances in any area. The wells drilled in areas subjected to such kind of unbalanced stress system often exhibit two types of borehole failures, shear failure and tensile failure, when the drilled rocks are replaced with the drilling mud (Lehne & Aadnoy 1992, Aadnoy & Bell 1998). The rocks can bear both compressive and shear stresses but the fluid filling the borehole can bear only compressive stress and not shear stress. Consequently, concentration of stresses takes place around the borehole in the form of hoop stress or tangential stress. When the mud weight is too low (i.e., radial stress = mud weight minus pore pressure), the maximum hoop stress becomes much higher than the radial stress. Consequently, a shear failure of rocks exposed to the borehole takes place, which is exhibited in the form of borehole elongation on the orthogonal calipers of dip meters (Cox 1983) or borehole images (Ma 1993, Aadnoy & Bell 1998) as long dark regions on the borehole images that are 180 degrees apart. On the contrary, when the mud weight is too high, the radial stress increases and the hoop stress decreases; consequently rock around the borehole comes under tension and fails in tension; the fractures so created are called
drilling induced fractures. It is manifested in the form of a fracture seen by the images oriented at 180 degrees from each other. Figure 17 gives schematics for the development of shear and tensile failures in boreholes. Generally, in vertical wells and those with smaller deviation, the orientation of borehole elongation is aligned with the trend of minimum horizontal stress. Similarly, the strike of drilling induced is aligned with the trend of maximum horizontal stress. However, it may not be the case with the deviated wells and particularly those wells that are not aligned with either of the two horizontal stresses. Orientations of regional stresses in most part of the Arabian Peninsula (Akbar, 1994) and Iran are NE-SW for the maximum horizontal stress (σHmax) and NW-SE for the minimum horizontal stress (σhmin), which is considered as Zagros stress. Figure 18 shows map of regional stress orientations in most part of Iran, Qatar, UA.E., Oman, and Yemen.OBM borehole imaging, particularly the acoustic (UBI) part of it, has been used to determine orientation of in-situ stresses in a number of fields of Dezful embayment. Most wells imaged with UBI showed that the present day stress orientation in the Dezful embayment tends to follow the regional Zagros stress. However, variation in this trend is also observed. For instance, two wells, X and Y, from the Marun Field, well ‘X’ being located near the crestal region and well ‘Y’ in the northwestern plunge area, show a 40 degrees change in the orientation of maximum horizontal stress (Figure 19). Well ‘Y’ follows the Zagros stress while well ‘X’ does not. The change in stress orientation at well ‘X’ is possibly a result of local structural feature. The information on stress orientation can further be used in well planning, oriented perforations, sanding control, and hydraulic fracture planning.
3.4. Sedimentology Geologists assess vertical and lateral changes in the reservoir by identifying and characterizing large-scale depositional events and sequence stratigraphic boundaries across fields. Using borehole-image data, they also define and determine the orientation of smaller depositional features to understand stratigraphically controlled reservoirs. A close examination of bedding reveals the depositional history in vertical successions of sediment types and grain sizes, helping to answer questions about the reservoir’s origin. Was it deposited by the wind, in a freshwater system, a marine system or in a combination of environments? Was it deposited in deep or shallow water? In what direction was the depositional system prograding? In what direction should the reservoir thicken or thin? Answers to questions like these help geologists determine the potential size of the reservoir, the best drilling locations and whether additional wells are needed for efficient reservoir exploitation (Serra 1989).Asmari sandstone, Oligocene-Miocene age, is largely deltaic in origin and comprises quite a prolific reservoirs in some fields of the Dezful embayment like Ahwaz, Mansuri, and Marun Fields. OBM images and cores from well ‘X’ of the Marun Field showed well-developed cross-beds in the upper sandstone reservoir (Figure 20). Cross-beds are identified as thin surfaces dipping at higher angle than the structural dip and dipping in the same or different direction from the bedding planes. Their dips are also computed interactively from the images to be used for paleocurrent direction determination. The information can be used in sedimentological studies carried out to make a depositional model for the formation under study. The OBMI images indicated NNW direction for the paleo currents of the delta water that deposited it. The images from the similar sandstone reservoir of
another well ‘Y’, which is located about 19 km northwestward with respect to well ‘X’, indicated more northward paleo-current direction (Figure 21). More data on paleo-current direction across the Marun field or other fields of the Dezful embayment would help the sedimentologists to delineate different distributaries of the delta that deposited upper sandstone reservoirs of the Asmari Formation.
3.5. Petrophysical Analysis OBMI provides quantitative measure of Rxo (resistivity of invaded zone) from four directions of borehole at 1.2-inch vertical resolution with a depth of investigation of 3.5 inch (Cheung 2001, Tabanou 2002). In conjunction with induction resistivity and porosity logs, it provides information on reservoir anisotropy for petrophysical evaluation and to compute refined hydrocarbon reserves for thinly laminated sandstone reservoirs (Tabanou 2002).In the Dezful embayment of Iran, OBMI Rxo is being used, along with induction and porosity logs, to indicate zones of higher and lower permeability so that surveys for formation pressure and reservoir fluid mobility with tools like RFT (reservoir formation tester) or MDT (modular dynamics tester) could be accurately planned and the corresponding results could be better explained. It is applicable to the zones that have high water saturation and not for the zones that have much higher hydrocarbon saturation. In a well ‘X’ from the Marun Field, large separation between Rxo and induction (deep and shallow) logs was observed in higher water saturation zones of the Asmari sandstone reservoir (Figure 22). In these zones, Rxo read much higher than the induction logs indicating invasion of oil-based -mud, hence increased resistivity of the invaded zone. The larger separation between the two resistivities (Rxo and induction logs) indicated higher permeability and smaller or no separation between them indicated lower permeability. Measurements of reservoir fluid mobility with the RFT confirmed permeable or less permeable nature of those zones in that well (Figure 22). Locations of dry test points (i.e., points where no formation pressure could be measured because of no permeability for the piece of formation exposed there) measured with the RFT corresponded with the thin shale streaks, highlighted by OBMI images due to its higher vertical resolution, of very low or no permeability in a carbonate section of Asmari reservoir (Figure 23). Thin shale streaks were also indicated by OBMI in the sandstone sections (Figure 22). If laterally continuous, such shale streaks might act as pressure barrier, hence might complicate the pattern of formation pressures measured with the RFT / MDT or any other formation pressure tester.
4. Comparison between oil based mud imaging with water based mud imaging in Dezful Embayment • OBM imaging includes both resistivity & acoustic sensors and OBMI provides the only quantitative Rxo measurement in OBM drilled wells with comparison to water based mud imaging. •
OBMI-Rxo is a high resolution curve that is sensitive to fluid mobility near to the borehole wall and which indicates invasion and indirectly lithology.
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In OBM the Rxo curves from OBMI are being used to provide an enhanced lithological description.
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The UBI therefore provides full bore images which are sensitive to fractures and other borehole wall irregularities as well as providing the caliper data, so OBM images can be better than WBM images for fracture evaluation. Fracture density from OBMI-UBI is very important parameter to increase production in fractured reservoir.
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The most comprehensive image log data for wellbore stability analysis comes from UBI. The pad tools (WBM imaging (FMI)) include a mechanical caliper measurement in 4 directions whereas the acoustic tools (UBI) emit ultrasonic echos that provide a full bore caliper, ie 180 azimuthal samples per depth interval.
5. Conclusion •
The use of OBMI-UBI provided information on structural dip and sedimentary dips and structures. The OBMI images revealed layering / bedding much clearer than by ultrasonic borehole images (UBI) .OBMI / UBI images in wells drilled with OBM (oil-based mud) helped to measure Structural Dip in simple (like Marun) and complex fields (like Gachsaran) of NISOC. OBMI / UBI images helped to resolve structural complexity in that section of the field by providing dips data of bedding, faults and fractures and by using these data exact location of the well within the Asmari reservoir was known.
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OBMI/UBI images provided information on fractured zones in the tight / low porosity carbonates of Sarvak Formation. Information on open fractures in low permeability reservoirs (whether low porosity or high porosity) can be used to predict production or injection profiles in fractured carbonates.
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UBI provided data on in-situ stresses and borehole damage information that can be used for drilling optimization, well planning (for high inclination wells), hydraulic fracturing, perforations stability, sanding prediction, and permeability modeling for reservoir simulation particularly for fractured reservoirs. Borehole images helped to predict the orientation of fractures formed by the hydraulic / acid fracturing campaign.
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OBMI Rxo (resistivity of invaded zone) helped to identify zones of higher permeability when combined with conventional induction logs and porosity logs.
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So, switching the drilling mud from oil to water based in order to run FMI costs money, time and can be risky.
References Aadnoy, B. and Bell, S., 1998. “Classification of Drilling-Induced Fractures and Their Relationship to In-Situ Stress Directions,” The Log Analyst, v. 39, no. 6, pp. 27-42. Akbar, M., 1994.: “In-Situ Stresses in the Subsurface of Arabian Peninsula and their Effect on Fracture Morphology and Permeability,” 6th ADIPEC, ADSPE 99, Abu Dhabi, U.A.E. Bell, J.S., 1990. "Investigating Stress Regimes in Sedimentary Basins Using Information from Oil Industry Wireline Logs and Drilling Records," Geological Applications of Wireline Logs, Geological Society Special Publication No. 48, published by The Geological Society London, pp. 305-325. Cheung, P., 2001. “Field test results of a new oil-based mud formation imager tool”, Transaction of the SPWLA 42nd annual logging symposium, Houston, Texas. Cox, J. W., 1983. "Long-Axis Orientation in Elongated Boreholes and its Correlation with Rock Stress, data," Transactions of the SPWLA 24th Annual Logging Symposium, Calgary, Canada. Lehne, K.A. & Aadnoy, B.S., 1992.: "Quantitative Analysis of Stress Regimes and Fractures from Logs and Drilling Records of a North Sea Chalk Field," The Log Analyst, pp.351-361. Ma, T.A., 1993. "Natural and Induced Fracture Classification Using Image Analysis," SPWLA 34th Annual Logging Symposium. Motiei, H., (1995): "Petroleum Geology of Zagros," Geological Survey of Iran with cooperation of Deputy Ministry of project and planning, no. 25. Mount, V.S. and Suppe, J. 1987: "State of Stress near the San Andreas Fault: Implications for Wrench Tectonics." Geology, 15, pp.1143-1146. Serra, O.,1989: “Sedimentary Environments from Wireline Logs,” 2nd ed. Sugar Land, Texas, USA: Schlumberger Educational Services, pp. 119–233. Stocklin, J., 1968: “Structural history and tectonics of Iran,” a review, AAPG, Bull.52, pp. 509-526. Szabo, F. and Kheradpir, A. 1989: “Permian and Triassic stratigraphy, Zagros Basin, South-West Iran, “Journal of Petroleum Geology, 1,2, pp. 57-82. Tabanou, J.R., 2002: “Thinly laminated reservoir evaluation in oil-based mud: high resolution versus bulk anisotropy measurement- a comprehensive evaluation,” SPWLA 43rd Annual Logging Symposium. Tatar M., Hatzfeld D., and Ghafory-Ashtiany M., 2004: “Tectonics of the Central Zagros (Iran) deduced from micro-earthquake seismicity,” Geophysical Journal International, v.156, pp. 255-266.
Figures
Figure 1: Satellite image of Iran and part of the Arabian plate highlighting intense folding in the southwest part of Iran.
Figure 2: Tectonics map showing location of Iran between the Asian and Arabian tectonic. Foreland folding in the south west of Zagros convergence and large-scale strike-slip faults are indicated in Iran.
NW
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Lurestan
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Izeh Fars
Dezful Embayment
Iraq
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Lurestan Izeh
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ait Kuw
Dezful Embayment Abadan Plain
Saudi Arabia
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Figure 3: NW-SE trending major anticlinal structures in the Foreland basin of Zagros Mountains.
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Figure 4: Location of Dezful Embayment in the southwest of Iran (Motiei 1995).
Dynamic Normalized OBMI Images
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Figure 5: Bedding shown by OBMI images in a section of Asmari Formation from well ‘X’ of Marun Field.
(c) Strike Rosette
(d) Dip Azimuth Rosette Dip histogram
(b)
Figure 6: Depth plot of OBMI image, bedding dips and lithology from well ‘X’ (a); polar plot (b), strike rosette (c), dip azimuth rosette (d) and dip inclination histogram (e) of bedding dips from the same well.
(a) C1,C2, CGR OBMI Image 0
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Figure 7:Structural model for well ‘A’ before drilling the main hole indicates possibility of a reverse fault.
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Figure 8: Structural model made after the main and the side-track holes were drilled.
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Figure 9: OBMI and UBI images showing high angle bedding and low angle open fractures dipping opposite to it in well ‘A’ of Gachsaran Field.
S20W
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Bedding
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(b) Open Fractures Bedding
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(d) Figure 10: Computer generated structural cross-section along N20E-S20W plane using bedding dip data from well ‘A’.
Figure 11: Statistical plots of bedding dips and open fractures, well ‘A’.
Well ‘A’ er sv an Tr se es ur ct fra
res ct u l f ra a n di gitu Lon
Open fractures From OBM Images
ri ma As
Figure 12: Schematic model of a section of the Gachsaran field around wells ‘A’ . OBMI – Closed fracture shown as resistive due to anhydrite filling of its aperture
Closed Fracture
UBI – The same closed fracture has a lighter shade trace on the UBI amplitude image due to higher amplitude caused by denser filling material
2m
Closed Fracture
Figure 13: Example of closed fractures in Asmari carbonates, from a well of Marun Field, indicated by OBMI and UBI images.
OBMI - Open fracture appears resistive due to invasion of oil-based -mud along its plane
UBI - Same fracture have low amplitude on UBI amplitude image, indicating its open nature
Open Fractures Open Fractures
2m
Figure 14: Example of open fractures indicated by OBMI and UBI in Asmari carbonates from a well of Marun Field.
Dynamic Norm. UBI Images
Dynamic Norm. OBMI Images
0
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Open Fractures
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(b)
Figure 15: OBMI and UBI, from well ‘C’, showing fractures (b) in Sarvak limestone of Gachsaran Field. Fracture occurrence log (a) is showing fracture dips and fracture density in relation to log porosity and resistivity.
C al & D rriifftt
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In te r p r e te d d ip s
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0 .2 - 2 0 0 0 o h m
Ve r tic a l sc s c a le o f lo g d isp is p la y = 1:8
Figure 16: Major open fracture cross-cutting bedding plane, possible slickenside on core along the fracture plane.
Minimum Stress
Hydraulic Fracture Shear Failure
Pmud
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Figure 17: Tensile failure (hydraulic fracture) and shear failure (which leads to formation of borehole breakouts) are shown with respect to radial and hoop stresses; and far field minimum and maximum horizontal stresses.
Hoop Stresses
Maximum Stress
Figure 18: Orientation of regional stresses in the Arabian Peninsula and Iran based on borehole breakouts and drilling induced fracture data – Akbar, 1994.
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σHmax
N
Well ‘X’
Well ‘Y’
σHmax = Maximum Horizontal Stress Figure 19: UBI and borehole radii plots show NW-SE trending borehole enlargements (due to shear failure of the rocks exposed on NW and SE sides of the borehole) in well ‘Y’ of Marun Field. Similar plots show nearly N-S trending borehole enlargements in well ‘X’, which is located about 19 Km away from well ‘Y’. Hydraulic fractures trending E-W are also indicated by UBI in well ‘X’. Borehole breakouts and hydraulic fractures indicate that orientation of maximum horizontal stress is changing across the Marun field.
After SD Removed
Before SD Removed
Figure 20: Cross-beds identified by OBMI and cores in the upper sands (deltaic origin) of Asmari Formation in well MN-292. After correcting their dips for the structural dip, the paleo-current direction, which deposited these sands, becomes NNW.
leo Pa low tF en urr -C
Well ‘Y’
N Pa leo
-C ur r
en tF
low
Well ‘X’
Figure 21: Structure contour map of Marun Field showing paleo-current directions (measured from OBMI) of deltaic sands of Asmari Formation at MN-292 and MN-297. The two wells are about 19 kilometers apart.
GR, Depth
OBMI Static Norm. Images
Resistivity (ohmm) / Mobility (md/cp) 0.2
2000
OBMI LQC Image
1.95 RHOB (g/cc) 2.95 0.45 NPHI (v/v) -0.15 0 PEF 20
Litho
RFT Mobility
LLD
Figure 22: OBMI Rxo showing large separation from induction resistivity logs in a section of Asmari sandstone sections of well ‘X’ of Marun Field indicating large invasion due to permeability. RFT mobility (blue circles) also compute higher mobility in the same sections.
OBMI Static Norm. Images
0.2
ohmm & md/cp
2000
Litho
Figure 23: Locations of dry test points (yellow circles) of RFT in a section of Asmari of well ‘X’ of Marun Field, correspond to shale streaks or very low permeability streaks indicated by OBMI images. No invasion in such streaks is observed on Rxo and induction curves indicating their very low permeability. RFT formation mobility is indicated by blue circles.
Evaluation in Dezful Embayment of Iran Using Oil-Based -Mud Imaging Techniques Zohreh Movahed, Radzuan Juninb, Zeynalabedin Safarkhanlou, Mahmood Akbar
www.elsevier.com/locate/petrol
PII: DOI: Reference:
S0920-4105(14)00136-3 http://dx.doi.org/10.1016/j.petrol.2014.05.019 PETROL2668
To appear in:
Journal of Petroleum Science and Engineering
Received date: 16 October 2012 Revised date: 20 May 2014 Accepted date: 24 May 2014 Cite this article as: Zohreh Movahed, Radzuan Juninb, Zeynalabedin Safarkhanlou, Mahmood Akbar, Evaluation in Dezful Embayment of Iran Using Oil-Based -Mud Imaging Techniques, Journal of Petroleum Science and Engineering, http://dx.doi.org/10.1016/j.petrol.2014.05.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Formation Evaluation in Dezful Embayment of Iran Using Oil-Based -Mud Imaging Techniques Zohreh Movaheda, Radzuan Juninbb, Zeynalabedin Safarkhanlouc, Mahmood Akbarc a
UTM University and Schlumberger UTM University c Schlumberger b
Abstract Structural delineation and fracture characterization are two of the main issues for the evaluation of carbonate reservoirs in structurally complex areas. Some of such fields are located in the southwest (Dezful embayment) of Iran. Getting to the Asmari reservoir is not so easy in some cases due to structural complexities where a thick pile of evaporites of Gachsaran Formation overly the reservoir. In such cases structural dip information plays an important role in mapping the structure in the area around the wells. In addition to structural complications, fracture characterization is quite important for Asmari carbonates and in particular for Sarvak carbonates. In most fields of the Dezful embayment, it is not feasible to drill wells with water-based mud, hence getting information on structural dip and fractures is not so easy with conventional borehole imaging tools. A combination of oil-based -mud imaging tools provided a new way of imaging the characteristics of formations drilled with oil-based -mud. Complicated structures were resolved utilizing the dip data gathered with such techniques. Fractures were characterized for their aperture (open or closed), intensity of fracturing, and directional attributes. The information helped to understand reservoir characteristics of tight carbonates of Sarvak formation. In addition to structural details, the OBM imaging provided quantitative Rxo (resistivity of invaded zone) measurement in oilbased -mud environment. It helped to understand invasion profiles, which generally is a function of permeability, in carbonate and sandstone reservoirs of Asmari Formation, and helped reservoir engineers and petrophysicists to understand response of RFT (reservoir formation tester) measurements for formation pressure and reservoir fluid mobility. The paper describes applications of OBM imaging in Asmari and Bangestan reservoirs of Dezful embayment of Iran.
Highlights •
A complete understanding of these reservoirs including structural, fractures.
•
Depositional Bedding / Layering and resolve complex structural problems.
•
Fracture Occurrence, Orientation & Characteristics and Geomechanics & Stresses.
•
Depositional Trend for Sandstone Reservoirs.
•
Evaluation of permeable zones for selection of MDT points.
Keywords Structural delineation, fracture characterization, oil-based -mud imaging, carbonate and sandstone, FMI, OBMI, UBI 1. Introduction The structural style of the fields located in the southern basin of Iran is quite complex due to compression along the northern edge of the Arabian plate marked by Zagros Mountain belt of 200300 km width (Figure 1&Figure 2). It stretches almost 1200-1400 km and crosses most of Iran (Stocklin 1968, Szabo & Kheradpir 1989, Tatar et al, 2004). It was caused by Arabian plate’s collision with the continental blocks of Central Iran during the Zagros orogeny, which began in Miocene time and continuing today (Stocklin 1968). It was the strongest tectonic event to affect southwest Iran (Figure 1&Figure 2&Figure 3). So keeping in view the complex nature of structures (like Gachsaran, Agha Jari, Bibi Hakimeh and etc.) comprising the Zagros Mountain belt, precise information on the structural dip and fault pattern in the subsurface is mandatory to plan development / infill wells successfully. In some wells higher than expected thickness of formations is found. In some cases, it is caused by steeper bedding dip and in some it is due to reverse faults. In some cases it is not so easy to determine the exact cause of unexpectedly higher thickness. Apart from structural complexities, it is also desirable to know whether productive fractures ahre present in a well which is penetrating a reservoir of very low matrix permeability, such as, Sarvak carbonates in the Dezful embayment (Figure 4). Since most reservoirs in this basin are comprised of carbonates and have a complex tectonics history, therefore the chances of finding good or bad fractures are quite high in these reservoirs and the main challenge is to find out where they are more concentrated in the reservoir and what orientations they have in relation to the structural axis, prevailing stress regime and gas-oil or oilwater contacts. But, because of certain drilling limitations, it is not possible to always drill through such complex structures and reservoirs with water-based mud. Therefore wells are started to drill with oil-based
mud. Which means getting information on structural dip and fracture occurrence
through conventional ways (water based mud imaging) is not so easy. In some cases it is possible but it requires lots of effort and resources to achieve that. In such cases the oil-based mud has to be completely replaced with water-based mud. But still the chances are not high to get the optimal results. To avoid such procedure, which costs considerable amount of extra money, a new combination of borehole imaging tools is being used to map fractures and measure structural dip in the wells drilled with oil-based -mud in the Dezful embayment of Iran and the paper discusses following different applications in oil based mud imaging:
•
Structural dip analysis
•
Fracture analysis using OBMI-UBI images and open hole logs
•
In-situ stress analysis
•
Sedimentology
•
Petrophysical Analysis
2. Used Imaging Tools So far borehole imaging is considered to be the best technique to identify and measure orientation of planar geological features in the subsurface. However, the increasing use of oil and synthetic based mud systems to limit drilling risks and improve efficiency poses many challenges for formation imaging. Even a thin film of nonconductive mud is essentially an opaque curtain, preventing conventional micro-resistivity imagers from measuring the formation. The presence of nonconductive mud cake or mud filtrate further complicates the situation. Oil-based mud can be displaced with water-based mud at considerable expense, but there is no guarantee that measurement will be possible. Addressing the need for images in this difficult environment clearly demanded a novel approach, which was implemented in the Iranian fields of the Dezful embayment. The approach used in this part of Iran is comprised of two independent techniques that are used in an integrated way to describe the geological features of formations traversed by the wells. The first technique gives acoustic ((UBI (Ultrasonic Borehole Imager Tool) amplitude and transit-time)) map of the features exposed to the borehole surface and the second technique gives the resistivity map ((OBMI (Oil Based Mud Micro Imager Tool)) of the same features. ultrasonic images tend to be relatively straightforward to interpret since they are usually quite simple and they are best for identification of features with high acoustic contrast and high apparent dip. In comparison micro resistivity images tend to be much more complex and can be quite difficult to interpret and they are best for revealing the low angle sedimentary structure with small conductivity contrasts. They are in most cases rich with information and it is well worth investing the time and effort to extract the details. The combination of acoustic(UBI) and electrical(OBMI) borehole imaging devices provided high-resolution pictures of the geological features exposed to the borehole surfaces in the wells drilled in the Dezful embayment. Main applications of these tools in this part of Iran are discussed in the following:
3. Discussion 3.1. Structural Dip Analysis OBMI (a laterolog type electrical imaging tool) is the best tool to provide an image of depositional bedding / layering in the wells drilled with oil-based -mud (OBM). The use of which provides information on structural dip and sedimentary dips and structures. The OBMI images reveal layering / bedding much clearer than the one shown by ultrasonic borehole images (UBI) from the same intervals (Figure 5). OBMI / UBI images in wells drilled with OBM (oil-based mud) helped to measure structural dip in simple (like Marun) and complex fields (like Gachsaran) of NISOC. Layer / bed boundaries in carbonate sequences are not always sharp and planar, due to digenetic processes, to be used for structural dip determination. Therefore while interpreting OBMI / UBI images, the dips from such boundaries are computed interactively with a geological workstation and categorized into two types based on their sharpness and planarity. The dips corresponding to sharp and well planar bed / layer boundaries are categorized as High Confidence. While the dips corresponding to vague and uneven bed / layer surfaces are categorized as Low Confidence. The bedding dips identified on the OBMI and UBI images from the study well were analyzed with the same approach. See Figure 5 and Figure 6 to see difference between high and low confidence bed / layer boundaries Low to high angle structural dips were measured in wells from different fields to understand the structure around
them. Figure 5and Figure 6 show low angle structural dip in well ‘X’ from Marun field and high angle structural dips in well ‘A’ of Gachsaran field, seen clearly by the UBI amplitude and OBMI electrical images. In a case study where a well ‘A’ was drilled in the Gachsaran field according to a pre-drill plan showing a possibility of a reverse fault in the vicinity of the well at the Cap Rock and Asmari reservoir levels (Figure 7). When drilled, the well penetrated the Cap Rock more than 200m deeper than the expected depth (Figure 8). Further drilling to hit the Asmari reservoir was stopped because of the shallow depth of oil-water contact. A side-tract hole had to be drilled towards northeast to penetrate the oil pool of the reservoir (Figure 8). The well successfully entered the reservoir but much larger thickness of it was encountered. The unexpectedly deeper level of the Asmari reservoir in the original hole and its much larger thickness than expected was related to higher structural dip of the Cap Rock and Asmari reservoir or a reverse fault. To investigate the real cause of the problem, OBM imaging was carried out. No indication of faulting was observed, however, a structural dip of 75 degrees SSW was computed .So the increased thickness of the Asmari Reservoir and more than 200m deeper than the expected depth of it in the original hole was a result of much higher structural dip than originally modeled(Figure 9&Figure 10&Figure 11&Figure 12).
3.2. Fracture analysis using OBMI-UBI images and open hole logs Fractures are planar features with no apparent displacement of blocks along their planes. Generally, they have a steep dip in tensional and wrench regimes. Whereas in compressional regimes, they may have high to low angle dips. Their aperture may be open, tight (closed) or filled with some minerals like clays, calcite, anhydrite, pyrite and etc. On the OBMI and UBI images, fractures tend to occur as linear features that generally have a dip steeper than the structural dip. The criteria to differentiate between open and closed fractures are different for oil-based mud imaging tools (OBMI and UBI) than the water-based mud imaging tools (FMI and FMS). On the OBMI images, open fractures and the fractures with their apertures filled with resistive material, like calcite and anhydrite, have the same resistive appearance. It is because of the fact that the open fractures invaded with the oil-based mud have the same resistive appearance as the one filled with resistive minerals like calcite and anhydrite. Such closed fractures can be differentiated from the open fractures using the amplitude image of UBI. The amplitude of the acoustic pulse sent by the UBI decrease in front of the open fractures filled with oil-based mud, thus open fractures appear as darker linear features on the UBI amplitude image. Whereas the calcite / anhydrite filled fractures do not affect the amplitude image because the rock matrix and the fracture filling material have more or less the same amplitude range. However, in some cases such filled fractures can be seen on the amplitude images when there is some amplitude contrast between the rock matrix and the fracture filling material (Figure 13&Figure 14). Fractures play an important role in reservoir characterization to determine reservoir potential, for production management, injection planning, well planning (vertical, inclined or horizontal), and in the end for reservoir simulation, in particular for carbonate reservoirs of low matrix permeability. Sometimes it is desirable to have fractures in a reservoir and sometime it is not. The choice is a direct function of the objectives (producer or injector) of the well, reservoir quality, proximity of the well to contacts between different reservoirs fluids (gas-oil, gas-water and oil-water)
and so on. In general Sarvak carbonates, Cinomanian to Turonian age, of the Dezful embayment of Iran are tight with very low matrix permeability. Therefore it was desirable to have some knowledge of open fracture occurrences in this reservoir. In pursuant of which, OBM imaging was carried out in some wells of the Gachsaran Field. The technique successfully indicated occurrence of open and closed fractures in some wells (Figure 13&Figure 14). Their orientation and distribution across the Sarvak reservoir in those wells were measured. Large variation in their distribution was observed. Some wells intersected no open fractures while some intersected hundreds of them. The well ‘X’ intersected more than 560 open fractures in Sarvak reservoir (420m) (Figure 15). Fracture distribution in the reservoir was computed to indicate zones of high density of open fractures and zones with no such fractures. Nearly entire logged interval of Sarvak formation is fractured with open fractures. The fractures tend to occur in large clusters of 10-20 m length. These clusters of fractures alternate with zones of no fractures or very small number of fractures. The highest density (i.e., number of fractures per meter) of fractures is present in zones at 2470-2485m, 2500-2520m, 2585-2595m, 26202630m,2640-2650m, 2670-2680m, and 2830-2850m (Figure 15).Cores from the same zones also indicated similar distribution of fractures. In a recent example from the Gachsaran field a core was taken from the Sarvak formation. Subsequently the borehole was logged using OBM imaging services to complete a comprehensive dataset for the analysis of fractures.
The core and log images from
this example well were matched in order to verify the log (Figure 16). 3.3. In-Situ Stress Analysis The subsurface of the continental crust rarely stays at hydrostatic stress condition, the stress state under which all points in the crust are subjected from all directions to equal stresses (σ1 = σ2 = σ3). However, such stress conditions are rarely met in the earth's subsurface as many structural movements keep taking place in it. The large portion of the disturbance in the equilibrium in the stress state is contributed by the plates' movements that ultimately result in the formation of regional stress system for the area bounded by them. However, sometimes the regional stress is completely overprinted due to stresses localized to a certain area (Mount & Suppe 1987, Bell 1990). The source of local stress system may be associated with faults, folding, diapirism and so forth. The orientation of such local stresses may be changed abruptly over short distances in any area. The wells drilled in areas subjected to such kind of unbalanced stress system often exhibit two types of borehole failures, shear failure and tensile failure, when the drilled rocks are replaced with the drilling mud (Lehne & Aadnoy 1992, Aadnoy & Bell 1998). The rocks can bear both compressive and shear stresses but the fluid filling the borehole can bear only compressive stress and not shear stress. Consequently, concentration of stresses takes place around the borehole in the form of hoop stress or tangential stress. When the mud weight is too low (i.e., radial stress = mud weight minus pore pressure), the maximum hoop stress becomes much higher than the radial stress. Consequently, a shear failure of rocks exposed to the borehole takes place, which is exhibited in the form of borehole elongation on the orthogonal calipers of dip meters (Cox 1983) or borehole images (Ma 1993, Aadnoy & Bell 1998) as long dark regions on the borehole images that are 180 degrees apart. On the contrary, when the mud weight is too high, the radial stress increases and the hoop stress decreases; consequently rock around the borehole comes under tension and fails in tension; the fractures so created are called
drilling induced fractures. It is manifested in the form of a fracture seen by the images oriented at 180 degrees from each other. Figure 17 gives schematics for the development of shear and tensile failures in boreholes. Generally, in vertical wells and those with smaller deviation, the orientation of borehole elongation is aligned with the trend of minimum horizontal stress. Similarly, the strike of drilling induced is aligned with the trend of maximum horizontal stress. However, it may not be the case with the deviated wells and particularly those wells that are not aligned with either of the two horizontal stresses. Orientations of regional stresses in most part of the Arabian Peninsula (Akbar, 1994) and Iran are NE-SW for the maximum horizontal stress (σHmax) and NW-SE for the minimum horizontal stress (σhmin), which is considered as Zagros stress. Figure 18 shows map of regional stress orientations in most part of Iran, Qatar, UA.E., Oman, and Yemen.OBM borehole imaging, particularly the acoustic (UBI) part of it, has been used to determine orientation of in-situ stresses in a number of fields of Dezful embayment. Most wells imaged with UBI showed that the present day stress orientation in the Dezful embayment tends to follow the regional Zagros stress. However, variation in this trend is also observed. For instance, two wells, X and Y, from the Marun Field, well ‘X’ being located near the crestal region and well ‘Y’ in the northwestern plunge area, show a 40 degrees change in the orientation of maximum horizontal stress (Figure 19). Well ‘Y’ follows the Zagros stress while well ‘X’ does not. The change in stress orientation at well ‘X’ is possibly a result of local structural feature. The information on stress orientation can further be used in well planning, oriented perforations, sanding control, and hydraulic fracture planning.
3.4. Sedimentology Geologists assess vertical and lateral changes in the reservoir by identifying and characterizing large-scale depositional events and sequence stratigraphic boundaries across fields. Using borehole-image data, they also define and determine the orientation of smaller depositional features to understand stratigraphically controlled reservoirs. A close examination of bedding reveals the depositional history in vertical successions of sediment types and grain sizes, helping to answer questions about the reservoir’s origin. Was it deposited by the wind, in a freshwater system, a marine system or in a combination of environments? Was it deposited in deep or shallow water? In what direction was the depositional system prograding? In what direction should the reservoir thicken or thin? Answers to questions like these help geologists determine the potential size of the reservoir, the best drilling locations and whether additional wells are needed for efficient reservoir exploitation (Serra 1989).Asmari sandstone, Oligocene-Miocene age, is largely deltaic in origin and comprises quite a prolific reservoirs in some fields of the Dezful embayment like Ahwaz, Mansuri, and Marun Fields. OBM images and cores from well ‘X’ of the Marun Field showed well-developed cross-beds in the upper sandstone reservoir (Figure 20). Cross-beds are identified as thin surfaces dipping at higher angle than the structural dip and dipping in the same or different direction from the bedding planes. Their dips are also computed interactively from the images to be used for paleocurrent direction determination. The information can be used in sedimentological studies carried out to make a depositional model for the formation under study. The OBMI images indicated NNW direction for the paleo currents of the delta water that deposited it. The images from the similar sandstone reservoir of
another well ‘Y’, which is located about 19 km northwestward with respect to well ‘X’, indicated more northward paleo-current direction (Figure 21). More data on paleo-current direction across the Marun field or other fields of the Dezful embayment would help the sedimentologists to delineate different distributaries of the delta that deposited upper sandstone reservoirs of the Asmari Formation.
3.5. Petrophysical Analysis OBMI provides quantitative measure of Rxo (resistivity of invaded zone) from four directions of borehole at 1.2-inch vertical resolution with a depth of investigation of 3.5 inch (Cheung 2001, Tabanou 2002). In conjunction with induction resistivity and porosity logs, it provides information on reservoir anisotropy for petrophysical evaluation and to compute refined hydrocarbon reserves for thinly laminated sandstone reservoirs (Tabanou 2002).In the Dezful embayment of Iran, OBMI Rxo is being used, along with induction and porosity logs, to indicate zones of higher and lower permeability so that surveys for formation pressure and reservoir fluid mobility with tools like RFT (reservoir formation tester) or MDT (modular dynamics tester) could be accurately planned and the corresponding results could be better explained. It is applicable to the zones that have high water saturation and not for the zones that have much higher hydrocarbon saturation. In a well ‘X’ from the Marun Field, large separation between Rxo and induction (deep and shallow) logs was observed in higher water saturation zones of the Asmari sandstone reservoir (Figure 22). In these zones, Rxo read much higher than the induction logs indicating invasion of oil-based -mud, hence increased resistivity of the invaded zone. The larger separation between the two resistivities (Rxo and induction logs) indicated higher permeability and smaller or no separation between them indicated lower permeability. Measurements of reservoir fluid mobility with the RFT confirmed permeable or less permeable nature of those zones in that well (Figure 22). Locations of dry test points (i.e., points where no formation pressure could be measured because of no permeability for the piece of formation exposed there) measured with the RFT corresponded with the thin shale streaks, highlighted by OBMI images due to its higher vertical resolution, of very low or no permeability in a carbonate section of Asmari reservoir (Figure 23). Thin shale streaks were also indicated by OBMI in the sandstone sections (Figure 22). If laterally continuous, such shale streaks might act as pressure barrier, hence might complicate the pattern of formation pressures measured with the RFT / MDT or any other formation pressure tester.
4. Comparison between oil based mud imaging with water based mud imaging in Dezful Embayment • OBM imaging includes both resistivity & acoustic sensors and OBMI provides the only quantitative Rxo measurement in OBM drilled wells with comparison to water based mud imaging. •
OBMI-Rxo is a high resolution curve that is sensitive to fluid mobility near to the borehole wall and which indicates invasion and indirectly lithology.
•
In OBM the Rxo curves from OBMI are being used to provide an enhanced lithological description.
•
The UBI therefore provides full bore images which are sensitive to fractures and other borehole wall irregularities as well as providing the caliper data, so OBM images can be better than WBM images for fracture evaluation. Fracture density from OBMI-UBI is very important parameter to increase production in fractured reservoir.
•
The most comprehensive image log data for wellbore stability analysis comes from UBI. The pad tools (WBM imaging (FMI)) include a mechanical caliper measurement in 4 directions whereas the acoustic tools (UBI) emit ultrasonic echos that provide a full bore caliper, ie 180 azimuthal samples per depth interval.
5. Conclusion •
The use of OBMI-UBI provided information on structural dip and sedimentary dips and structures. The OBMI images revealed layering / bedding much clearer than by ultrasonic borehole images (UBI) .OBMI / UBI images in wells drilled with OBM (oil-based mud) helped to measure Structural Dip in simple (like Marun) and complex fields (like Gachsaran) of NISOC. OBMI / UBI images helped to resolve structural complexity in that section of the field by providing dips data of bedding, faults and fractures and by using these data exact location of the well within the Asmari reservoir was known.
•
OBMI/UBI images provided information on fractured zones in the tight / low porosity carbonates of Sarvak Formation. Information on open fractures in low permeability reservoirs (whether low porosity or high porosity) can be used to predict production or injection profiles in fractured carbonates.
•
UBI provided data on in-situ stresses and borehole damage information that can be used for drilling optimization, well planning (for high inclination wells), hydraulic fracturing, perforations stability, sanding prediction, and permeability modeling for reservoir simulation particularly for fractured reservoirs. Borehole images helped to predict the orientation of fractures formed by the hydraulic / acid fracturing campaign.
•
OBMI Rxo (resistivity of invaded zone) helped to identify zones of higher permeability when combined with conventional induction logs and porosity logs.
•
So, switching the drilling mud from oil to water based in order to run FMI costs money, time and can be risky.
References Aadnoy, B. and Bell, S., 1998. “Classification of Drilling-Induced Fractures and Their Relationship to In-Situ Stress Directions,” The Log Analyst, v. 39, no. 6, pp. 27-42. Akbar, M., 1994.: “In-Situ Stresses in the Subsurface of Arabian Peninsula and their Effect on Fracture Morphology and Permeability,” 6th ADIPEC, ADSPE 99, Abu Dhabi, U.A.E. Bell, J.S., 1990. "Investigating Stress Regimes in Sedimentary Basins Using Information from Oil Industry Wireline Logs and Drilling Records," Geological Applications of Wireline Logs, Geological Society Special Publication No. 48, published by The Geological Society London, pp. 305-325. Cheung, P., 2001. “Field test results of a new oil-based mud formation imager tool”, Transaction of the SPWLA 42nd annual logging symposium, Houston, Texas. Cox, J. W., 1983. "Long-Axis Orientation in Elongated Boreholes and its Correlation with Rock Stress, data," Transactions of the SPWLA 24th Annual Logging Symposium, Calgary, Canada. Lehne, K.A. & Aadnoy, B.S., 1992.: "Quantitative Analysis of Stress Regimes and Fractures from Logs and Drilling Records of a North Sea Chalk Field," The Log Analyst, pp.351-361. Ma, T.A., 1993. "Natural and Induced Fracture Classification Using Image Analysis," SPWLA 34th Annual Logging Symposium. Motiei, H., (1995): "Petroleum Geology of Zagros," Geological Survey of Iran with cooperation of Deputy Ministry of project and planning, no. 25. Mount, V.S. and Suppe, J. 1987: "State of Stress near the San Andreas Fault: Implications for Wrench Tectonics." Geology, 15, pp.1143-1146. Serra, O.,1989: “Sedimentary Environments from Wireline Logs,” 2nd ed. Sugar Land, Texas, USA: Schlumberger Educational Services, pp. 119–233. Stocklin, J., 1968: “Structural history and tectonics of Iran,” a review, AAPG, Bull.52, pp. 509-526. Szabo, F. and Kheradpir, A. 1989: “Permian and Triassic stratigraphy, Zagros Basin, South-West Iran, “Journal of Petroleum Geology, 1,2, pp. 57-82. Tabanou, J.R., 2002: “Thinly laminated reservoir evaluation in oil-based mud: high resolution versus bulk anisotropy measurement- a comprehensive evaluation,” SPWLA 43rd Annual Logging Symposium. Tatar M., Hatzfeld D., and Ghafory-Ashtiany M., 2004: “Tectonics of the Central Zagros (Iran) deduced from micro-earthquake seismicity,” Geophysical Journal International, v.156, pp. 255-266.
Figures
Figure 1: Satellite image of Iran and part of the Arabian plate highlighting intense folding in the southwest part of Iran.
Figure 2: Tectonics map showing location of Iran between the Asian and Arabian tectonic. Foreland folding in the south west of Zagros convergence and large-scale strike-slip faults are indicated in Iran.
NW
SE
N
ZAGROS
Lurestan
THRUST
Izeh Fars
Dezful Embayment
Iraq
Oman
Thrust Zone
Persian
Lurestan Izeh
Gulf
ait Kuw
Dezful Embayment Abadan Plain
Saudi Arabia
Fars 0
Figure 3: NW-SE trending major anticlinal structures in the Foreland basin of Zagros Mountains.
100
200
300 Km
Figure 4: Location of Dezful Embayment in the southwest of Iran (Motiei 1995).
Dynamic Normalized OBMI Images
Dips and Borehole deviation 0
deg.
90
Static Normalized OBMI Images
Dynamic Normalized UBI Amplitude Image
Static Normalized UBI Amplitude Image
Lithology
Depth, C1,C2, CGR
Figure 5: Bedding shown by OBMI images in a section of Asmari Formation from well ‘X’ of Marun Field.
(c) Strike Rosette
(d) Dip Azimuth Rosette Dip histogram
(b)
Figure 6: Depth plot of OBMI image, bedding dips and lithology from well ‘X’ (a); polar plot (b), strike rosette (c), dip azimuth rosette (d) and dip inclination histogram (e) of bedding dips from the same well.
(a) C1,C2, CGR OBMI Image 0
(e)
deg.
90
Litho
Meters
NE
SW
SW
-1000
Well ‘A’
1000
NE
Well ‘A’
500
Gs .M Gs . M b. 3 b. 2
Fault F Gachsaran Fm.
p Ca
ck
ri ma As
-1250
Original hole
Sea Level
Ro
. Fm
Gachsaran Fm.
-500
e bd Pa
-1500
Cap Rock
m. hF
-1000
ran Ga
?
M b.
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Gs .
-2000
4-2
-1750
F
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chs a
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Fm .
m. ari F Asm Fm. deh Pab
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-2500
Figure 7:Structural model for well ‘A’ before drilling the main hole indicates possibility of a reverse fault.
C1,C2 GR
Static Norm. OBMI Image
Dyn.Norm. OBMI Images
0
Dip (deg)
90
Figure 8: Structural model made after the main and the side-track holes were drilled.
Dynamic Norm. UBI Amplitude Image
Static Norm. UBI Amplitude Image
Litho
Bedding
DEVI
Open Fractures
Figure 9: OBMI and UBI images showing high angle bedding and low angle open fractures dipping opposite to it in well ‘A’ of Gachsaran Field.
S20W
Asmari Top
N20E
Open Fractures
Bedding
(a)
(c)
(b) Open Fractures Bedding
Bedding Open Fractures
(d) Figure 10: Computer generated structural cross-section along N20E-S20W plane using bedding dip data from well ‘A’.
Figure 11: Statistical plots of bedding dips and open fractures, well ‘A’.
Well ‘A’ er sv an Tr se es ur ct fra
res ct u l f ra a n di gitu Lon
Open fractures From OBM Images
ri ma As
Figure 12: Schematic model of a section of the Gachsaran field around wells ‘A’ . OBMI – Closed fracture shown as resistive due to anhydrite filling of its aperture
Closed Fracture
UBI – The same closed fracture has a lighter shade trace on the UBI amplitude image due to higher amplitude caused by denser filling material
2m
Closed Fracture
Figure 13: Example of closed fractures in Asmari carbonates, from a well of Marun Field, indicated by OBMI and UBI images.
OBMI - Open fracture appears resistive due to invasion of oil-based -mud along its plane
UBI - Same fracture have low amplitude on UBI amplitude image, indicating its open nature
Open Fractures Open Fractures
2m
Figure 14: Example of open fractures indicated by OBMI and UBI in Asmari carbonates from a well of Marun Field.
Dynamic Norm. UBI Images
Dynamic Norm. OBMI Images
0
Dips 90
Open Fractures
(a)
(b)
Figure 15: OBMI and UBI, from well ‘C’, showing fractures (b) in Sarvak limestone of Gachsaran Field. Fracture occurrence log (a) is showing fracture dips and fracture density in relation to log porosity and resistivity.
C al & D rriifftt
OBMI
In te r p r e te d d ip s
O B M I-R x o
U B I-A m p
U B I - R a d iu s
0 .2 - 2 0 0 0 o h m
Ve r tic a l sc s c a le o f lo g d isp is p la y = 1:8
Figure 16: Major open fracture cross-cutting bedding plane, possible slickenside on core along the fracture plane.
Minimum Stress
Hydraulic Fracture Shear Failure
Pmud
Radial Stress
Figure 17: Tensile failure (hydraulic fracture) and shear failure (which leads to formation of borehole breakouts) are shown with respect to radial and hoop stresses; and far field minimum and maximum horizontal stresses.
Hoop Stresses
Maximum Stress
Figure 18: Orientation of regional stresses in the Arabian Peninsula and Iran based on borehole breakouts and drilling induced fracture data – Akbar, 1994.
N
E
S
W
N
N
E
S
W
Borehole Breakouts
N (M)
(M) N
σH σH Hydraulic fracture
σH
σH Borehole Breakouts
GR
GR
Well ‘Y’
Well ‘X’
σHmax
σHmax
N
Well ‘X’
Well ‘Y’
σHmax = Maximum Horizontal Stress Figure 19: UBI and borehole radii plots show NW-SE trending borehole enlargements (due to shear failure of the rocks exposed on NW and SE sides of the borehole) in well ‘Y’ of Marun Field. Similar plots show nearly N-S trending borehole enlargements in well ‘X’, which is located about 19 Km away from well ‘Y’. Hydraulic fractures trending E-W are also indicated by UBI in well ‘X’. Borehole breakouts and hydraulic fractures indicate that orientation of maximum horizontal stress is changing across the Marun field.
After SD Removed
Before SD Removed
Figure 20: Cross-beds identified by OBMI and cores in the upper sands (deltaic origin) of Asmari Formation in well MN-292. After correcting their dips for the structural dip, the paleo-current direction, which deposited these sands, becomes NNW.
leo Pa low tF en urr -C
Well ‘Y’
N Pa leo
-C ur r
en tF
low
Well ‘X’
Figure 21: Structure contour map of Marun Field showing paleo-current directions (measured from OBMI) of deltaic sands of Asmari Formation at MN-292 and MN-297. The two wells are about 19 kilometers apart.
GR, Depth
OBMI Static Norm. Images
Resistivity (ohmm) / Mobility (md/cp) 0.2
2000
OBMI LQC Image
1.95 RHOB (g/cc) 2.95 0.45 NPHI (v/v) -0.15 0 PEF 20
Litho
RFT Mobility
LLD
Figure 22: OBMI Rxo showing large separation from induction resistivity logs in a section of Asmari sandstone sections of well ‘X’ of Marun Field indicating large invasion due to permeability. RFT mobility (blue circles) also compute higher mobility in the same sections.
OBMI Static Norm. Images
0.2
ohmm & md/cp
2000
Litho
Figure 23: Locations of dry test points (yellow circles) of RFT in a section of Asmari of well ‘X’ of Marun Field, correspond to shale streaks or very low permeability streaks indicated by OBMI images. No invasion in such streaks is observed on Rxo and induction curves indicating their very low permeability. RFT formation mobility is indicated by blue circles.