SPE 139942

Using Electrical Borehole Image Log to Optimize Formation Pressure Sampling and Its Integration to Determine Structural/Stratigraphic Break in a Tight Carbonate Reservoir at Karachaganak Field Juandi, D.,El-Battawy, A., SPE, Schlumberger, Russo, J., Chittick, S., SPE, KPO BV.

Copyright 2010, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Caspian Carbonates Technology Conference held in Atyrau, Kazakhstan, 8–10 November 2010. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract Karachaganak field hosts world-class gas and liquid reserves in an Upper Devonian to Lower Permian isolated carbonate platform. A complex diagenesis over carbonate rocks extends from syndepositional to deep burial settings. Effective porosity in most parts of the reservoir is secondary in nature, such as vugs and fractures, with average porosity considered as low. The complexity of carbonate reservoirs is well known owing to the nature of their heterogeneity and cannot be resolved by conventional openhole logs because of limited vertical resolution and large azimuthal dependency.

Electrical borehole image logs provide both the small-scale resolution and azimuthal coverage to quantitatively resolve the heterogeneous nature and are used routinely to determine subseismic structural or stratigraphic events, as well as to optimize the selection of formation pressure and sampling points. The use of a high-resolution electrical borehole image log helps place the tool probes at optimum formation and depth locations, thereby reducing risk and operating time. Analysis of pressure gradients can confirm subseismic reservoir barriers that had been earlier interpreted as structural or stratigraphic breaks. Two case studies from nearby vertical Wells A and B are described. Well A has a fieldwide unconformity surface picked at a depth of X610 m MD, indicated by a drag pattern in the overlying zone. The carboniferous section in that well is composed of numerous shoaling upward cycles. Any of these cycle tops can act as a vertical permeability barrier and result in a step in the pressure profile. Meanwhile, Well B has an unconformity within the internal Permian at a depth of X142.5 m MD, indicated by the drag pattern. The fieldwide unconformity and a stratigraphic barrier can be proved by a shift of pressure gradient at a depth of X608 m MD, indicated by a change of dip attitude. Introduction In tight carbonate reservoir rocks, it is always a challenge to pinpoint the “sweet spots” suitable to achieving the best selection of formation pressure and sampling points. Karachaganak field, like many productive carbonate fields, has dual porosity systems with widely varying proportions of primary and secondary porosity. The secondary porosity may contain vugs, molds, and fractures. Most interpretation methods rely on traditional resistivity and porosity logs. These often fail to produce results consistent with production because of the complex nature of the dual-porosity carbonate (Newberry et al., 1996). Borehole electrical images provide both the small-scale resolution and azimuthal borehole coverage to quantitatively resolve the heterogeneous nature of the porosity components.

According to Elshahawi et al. (2006) 1-, single- and multi-well pressure gradient analysis play an important role in reservoir characterization. Even subtle discontinuities in pressures and pressure gradients can be indicative of reservoir compartmentalization. Success in wireline formation tester operations is greatly increased by accurate planning that includes more

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accurately placing the tester probes and packers in the borehole. A key element is the accurate understanding of subsurface lithology, including subtle reservoir internal baffles and barriers. Identifying reservoir compartments and flow barriers, and unraveling reservoir architecture are critically important to reservoir management.

Technical Approach The electrical borehole image log was fully processed before the interpretation. The image processing included tool speed correction to correct variation relative to cable speed and sticking of the tool, equalization, and normalization to enhance the image contrast. The formation pressure points and sampling intervals were selected based on the processed borehole images. The preferred depths to acquire formation pressure and sampling data were in the zones with fractures and vuggy porosity development (Figure 1), based on our local experience. In addition, manual dip interpretation on the images was also performed to determine structural dip, fracture/fault, and unconformity.

The electrical borehole image logging tool (Figure 2) provides the microresistivity image of the breohole wall which is created from the current measured by the array of buttons. There are 192 microresistivity arrays totaling from all of pads & flaps. The high-resolution component dominates the image because its value varies from button to button. The low resolution component appears only as a gradually changing background. Microresistivity changes related to lithological and petrophysical variations in the rock, which are conveyed mainly by the high-resolution current component, are interpreted in the image in terms of rock texture, stratigraphic and structural features and fractures4-. Formation pressure is measured using a probe tool (Figure 3). The probe is set in the wellbore and a short test, called a pretest, measures formation pressure 4-,6- Up to 20 cc of fluid maybe withdrawn from the formation during the pretest. Both drawdown and buildup data are acquired for each pretest. Formation pressure is taken to be the final pressure measured. Points with representative stable final pressure were marked with a green color. If the final pressure was not considered stable or representative, it was marked on the plot by yellow color, e.g.“supercharged,” or by red color for “incomplete buildup” or “dry test or tight formation”. Typically, supercharged points may be recognized by their position to the right of a reliable gradient line on the pressure depth plot. If there was a consistent relationship between low mobility and supercharging, the mobility cutoff was marked in the mobility track as a shaded band. Formation pressure data plots (in TVD) for both Wells A and B are presented in Figure 4 and Figure 6. The trend break or variation in the formation pressure gradient profile provided important information for the reservoir depletion mechanism and helped to imply the vertical reservoir communication across substrata (Grayson et al., 2000) 3-. While a continuous pressure trend does not neccessarily imply hydraulic communication, a break in the trend indicated by a step change or shift in pressures or pressure gradients implies the existence of a pressure barrier and no hydraulic communication. Case Study One from Well A There were only 3 lost seal data points out of the 40 attempted pressure tests, suggesting a high success ratio of formation pressure data acquisition with 4 tight points (Figure 4). The quality of pressure data allowed a detailed reliable pressure gradient profile. Based on the dip data from the electrical borehole image logs, Well A showed a few shoaling upward cycles (also called “cyclic platform”), indicated by the repetition of a dip decreasing upward pattern. The cycle tops are often exposure surfaces (short-lived) with abundant cementation indicated by tight streaks on density and image logs (Figure 5). As seen on the right plot in Figure 4, the formation pressure gradient profile along with the magnitude of the pressure steps reflects the dynamic nature of the system, which has gas injection taking place in a few nearby wells and several producers at various levels. The dip patterns suggested a possible small scale unconformity at depth of X752 m MD, confirmed by a trend shift of formation pressure gradient (Figure 4). A fieldwide unconformity (C1 horizon) separating Carboniferous with the overlying Permian formations at depth X610 m MD can be determined accurately by the dramatic change of the dip magnitude trends below and above the surface which also corresponds to a shift in pressure data.

Case Study Two from Well B The electrical borehole image log and formation pressure data were acquired mostly in the uppermost part of Carboniferous formation and the lower part of Permian formation. There are 11 good-quality formation pressure data points acquired out of 16 attempted pressure tests. All pressure data were plotted in green dots in Figure 6.

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The dip pattern and a shift of pressure gradient profile indicated a downlapping sequence atop the Carboniferous formation boundary at depth of X608 m MD in Well B. It was also marked as a field scale pressure barrier (C1 horizon). A possible internal unconformity within the Permian was interpreted at depth of X142.5 m MD by dip pattern change. Conclusions Electrical borehole image logs helped geoscientists to select formation pressure and sampling points accurately in the highly heterogeneous carbonate rocks presented in this study. The zones with fracture and vugs interpreted on borehole images were recommended for taking pressure points and samples, which resulted in good-quality reservoir engineering data acquisition. The zones with resistive spots or healed (closed) fractures corresponding to tighter formation were also mostly avoided for taking pressure data points. The integration between electrical borehole image log and formation pressure results provided a much better understanding on reservoir compartmentalization. A fieldwide unconformity was interpreted at a depth of X610 m MD in Well A using dip data. In addition, a few shoaling upward cycles were also recognized in the Carboniferous section in the well. Most of the cycle boundaries were detected as vertical permeability barriers confirmed by dynamic pressure data. In Well B, an unconformity within the Permian formation was picked at a depth of X142.5 m MD by dip data. The fieldwide unconformity was recognized at X608 m MD that was also confirmed as a reservoir barrier by the shift in pressure gradient data. Acknowledgement The authors wish to thank Karachaganak Petroleum Operating B.V. (KPO B.V) for cooperation and for releasing this data for publication. Thanks are also extended to Mr. Benjamin Robertson (Res. Devel. Geology Section Head, RPE, KPO B.V.), who provided his important geological input for Karachaganak field, and to Elena Mitrofanskaya and Saltanat Sargunanova of Schlumberger for their geological workstation interpretation support. References

1- Elshahawi, H., Donaghy, E., Guillory, C., Kear, G.R., Kumar, A., Ardila, M., Williamson, D., Cantwell, W., and Briscoe, B. 2006. Enhancing Formation Testing and Sampling Operations Through the Use of Log-Derived High Resolution Mineral-Based Lithofacies Mapping. Paper SPE 100738 presented at the SPE Annual Technical COnfrence and Exhibition, San Antonio, Texas, 24–27 September.

2- FMI, 2002, Schlumberger publication. 3- Grayson, C.T., Morris, C.W., and Blume, C.R.: 2000. Fluid Identification and Pressure Transient Analysis in the Fractured Monterey

Using the Modular Dynamics Tester. Paper SPE 62532 presented at the SPE/AAPG Western Regional Meeting, Long Beach, California.

4- MDT, 1995, Schlumberger publication. 5- Newberry, B.M., Grace, L.M., and Stief, D.D. 1996. Analysis of Carbonate Dual Porosity Systems from Borehole Electrical Images.

Paper SPE 35158 presented at the Permian Basin Oil & Gas Recovery Conference, Midland, Texas, 27–29 March. 6- Wireline Formation Testing and Sampling, 1996, Schlumberger publication.

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Figure 1: An example of expanded log images from Well B, showing a small zone (fractured and vugs) where a formation pressure point was taken at depth X532.44 m MD with good data.

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Figure 2. The illustration of the electrical borehole imager tool shows the current path (yellow shade) from the pads and flaps. An applied voltage causes an alternating current (AC) to flow from each electrode button in the lower pads/flaps section through the formation to the electrode on the upper cartridge.

Figure 3: Example of a typical probe tool used for pressure measurement

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Figure 4: An example from Well A in the lowermost part of Permian and the upper part of Carboniferous formations. Composite on the left side shows the dip pattern (fourth track from left) from manual dip picking result on the electrical borehole image. The right plot shows the formation pressure results and its pressure gradient profile (first track from left). The dip pattern shows the repetition of dip decreasing upward representing shoaling upward cycle over the Carboniferous section. Meanwhile the formation pressure gradient profile showing the step of formation pressure and confirming the possible small scale unconformity at depth X752 m MD where there is a big shift of formation pressure gradient.

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Figure 5: Cemented band in Well A over interval X807.5-X806.2 m MD represents one of cycle tops are often exposure surfaces (short-lived) with abundant cementation indicated by tight streaks on density and image logs. Few shoaling upward cycles (also called as ‘cyclic platform’) indicated by the repetition of dip decreasing upward pattern and the steps of formation pressure gradient profile as seen in Figure 4.

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Figure 6: An example from Well B. The far left side shows the dip pattern (fifth track from left) extracted from electrical borehole image and showing an indication of faulting and an unconformity. The shift of pressure gradient (first track of far right composite) confirmed an exisisting of fieldwide unconformity at depth X608 mMD which acts as reservoir barrier.

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