- Email: [email protected]
Wireline Logs for Coalbed Evaluation
C H A P T E R
5
Wireline Logs for Coalbed Evaluation Todd Sutton Schlumberger, Pittsburgh, PA, USA
5.1 BASIC COALBED LOG EVALUATION Most basic log delineation of coalbeds involves running the caliper— density-gamma ray measurement. Some operators prefer to supplement that with one or more of either the compensated neutron, photoelectric factor (PEF), or resistivity logs. More precise vertical coalbed determination comes with acquiring high-resolution data. In “dry” coalbed regions drilled on air, audio, and temperature log measurement are used to identify gas entry, and thereby qualitative permeability. The 33⁄8″ Platform Express and new 2¼″ Multi Express (MEX) have provided all of these essential coalbed measurements. See Figure 5.1 for a typical coalbed methane (CBM) log presentation. The standard Borehole Compensated Sonic tools have been used to identify coals and tie-in to surface seismic, also. Resistivity measurements offer a look at the presence of an invasion profile (fluid-filled boreholes), thereby indicating qualitative permeability. The Array Induction Tool with its five depths of investigation as well as micrologs have successfully identified invasion in coalbeds. This drilling fluid invasion suggests cleating. In some cased holes, coalbed identification is acquired with pulsed neutron tools, which provide a gamma ray, neutron porosity, sigma measurement, plus near/far count rate qualitative gas identification. The Reservoir Saturation Tool (RST) has answered the need for cased hole evaluation. See Figure 5.2 for the basic log values for coal identification. In addition to providing improved slim and 3″ core hole access, the new MEX is notable for it’s efficiency in obtaining high-quality, comprehensive data in all formations. The lightweight, small-diameter, compact logging string can log at speeds as high as 4500 ft/h [1372 m/h] recording
Coal Bed Methane http://dx.doi.org/10.1016/B978-0-12-800880-5.00005-X
93
Copyright © 2014 TODD SUTTON. Published by Elsevier Inc. All rights reserved.
94
5. WIRELINE LOGS FOR COALBED EVALUATION
Time mark every 60 s COAL TMT differential gas temperature (TDELTAT) –1 1 (DC/K) GR > 400 From LHT1 to GR2
0
TMT upper audio (AUD1) (MV)
500
500
TMT lower audio (AUD2) (MV)
0
70
Gas temperature (TGTEM) (DEGF)
80
Bulk density (RHOB) (G/C3)
3
GR > 200 From LHT1 to GR1
10000
Tension (TENS) (LBF)
0
Gamma ray (GR) (GAPI)
5
Caliper (CALI) (IN)
Tool/tot. drag 0 from D3T 2 to STIA
Cable Photoelectric Factor (PEF) drag 200 From STIA 0 (....) to STIT Stuck Bulk density correction (DRHO) stretch 15 (STIT) –0.05 (G/C3) 0.45 0 (F) 50
10
X200
FIGURE 5.1 Platform Express in coal with low density plus gas entry temperature and audio response in air-filled borehole.
FIGURE 5.2 Though the formation presence of water and varying salinity influences log response, pure log responses in various types of coal are as follows. PEF (photoelectric factor), CNL, Compensated Neutron Log.
all sensors simultaneously. This newest tool can be run in real-time or memory mode, and offers through drill pipe access for vertical and horizontal wellbores.
5.2 Advanced Coal Analysis
95
The MEX platform is uniquely and fully characterized for CBM wells to account for the characteristically low density and low PEF of CBM formations. Standard logging tools are not characterized for the low density of coal and PEF’s lower than 0.9. In addition, the unique articulated configuration of the Litho-Density* pad improves pad application to reduce the effects of hole rugosity, which is oftentimes associated with coalbeds (Herron and Herron, 1996).
5.2 ADVANCED COAL ANALYSIS As stated above, the simplest log method of qualitative analysis is to use the bulk density to approximate ash content, which correlates to coal rank. The addition of the compensated neutron, PEF, and gamma ray complements local correlations. This technique can be complicated by the tendency of coalbeds to washout and thus creating uncertainty on the absolute measurement. In addition, the coal components, particularly ash can vary, which adds to parameter selection uncertainty. An established alternative coal evaluation method is based on elemental analysis from neutron-induced gamma ray spectroscopy. The advantage is that the preponderance of formation signal arises from the formation elements, not the borehole in the case of washout; plus gamma ray spectroscopy yields discrete elemental signatures that more precisely define mineralogy such as ash content. Both the Elemental Capture Sonde (ECS) and RST have been used successfully for this quantitative coal analysis technique (Ahmed et al., 1991). Neutron-induced gamma ray spectroscopy tools emit high-energy neutrons that are principally slowed down and then captured by the formation elements. Upon capture, the formation element emits a gamma ray at a unique energy level that is characteristic of that element. A gamma ray detector detects the energy level and also the count rate, called the energy spectrum, which reflects the type and quantity of element. Figure 5.3 represents the Coal Advisor process, which begins by calculating the relative gamma ray yield by comparing the measured spectrum with the theoretical spectrum of each element. A mathematical inversion provides a percentage of the principal elements such as silicon, calcium, iron, sulfur, and hydrogen. The yields obtained at well site are only relative measures, because the varying borehole environment affects the total signal. Additional information is needed to gain the absolute elemental concentrations. This is accomplished by using the oxide closure model, which states that dry rock consists of a set of oxides, the sum of whose concentrations must equal unity. The relative oxide yield allows total yield calculation, and hence the factor needed to convert to unity. This normalization factor then converts each relative yield to a dry weight elemental concentration.
96
5. WIRELINE LOGS FOR COALBED EVALUATION
FIGURE 5.3 Mineralogy interpretation steps—from gamma ray spectra to SpectroLith model.
The final step in coal analysis is to transform elemental concentration to mineral concentrations via the SpectroLith model by using empirical correlations based on more than 400 core samples from varied clastic environments. The major advantage of this method is that it is virtually automatic and does not depend heavily on user intervention. Results are presented as dry weight percentages of clay, coal, the aggregate of quartz—feldspar—mica, and other minerals such as pyrite and siderite. Coals are easily identified by their high hydrogen concentration. The challenge to quantify amounts of fixed carbon, volatile material, and moisture is accomplished with two assumptions. First, other sources of hydrogen in coal such as water in cleats, formation moisture, clay bound water, and borehole water (unless drilled on air), tend to have a consistent background and thus are subtracted to yield the actual hydrogen concentration in coal. Second, for a given area or formation, hydrogen coal concentration can be sufficiently consistent to allow for a conversion to coal percentage. Total ash content is obtained from its components: quartz, clay, carbonates, and pyrite. Fixed carbon and volatile material can be estimated from ash content correlations. As mentioned, many correlations have been established for specific areas or formations. ECS Coal mineralogy is further enhanced with other log information with the ELAN computation. Detailed ash description from the ECS, combined with the density, PEF, and neutron data, which
5.2 Advanced Coal Analysis
97
FIGURE 5.4 Coal Advisor example using openhole logs and Elemental Capture Spectroscopy Tool.
identifies fixed carbon from volatile matter; all contributes to a more precise coal analysis. See a finalized Coal Advisor in Figure 5.4. Cleating can be identified with the presence of calcite and pyrite, which suggests formation water flow causing secondary mineralization. High quantities of calcite and pyrite may indicate filled cleats or low-grade coal. Quartz and clay have been observed in cleats, also. Large quantities of such minerals plus a large total ash content indicate a lower ranked coal. This coal type will have fewer cleats since less water and volatile matter was lost during coalification. By observation, well-cleated coal has between 2 and 7% calcite and 0.5–5% pyrite. Poorly cleated coals have total ash above 45%, clay above 25%, and quartz above 10%. Partly cleated coals will have percentages that run in between, but cutoffs should be tempered by local production experience. In summary, neutron-induced gamma ray spectroscopy as with the ECS, in combination with other logs provides a continuous record of the major variables needed for coal seam evaluation and surrounding formations. Cleat grading indicates permeability and ash content lends to coal grading and gas content. Similar elemental analysis can be done in cased holes not only with the ECS, but also with the Reservoir Saturation Tool (RST), which has the additional benefit of identifying coal zones with the carbon/oxygen measurement.
98
5. WIRELINE LOGS FOR COALBED EVALUATION
5.3 IMAGING AND MECHANICAL PROPERTIES Further detailed coal analysis can be achieved with the Formation Micro Imager Log (FMI) with its high 0.2″ (0.5 cm) vertical resolution. This downhole tool makes circumferential microresistivity measurements and responds to the various conductivities of laminations, pore space, and some minerals. FMI offers thin coalbed resolution, identification of fractures, cleat type, faults, drilling-induced fractures, dip and strike orientation, and in situ stress determination that can supplement advanced coal analysis. See Figure 5.5 for an FMI example in coal. In addition, dipole type sonic tools attain the formation mechanical properties for understanding fracture behavior and for designing optimum
FIGURE 5.5 High resolution coalbed image using the FMI (Formation Micro Imager Log). API, American Petroleum Institute.
5.3 Imaging and Mechanical Properties
99
fracture jobs. Coal has a higher Poisson’s ratio and lower Young’s Modulus oftentimes than surrounding beds, so coals tend to transfer overburden stress laterally and maintain higher fracture gradients. The Sonic Scanner (SSCAN) has provided these critical formation property measurements and has shown that coals are typically more stressed than surrounding beds, thereby indicating more fracture growth outside of the coalbed than within. The SSCAN not only determines Young’s Modulus, Poisson’s Ratio, and Closure Stress Gradient, but also the in situ stress magnitudes and directions to improve hydraulic fracturing results. See Figure 5.6 for a Coal Advisor plus SSCAN-Mechanical Properties Log in coal.
FIGURE 5.6 Coal Advisor example with stress profile using Sonic Scanner log. AIT, Array Induction Tool.
100
5. WIRELINE LOGS FOR COALBED EVALUATION
5.4 SUMMARY Formation evaluation techniques for bulk density, spectral analysis, imaging, and mechanical properties; tested by extensive core study help the industry understand coalbed reservoirs. Log processing methods uncover lithology, coal quality, proximate analysis and permeability data. Local stress information from imaging and dipole sonic tools reveal more detailed variables associated with permeability and fracture behavior. All logging techniques contribute to more precise reserve calculations and more effective completion methods (Anderson et al., 2003).
References Ahmed, U., Johnston, D., Colson, L. “An advanced and integrated approach to coal formation evaluation”, paper SPE 22736, presented at the SPE Annual Tech Conference and Exhibition, Dallas, TX, October 6–9, 1991. Anderson, J., Basinski, P., Beaton, A., Boyer, C., Bulat, D., Satyaki, R., Ronheimer, D., Schlachter, G., Colson, L., Olsen, T., Zachariah, J., Khan, R., Low, N., Ryan, B., Schoderbek, D., Autumn 2003. Producing natural gas from coal. Oilfield Review 15 (3), 8–31. Herron, S., Herron, M. “Quantitative lithology: an application for open and cased hole spectroscopy”, Transactions of the SPWLA 37th Annual Symposium, New Orleans, LA, June 16–19, 1996, paper E.
5
Wireline Logs for Coalbed Evaluation Todd Sutton Schlumberger, Pittsburgh, PA, USA
5.1 BASIC COALBED LOG EVALUATION Most basic log delineation of coalbeds involves running the caliper— density-gamma ray measurement. Some operators prefer to supplement that with one or more of either the compensated neutron, photoelectric factor (PEF), or resistivity logs. More precise vertical coalbed determination comes with acquiring high-resolution data. In “dry” coalbed regions drilled on air, audio, and temperature log measurement are used to identify gas entry, and thereby qualitative permeability. The 33⁄8″ Platform Express and new 2¼″ Multi Express (MEX) have provided all of these essential coalbed measurements. See Figure 5.1 for a typical coalbed methane (CBM) log presentation. The standard Borehole Compensated Sonic tools have been used to identify coals and tie-in to surface seismic, also. Resistivity measurements offer a look at the presence of an invasion profile (fluid-filled boreholes), thereby indicating qualitative permeability. The Array Induction Tool with its five depths of investigation as well as micrologs have successfully identified invasion in coalbeds. This drilling fluid invasion suggests cleating. In some cased holes, coalbed identification is acquired with pulsed neutron tools, which provide a gamma ray, neutron porosity, sigma measurement, plus near/far count rate qualitative gas identification. The Reservoir Saturation Tool (RST) has answered the need for cased hole evaluation. See Figure 5.2 for the basic log values for coal identification. In addition to providing improved slim and 3″ core hole access, the new MEX is notable for it’s efficiency in obtaining high-quality, comprehensive data in all formations. The lightweight, small-diameter, compact logging string can log at speeds as high as 4500 ft/h [1372 m/h] recording
Coal Bed Methane http://dx.doi.org/10.1016/B978-0-12-800880-5.00005-X
93
Copyright © 2014 TODD SUTTON. Published by Elsevier Inc. All rights reserved.
94
5. WIRELINE LOGS FOR COALBED EVALUATION
Time mark every 60 s COAL TMT differential gas temperature (TDELTAT) –1 1 (DC/K) GR > 400 From LHT1 to GR2
0
TMT upper audio (AUD1) (MV)
500
500
TMT lower audio (AUD2) (MV)
0
70
Gas temperature (TGTEM) (DEGF)
80
Bulk density (RHOB) (G/C3)
3
GR > 200 From LHT1 to GR1
10000
Tension (TENS) (LBF)
0
Gamma ray (GR) (GAPI)
5
Caliper (CALI) (IN)
Tool/tot. drag 0 from D3T 2 to STIA
Cable Photoelectric Factor (PEF) drag 200 From STIA 0 (....) to STIT Stuck Bulk density correction (DRHO) stretch 15 (STIT) –0.05 (G/C3) 0.45 0 (F) 50
10
X200
FIGURE 5.1 Platform Express in coal with low density plus gas entry temperature and audio response in air-filled borehole.
FIGURE 5.2 Though the formation presence of water and varying salinity influences log response, pure log responses in various types of coal are as follows. PEF (photoelectric factor), CNL, Compensated Neutron Log.
all sensors simultaneously. This newest tool can be run in real-time or memory mode, and offers through drill pipe access for vertical and horizontal wellbores.
5.2 Advanced Coal Analysis
95
The MEX platform is uniquely and fully characterized for CBM wells to account for the characteristically low density and low PEF of CBM formations. Standard logging tools are not characterized for the low density of coal and PEF’s lower than 0.9. In addition, the unique articulated configuration of the Litho-Density* pad improves pad application to reduce the effects of hole rugosity, which is oftentimes associated with coalbeds (Herron and Herron, 1996).
5.2 ADVANCED COAL ANALYSIS As stated above, the simplest log method of qualitative analysis is to use the bulk density to approximate ash content, which correlates to coal rank. The addition of the compensated neutron, PEF, and gamma ray complements local correlations. This technique can be complicated by the tendency of coalbeds to washout and thus creating uncertainty on the absolute measurement. In addition, the coal components, particularly ash can vary, which adds to parameter selection uncertainty. An established alternative coal evaluation method is based on elemental analysis from neutron-induced gamma ray spectroscopy. The advantage is that the preponderance of formation signal arises from the formation elements, not the borehole in the case of washout; plus gamma ray spectroscopy yields discrete elemental signatures that more precisely define mineralogy such as ash content. Both the Elemental Capture Sonde (ECS) and RST have been used successfully for this quantitative coal analysis technique (Ahmed et al., 1991). Neutron-induced gamma ray spectroscopy tools emit high-energy neutrons that are principally slowed down and then captured by the formation elements. Upon capture, the formation element emits a gamma ray at a unique energy level that is characteristic of that element. A gamma ray detector detects the energy level and also the count rate, called the energy spectrum, which reflects the type and quantity of element. Figure 5.3 represents the Coal Advisor process, which begins by calculating the relative gamma ray yield by comparing the measured spectrum with the theoretical spectrum of each element. A mathematical inversion provides a percentage of the principal elements such as silicon, calcium, iron, sulfur, and hydrogen. The yields obtained at well site are only relative measures, because the varying borehole environment affects the total signal. Additional information is needed to gain the absolute elemental concentrations. This is accomplished by using the oxide closure model, which states that dry rock consists of a set of oxides, the sum of whose concentrations must equal unity. The relative oxide yield allows total yield calculation, and hence the factor needed to convert to unity. This normalization factor then converts each relative yield to a dry weight elemental concentration.
96
5. WIRELINE LOGS FOR COALBED EVALUATION
FIGURE 5.3 Mineralogy interpretation steps—from gamma ray spectra to SpectroLith model.
The final step in coal analysis is to transform elemental concentration to mineral concentrations via the SpectroLith model by using empirical correlations based on more than 400 core samples from varied clastic environments. The major advantage of this method is that it is virtually automatic and does not depend heavily on user intervention. Results are presented as dry weight percentages of clay, coal, the aggregate of quartz—feldspar—mica, and other minerals such as pyrite and siderite. Coals are easily identified by their high hydrogen concentration. The challenge to quantify amounts of fixed carbon, volatile material, and moisture is accomplished with two assumptions. First, other sources of hydrogen in coal such as water in cleats, formation moisture, clay bound water, and borehole water (unless drilled on air), tend to have a consistent background and thus are subtracted to yield the actual hydrogen concentration in coal. Second, for a given area or formation, hydrogen coal concentration can be sufficiently consistent to allow for a conversion to coal percentage. Total ash content is obtained from its components: quartz, clay, carbonates, and pyrite. Fixed carbon and volatile material can be estimated from ash content correlations. As mentioned, many correlations have been established for specific areas or formations. ECS Coal mineralogy is further enhanced with other log information with the ELAN computation. Detailed ash description from the ECS, combined with the density, PEF, and neutron data, which
5.2 Advanced Coal Analysis
97
FIGURE 5.4 Coal Advisor example using openhole logs and Elemental Capture Spectroscopy Tool.
identifies fixed carbon from volatile matter; all contributes to a more precise coal analysis. See a finalized Coal Advisor in Figure 5.4. Cleating can be identified with the presence of calcite and pyrite, which suggests formation water flow causing secondary mineralization. High quantities of calcite and pyrite may indicate filled cleats or low-grade coal. Quartz and clay have been observed in cleats, also. Large quantities of such minerals plus a large total ash content indicate a lower ranked coal. This coal type will have fewer cleats since less water and volatile matter was lost during coalification. By observation, well-cleated coal has between 2 and 7% calcite and 0.5–5% pyrite. Poorly cleated coals have total ash above 45%, clay above 25%, and quartz above 10%. Partly cleated coals will have percentages that run in between, but cutoffs should be tempered by local production experience. In summary, neutron-induced gamma ray spectroscopy as with the ECS, in combination with other logs provides a continuous record of the major variables needed for coal seam evaluation and surrounding formations. Cleat grading indicates permeability and ash content lends to coal grading and gas content. Similar elemental analysis can be done in cased holes not only with the ECS, but also with the Reservoir Saturation Tool (RST), which has the additional benefit of identifying coal zones with the carbon/oxygen measurement.
98
5. WIRELINE LOGS FOR COALBED EVALUATION
5.3 IMAGING AND MECHANICAL PROPERTIES Further detailed coal analysis can be achieved with the Formation Micro Imager Log (FMI) with its high 0.2″ (0.5 cm) vertical resolution. This downhole tool makes circumferential microresistivity measurements and responds to the various conductivities of laminations, pore space, and some minerals. FMI offers thin coalbed resolution, identification of fractures, cleat type, faults, drilling-induced fractures, dip and strike orientation, and in situ stress determination that can supplement advanced coal analysis. See Figure 5.5 for an FMI example in coal. In addition, dipole type sonic tools attain the formation mechanical properties for understanding fracture behavior and for designing optimum
FIGURE 5.5 High resolution coalbed image using the FMI (Formation Micro Imager Log). API, American Petroleum Institute.
5.3 Imaging and Mechanical Properties
99
fracture jobs. Coal has a higher Poisson’s ratio and lower Young’s Modulus oftentimes than surrounding beds, so coals tend to transfer overburden stress laterally and maintain higher fracture gradients. The Sonic Scanner (SSCAN) has provided these critical formation property measurements and has shown that coals are typically more stressed than surrounding beds, thereby indicating more fracture growth outside of the coalbed than within. The SSCAN not only determines Young’s Modulus, Poisson’s Ratio, and Closure Stress Gradient, but also the in situ stress magnitudes and directions to improve hydraulic fracturing results. See Figure 5.6 for a Coal Advisor plus SSCAN-Mechanical Properties Log in coal.
FIGURE 5.6 Coal Advisor example with stress profile using Sonic Scanner log. AIT, Array Induction Tool.
100
5. WIRELINE LOGS FOR COALBED EVALUATION
5.4 SUMMARY Formation evaluation techniques for bulk density, spectral analysis, imaging, and mechanical properties; tested by extensive core study help the industry understand coalbed reservoirs. Log processing methods uncover lithology, coal quality, proximate analysis and permeability data. Local stress information from imaging and dipole sonic tools reveal more detailed variables associated with permeability and fracture behavior. All logging techniques contribute to more precise reserve calculations and more effective completion methods (Anderson et al., 2003).
References Ahmed, U., Johnston, D., Colson, L. “An advanced and integrated approach to coal formation evaluation”, paper SPE 22736, presented at the SPE Annual Tech Conference and Exhibition, Dallas, TX, October 6–9, 1991. Anderson, J., Basinski, P., Beaton, A., Boyer, C., Bulat, D., Satyaki, R., Ronheimer, D., Schlachter, G., Colson, L., Olsen, T., Zachariah, J., Khan, R., Low, N., Ryan, B., Schoderbek, D., Autumn 2003. Producing natural gas from coal. Oilfield Review 15 (3), 8–31. Herron, S., Herron, M. “Quantitative lithology: an application for open and cased hole spectroscopy”, Transactions of the SPWLA 37th Annual Symposium, New Orleans, LA, June 16–19, 1996, paper E.