Waterflood Improvement in the Permian Basin: Impact of In-Situ-Stress Evaluations
- R.C. Nolen-Hoeksema (Consultant) | J.M. Avasthi (Chevron Petroleum Technology Co.) | W.C. Pape (Chevron U.S.A. Inc.) | A.W.M. El Rabaa (Mobil E&P Technical Center)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1994
- Document Type
- Journal Paper
- 254 - 260
- 1994. Society of Petroleum Engineers
- 5.1.7 Seismic Processing and Interpretation, 5.5.11 Formation Testing (e.g., Wireline, LWD), 1.6 Drilling Operations, 5.1.2 Faults and Fracture Characterisation, 5.8.7 Carbonate Reservoir, 1.7.5 Well Control, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 3 Production and Well Operations, 1.6.9 Coring, Fishing, 5.6.5 Tracers, 5.7.2 Recovery Factors, 4.1.2 Separation and Treating, 3.3.2 Borehole Imaging and Wellbore Seismic, 5.4.1 Waterflooding, 1.7 Pressure Management, 5.1.1 Exploration, Development, Structural Geology, 5.6.1 Open hole/cased hole log analysis, 1.2.2 Geomechanics, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 6.5.2 Water use, produced water discharge and disposal
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We evaluated in-situ-stress magnitudes. and directions to support waterflood improvement programs in McElroy field and North Westbrook Unit. In-situ-stress and hydraulic-fracture directions coincided with directional floodwater effects. This information contributed to successful waterflood realignment programs.
Large oil reserves remain in the heterogeneous carbonate reservoirs of the Permian Basin in west Texas. Many of these reservoirs have been under continuous waterflooding since the 1950's and 1960's, but, historically, waterflood recovery efficiencies in these reservoirs have been low. Infill drilling and well-pattern modification help to accelerate oil production, to increase oil reserves, and to achieve better reservoir and operational control. Hydraulic fracturing helps to accelerate oil production by connecting large volumes of the reservoir formation to wells through hydraulic fractures. Historically, however, engineers undertook these strategies to improve waterflood recovery efficiencies with little regard for in-situ reservoir stresses, in particular, the stress directions.
In-situ reservoir stresses control the initiation, reopening, and propagation pressures and direction of hydraulic fractures. Fracture orientation and length affect the sweepout behavior in waterfloods.1-4 Consequently, knowledge of the in-situ-stress orientation and magnitude is important when establishing the well pattern and spacing for a waterflood that seeks high injection rates with efficient sweep from injection to production wells. We present two case studies from the McElroy field and North Westbrook Unit in the Permian Basin (Fig. 1). We evaluated in-situ-stress magnitudes and directions to aid waterflood improvement programs in these reservoirs.
The McElroy field encompasses 19,840 acres and straddles the Crane-Upton county line in west Texas (Fig. 1). It is on the eastern edge of the Central Basin Platform, at the western margin of the Midland basin. Discovered in 1926, the field has been under continuous waterflooding since the early 1960's.
The reservoir is 2,620 to 4,200 ft deep in dolostones of the Grayburg formation, a shoaling-upward sequence of open-shelf, shallow-shelf, and shallow-shelf-to-intertidal deposits. Oil production comes from a pay zone with a 400-ft gross thickness and an 80-ft average net thickness. Average reservoir porosity is ˜9%. Permeability typically ranges from 0.1 to 200 md, although permeability for a number of core samples in some pay zones reached several darcies. Oil recovery varies across the field, with the highest production coming from the central area of the field and the lowest from flanking areas. The lower recovery in flanking areas results from low reservoir quality (e.g., low permeabilities and high water saturations). Ease of water injectivity follows a similar pattern across the field.
Results of a recent waterflood performance evaluation found that the McElroy waterflood was inefficient and had poor injection support, which was indicated by declining production, low WOR's, and low reservoir pressures. Waterflood inefficiency and low reservoir pressures were attributable to high reservoir heterogeneity, irregular well-pattern geometries, variable well densities, and low injector/ producer ratios. These characteristics diminished effective water injection and inhibited reservoir pressure management.
The waterflood pattern needed realignment to improve production and waterflood sweep efficiencies and to prepare for tertiary recovery operations. Increasing well densities, the injector/producer ratio and reservoir pressures was necessary. New well locations would minimize interference between wells, and optimized water-injection rates would build reservoir pressures and balance production rates. We expected more well interference because hydraulic-fracture lengths would grow relative to the interwell distance as the water-injection pressures and well densities increased.
Of equal importance was a desire for the well-pattern realignment to take advantage of an apparent N78°W±33° (average±standard deviation) directional permeability trend (Fig. 2). This trend has been observable since the 1960's, when production-history plots first showed that producing wells offset from injection wells along this trend responded most rapidly to injection. Production data showed lower oil/water production-rate ratios (O/W in Fig. 2) and lower oil/water cumulative-production ratios parallel to this trend than perpendicular to it. Modern pulse-testing data confirmed this directional permeability trend.5 Whether this directional permeability trend resulted from natural fractures, induced hydraulic fractures, or some other cause was unclear.In-Situ-Stress Determination Program.
In 1987, Chevron drilled a 3,200-ft replacement injection well in Section 195 of the McElroy field. The main Grayburg pay zone occurred at 2,825 to 3,050 ft in this well. We coordinated the in-situ-stress measurement program with the drilling program for this replacement well.6 The field program included cutting 278 ft of oriented core,5 microfractures,7 anelastic strain recovery (ASR) measurements8,9 on 25 samples, and borehole televiewer (BHTV)10 and formation microscanner logging. After the field program, the core underwent laboratory analysis, including paleomagnetic core orientation,11,12 differential ultrasonic velocity analysis (DUVA),4 repressurization-stress relaxation (RSR),13 mechanical property testing, and petrography.Experimental Observations.
Ref. 6 gives detailed results of the field trial program in McElroy field. In this paper, we summarize the main conclusions of that program and present new in-situ-stress observations (Table 1) and results of the McElroy waterflood-realignment program.
From the Section 195 field-trial program, production maps, waterflood-surveillance programs, and published data of the Southern Great Plains in-situ-stress directions,14 we determined that the regional in-situ stress in McElroy field had its maximum horizontal stress, sH,max, oriented in a predominantly west-northwest direction (N75°W±15°). The total stresses in the reservoir pay zone are sH,max=2,050 to 3,300 psi, sH,min=1,700 to 2,300 psi, and sv=2,825 to 3,105 psi.
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