Key surface logging technologies in horizontal well geosteering

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 39, Issue 5, October 2012 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2012, 39(5): 660–666.

RESEARCH PAPER

Key surface logging technologies in horizontal well geosteering Li Yichao1,*, Wang Zhizhan2, Qin Liming2, Xu Hongze3 1. Sinopec management department of Petroleum Engineering, Beijing 100007, China; 2. Logging Department, Sinopec Research Institute of Petroleum Engineering, Beijing 100101, China; 3. Geologging Company, Shengli Petroleum Administration Bureau, Sinopec, Dongying 257064, China

Abstract: In the drilling of horizontal wells, the core task of surface logging is to guide geosteering while drilling by using collected data including electrical property, lithology, petrophysics, and oil/gas-bearing properties. Surface logging in the horizontal geosteering drilling have two key technologies: one is stratigraphic correlation and prediction technology before encountering the zone of interest, the other one is geological interpretation and geosteering technology after encountering the zone of interest. Based on a systematic analysis of difficult points in the two aspects, in combination with application examples, this paper deeply analyzes how to accurately drill into horizontal section by in-depth stratigraphic correlation and timely model modification in the case of inaccurate horizon depth design; and how to improve drilling-encounter ratio of oil layer and well bore quality by accurate interpretation and guidance in the case of encountering non-target zone (e.g., borehole deviates from right track, sedimentary facies changes in the target zone, faults or mudstone interlayers are encountered.). Key words: horizontal well; geosteering; mud logging; stratigraphic correlation and prediction; geological interpretation

Introduction Horizontal wells can greatly increase the length of borehole in production layer and drainage area. Although their costs are higher than that of vertical wells, their productions are several times more than that of vertical wells[1]. In some reservoirs such as thin, low permeable, heavy oil and shale gas reservoirs, and some reservoirs with active bottom water or gas cap, horizontal wells have been drilled extensively[24]. Currently, tight sand and shale gas reservoirs become the focus of oil exploration and development in China[5, 6]. Additionally, these unconventional resources only can be exploited by the horizontal wells to obtain good economic benefits. During the drilling of horizontal wells, geosteering while drilling plays an important role. Geosteering while drilling has been widely used abroad, such as Bakerhuges company’s Trak series, which includes AziTrak Deep Azimuthal Resistivity(AZiTrak), LithoTrak Bulk Density and Neutron Porosity (LithoTrak), MagTrak Magnetic Resonance (MagTrak), SoundTrak Acoustic Formation Measurements (SoundTrak), StarTrak High-Definition Resistivity Imaging (StarTrak), TesTrak Formation Pressure Testing (TesTrak), etc. In China, logging while drilling technology (LWD) is just starting to rise while mud logging (including comprehensive logging), MWD (Measure-

ment While Drilling) and so on are mainly used to geosteer while drilling. However, no matter what techniques are used in geosteering, the core of mud logging is utilizing much more information (lithology, resistivity, petrophysical and oil-bearing properties) obtained by logging while drilling to predict or geosteer. Prediction here is stratigraphic correlation and prediction before drilling into horizontal interval, and geosteering is geological interpretation and orientation during drilling in the horizontal interval. So, the former aims at drilling into oil layer and the latter aims at making sure the horizontal interval in the oil layer to improve the economic benefits. In this paper, two critical techniques—stratigraphic correlation and prediction, and geological interpretation and orientation—for geosteering while drilling horizontal wells are discussed.

1

Stratigraphic correlation and prediction

Stratigraphic correlation is an important mean and basis of geological research. The correlation, division and prediction of strata is an important technology of mud logging in oilfields, which play an important role in determining coring formation, buried-hill interface and finishing horizon. Furthermore, it is crucial in accurately predicting target layers when drilling horizontal wells or extended reach wells. Al-

Received date: 29 Nov. 2011; Revised date: 16 May 2012. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (41002034). Copyright © 2012, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

LI Yichao et al. / Petroleum Exploration and Development, 2012, 39(5): 660–666

though horizontal wells may be deployed when the formation have been identified clearly with many adjacent wells, due to the seismic data quality or resolution, it is not surprising the depth of target zone by geological design differs by several meters from actual drilling depth. Before drilling into horizontal interval, the deviation angle often reaches up to more than 70°. Now, if the vertical depth differs by one meter, the horizontal displacement would differ by dozens of meters or even over one hundred meters, so that the quality and the oil layer drilling-encounter ratio of horizontal wells probably reduce sharply, especially for thin layers. As soon as the bit penetrates the target layer into bottom coal or soft formation, the drilling would be finished prematurely, not meeting design requirement[7]. 1.1

Difficulties and measures

The application of PDC (Polycrystalline Diamond Compact) bits and underbalanced drilling and big deviation angles produces fine and mingled cuttings, posing challenges to lithology identification, and resulting in drop of oil bearing level. Additionally, due to the changes of structures, lithofacies and sedimentary facies, every two wells are different in formation condition and stratigrahpic correlation difficulty. Sometimes, some wells have no marker layers and index bed for correlating, making stratigraphic correlation and prediction while drilling very difficult. With progress of mud logging, the degrees of fining, quantification and generalization have been improved gradually. For example, flash chromatograph (analysis cycle of 30 s) and micro drilling time techniques (one point by 0.1 m) provide effective solutions for fine stratigraphic correlation and division; elemental mud logging (XRF element logging[8], laser induced element logging) and cuttings gamma logging provide valid solutions for stratigraphic correlation under special drilling conditions or cases without marker layers; NMR logging (core NMR and drilling fluid NMR), quantitative fluorescence logging and ion chromatography logging provide powerful means for quantification and comparison of oil bearing, water bearing and physical properties of the reservoir. Stratigraphic correlation should choose adjacent wells in the same fault block with the same material source and sedimentary facies, following the principle of cyclicity, similarity and coordination, and firstly big strata should be controlled then the small layers can be identified in detail. The basis of correlation is marker layer, sedimentary cycle, lithological association, element characters and gamma spectrometry features. And the method of correlation is comparing mud logging, MWD, LWD information of the well being drilled with those of the well used as design basis to predict the target zone’s depth while drilling, under constraint of seismic data calibrated by synthetic record and on the basis of understanding of strata distribution. 1.2

Application examples

Taking well L651-P1 in A oilfield as example, the target

zone of drilling designed is the oil layer in biogenic limestone of first member of Palaeogene Shahejie Formation, corresponding to the oil layer from 1 945.0 m to 1 949.3 m in well L651 with a thickness of 4.3 m. Both vertical depths of target A and target B are 1 945.5 m and the horizontal distance between target A and target B is 300 m. The drilling design requires that the horizontal interval should be drilled under the top of the target zone within distance of 1m at 90° deviation angle. In this area, the lithological association of first member in Shahejie Formation includes oil mudstone, oil shale interbed with dolomite and biogenic limestone, in which dolomite, biogenic limestone and oil shale are all marker layers. However, logging items of the well are only gas logging, cutting logging, LWD with GR and RILD logging (Fig. 1). Since oil shale and gray argillaceous dolomite in first member of Shehejie Formation have gas show, and values of resistivity curve are all high, the comparison should be in the principle of large-strata controlling. In adjacent Well L651, argillaceous dolomite appears 10 m above of the target zone top, which can be regarded as correlation basis. For stratigraphic correlation before drilling into oil layer, all LWD resistivity values at vertical depth of 1 935 m are low, but the Gamma curves show big jumping magnitude, making correlation difficult. However, cutting logging doesn’t show argillaceous dolomite at this depth, so it can be inferred that the target zone may be deeper. The comparison between vertical-depth logging curves show the target zone could be pushed back to 1 954.5 m. When continuing drilling at reduced inclination, the resistivity at the vertical depth of 1 945 m show high tips (Fig. 1b), and through comparison, this high resistivity interval is equal to that of the first argillaceous dolomite at the depth of 1 937.5 m of Well L651. Therefore, the top depth of oil layer is inferred at 1 957 m. When drilling to inclined depth of 2 092 m (vertical depth of 1 951.2 m), argillaceous dolomite appears without oil and gas show, so it is not target zone. When continuing drilling to inclined depth of 2 172 m (vertical depth of 1 956.2 m), argillaceous dolomite with oil patch is found. From the correlation of mud logging section, this is recognized as the target zone, which is 11.7 m deeper than the design depth (Fig. 2). The resistivity in this well is high at both oil shale and argillaceous dolomite so it is difficult to compare. However, from the resistivity characteristics in well L651, when drilling into target zone, the Gamma curve slips down firstly and increases in lime mudstone zone. But when drilling argillaceous dolomite, it shows high resistivity so the oil shale in the argillaceous dolomite displays lower Gamma value. Therefore, these characteristics provide evidences for predicting the target zone accurately. Another example is Well DP6 in B oilfield. The designed target zone is quartz sandstone layer in Shan 1 member. The vertical depth of target A is 2 874 m and the distance between target A and the bottom of the sandstone is 5.14 m. During drilling, based on the bottom depth of Shan 2 member and the coal layer depth of Shan 1 1 member marker, the mud logging

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Fig. 1

Stratigraphic correlation while drilling well L651 (a) and well L651-P1(b)

Fig. 2 Comparison between designed borehole trajectory and practical borehole trajectory of well L651-P1

workers predicted that the depth of target A would be drilled 7 m ahead of the designed depth. But the owner did not accept this prediction and continued to drill according to the original design. So, the target zone was drilled throughout earlier at the vertical depth of 2 871.53 m and then the drilling was delayed for three days to fill the well by cementing plug (Table 1). The repeat drilling cost 18 days and the total time delay reached 21 days. Actually, the bottom depth of target zone predicted by LWD differs from the actual drilling depth by

less than 0.5 m[7].

2

Geological interpretation and geosteering

Oil layer drilling-encounter ratio after drilling into the horizontal interval is a crucial indicator to judge whether the horizontal well is drilled successfully or not. Advanced geosteering techniques such as LWD imaging logging, azimuthal resistivity logging, NMR logging and remote boundary detection have been used extensively in horizontal wells

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Table 1 The comparison of designed and actual drilling depth of Well DP6 Vertical depth/m Compared Prediction Design Actual drilling Design Prediction items results results results error error Bottom of Shan 2 member

2 824.50

2 819.00

5.50

/

Bottom of target zone

2 872.00 2 879.14

2 871.53

7.61

+0.47

Target A

2 866.86 2 874.00

2 866.39

7.61

+0.47

/

Fig. 3 Some situations in which nontarget zones are drilled in the horizontal section

abroad. Currently, geosteering in horizontal wells in China has a wide gap from that overseas [7, 9]. For example, both LWD technique and interpretation lag far behind. However, lithology, physical properties and oil bearing data by mud logging while drilling and resistivity information by LWD/MWD[10], combined with seismic section, can still correct oil layer models at real time during drilling, realizing precise geosteering to increase oil layer drilling-encounter ratio. Although surface logging may be affected by borehole factors and lag behind a little, the information is direct and intuitive, which can reduce the multi-solution of the interpretation results. This is an advantage which LWD data doesn’t have. Furthermore, the lagging parameters in horizontal wells of middle or shallow layers are in real time so that the geological interpretation should combine these two methods. 2.1

Analysis and judgment of lithology in target zone

Seismic data cannot identify facies changes and small faults caused by thin layer’s variation and lithology changes, so it is common to encounter unexpected lithology and poorer oil and gas show at target zone during horizontal drilling. Lithology variation can be judged by many methods such as drillingtime logging, element logging [11], cutting logging, Gamma logging while drilling and resistivity curve while drilling. And variations in oil and gas show can be identified by changes of gas logging curve, oil content in drilling fluid [12] and resistivity characteristics. Usually, when drilling into horizontal interval, non-target-zone lithology (mudstones) is drilled in possible situations as follows: (1) Borehole deviates from the correct trajectory (Fig.3a). In this case, the angle between wellbore trajectory and formation should be analyzed by LWD information [13] (easily interpreted using azimuthal resistivity or imaging logging), and the drill bit deviation direction and distance need to be judged to adjust the drilling trajectory. (2) Sedimentary facies changes in target zone (Fig. 3b). Two possible reasons may cause this situation, one is facies changes or pinch out of sandbody, and the other is that there are some more sandbodies but they do not connect with each other. If the reason is the former, drilling should be stopped in time. If the reason is the latter, whether continue to drill or not should be determined by judging distance between

sandbodies according to well area information and seismic section. (3) Drilling faults (Fig. 3c). Under this situation, the fault types such as normal fault or reverse fault and fault throws should be interpreted accurately to determine if drilling inclination should be increased or reduced. (4) Encountering mudstone interbeds (Fig. 3d). Drilling can be continued under this situation. Only through accurate interpretation, can the direction of drill bit be adjusted exactly and can if and when the drilling should be finished [14] be determined. 2.2

Application examples

In well ZB3-P4 of A oilfield, the target zone is biogenic limestone layer in the first member of Shahejie Formation, and equal to the oil layer of the first member of Shahejie Formation in Well BN3-30 whose depth is from 1 412.1 m to 1 423.6 m with a thickness of 11.5 m. The design vertical depths of target A and B are 1 411.8 m and 1 414.8 m respectively. The horizontal distance between them is 300 m and the oil layer’s top depths of target A and B are 1409.8 m and 1408.8 m respectively. The drilling design requires that the horizontal interval should be drilled 26 m below the top of target zone at the deviation angle of 89.43°. In this area, the biogenic limestone belongs to biohermal sedimentary. Drilled vertical wells show the biogenic limestone layer is thick in some wells, but in other wells, the layer is thin or even missing, so it can be inferred that the biohermal sedimentary is discontinuous laterally. In drilling process, the target zone was drilled in the inclined depth of 1 515 m (vertical depth of 1 405.4 m), 4.4 m ahead of the designed depth. Based on the new top depth of oil shale, the vertical depths of target A and B were adjusted to 1 407.4 m. When drilling into horizontal interval, the cutting samples at the depth of 1 606 m show gray mudstones, but the seismic section (Fig. 4) showed the drilling trajectory extending in the oil layer. So, it was concluded through analysis that the discontinuity of biogenic limestone may explain why the mudstone was encountered, and the drilling should follow the designed trajectory. After drilling 49 m in mudstone layer to inclined depth of 1 655 m, oil patches of biogenic limestone were observed once again (Fig. 5). Then the horizontal interval was drilled and completed successfully according to the well design.

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a mudstone belt from E interval to F interval. Therefore, when drilling into non-target-zone lithology, the sedimentary faices and sub-facies should be analyzed by combining adjacent well data with seismic section in order to make accurate interpretation and correct geosteering.

3

Fig. 4

Seismic section of the target zone in well ZB3-P4

From the above example, the key in geologic interpretation of horizontal well is building an accurate geological model based on the understanding of sedimentary facies, available adjacent wells data and seismic section[15, 16]. Furthermore, in the drilling process, the model should be modified and improved in time; otherwise the judgment result would be false. For example, in Well P26 of B oilfield, the content curve of element Si by XRF logging showed 99 m thickness of mudstone from 2 851 m to 2 950 m (section E-F in Fig. 6). So, it was inferred that the target zone was drilled through and a wrong decision to modify inclination upward was made, resulting in deviation of wellbore from the target zone and sharp drop of the oil layer drilling-encounter ratio. In fact, the target zone of this well was controlled by three adjacent wells. Although the depths of target zones in them are different, the lithology is sandstone with no mudstone. In well P26, there is

Conclusions

While drilling horizontal wells, there are many uncertainties such as lithology and structure. Advanced techniques of logging and mud logging while drilling provide powerful means for horizontal well geosteering. Additionally, drilling workers should have rich geologic knowledge and experience. Only the close integration of petroleum geology and petroleum engineering technology can result in accurate stratigraphic correlation and target depth prediction before drilling into target zone, and correct geological interpretation and drilling geosteering also after drilling into the target zone, so that the borehole quality and oil layer drilling-encounter ratio can be improved. This study indicates that during geosteering in horizontal wells, besides information by logging and mud logging while drilling, seismic information should be considered to realize the combination of macroscopic and microcosmic, structure and sedimentary facies, lithology and resistivity property, physical property and oil bearing, fulfilling fine correlation, accurate prediction, correct interpretation and scientific decision.

Fig. 5 Borehole trajectory and horizontal interval profile in well ZB3-P4

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Fig. 6 Borehole trajectory and horizontal interval composite profile in well P26

Acknowledgement

fields. Petroleum Exploration and Development, 2010, 37(5):

This study was supported by Liu Caixia in Geologging Company in Shengli Oilfield of SINOPEC, Wang Feilong, Liu Jiangtao and Li Yonjie in North China Branch of SINOPEC. We appreciate them very much.

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