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Abstract This paper summarizes and contrasts the peripheral waterflood performance of two reservoirs that constitute the 31S project in the Elk Hills Field, Kern County, California. The combination of light oil (36 API), favorable mobility ratio (0.4) and an anticlinal structure makes these sandstones good candidates for peripheral flooding. From 1978 until December 1995 cumulative water injection in the 31S project was 371 million barrels and cumulative recovery was 155 million barrels of oil. Early initiation of a pressure maintenance project including 117 BCF crestal gas injection has improved oil recovery. The Main Body 'B' reservoir is exhibiting a distinctly layered response with high recovery from its more permeable layers and lower recovery from tighter layers. This flood has the potential for bypassed reserves. The Western 31S reservoir has moderate to high dips and is exhibiting a less layered flood response and very efficient displacement. The gravity drainage component from the secondary gas cap is playing a significant role in the process. Aside from high recovery, the project is remarkable in demonstrating successful resaturation of a secondary gas cap. Future work will include refinement of original-oil-in-place (OOIP) estimates with a new petrophysical model and improving tight layer management by pattern waterflooding and increasing productivity and injectivity with stimulation treatments. Introduction In the literature there are a few papers describing peripheral waterfloods. The 31S project in the Elk Hills Field, California provides an opportunity to study a large, mature peripheral waterflood and to see the effects of a long term injection into two very different reservoirs. The Elk Hills Field is located about 20 miles WSW of Bakersfield, California (Fig. 1). It is one of the 10 most productive oil fields in the United States with a cumulative recovery of over one billion barrels to date. The Main Body 'B' (MBB) and Western 31S (W31S) sands occur at an average depth of 6,500 ft. The zone was discovered in 1941. The field was held in reserve and only periodically produced until 1976 when it was opened up to production at capacity. The current production from the MBB and W31S reservoirs is about 11,300 BOPD and 3,500 BOPD respectively. The mechanism for primary production from for the MBB and W31S reservoirs was a combination of solution gas, partial water drive, and some contribution from gravity drainage. The primary recovery factor was estimated to be in the range of 12% of the OOIP. Since the onset of the exploitation, secondary recovery was considered essential for the MBB and W31S reservoirs. P. 227 |
