PSAnalysis of Low Permeability Intervals in a Heavy-Oil Braided Stream Deposit Using a Combination of Core and Log Analysis, Kern River field, California
By
Larry C. Knauer1, Robert Horton2, and Allen Britton3
Search and Discovery Article #50004 (2003)
*Adapted for online presentation from poster session presented at the AAPG Convention, Salt Lake City, Utah, May, 2003.
1ChevronTexaco, Bakersfield, CA ([email protected])
2California State University, Bakersfield, CA
3Core Laboratories, Bakersfield, CA
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The Kern River field is located in Kern County, California, immediately adjacent to the city of Bakersfield (Figure 1). This super-giant oil field has produced over 1.5 billion barrels of 12-degree-API gravity crude during the last 103 years from a Mio-Pleistocene braided stream deposit (the Kern River alluvial fan) (Figures 2, 3, and 4). Estimated reserves are still substantial. Production of over 100,000 BOPD places this field in the top five producing fields in the country. Zones of reduced reservoir quality due to poorly sorted sand, siltstones, and minor amounts of clay are resulting in unproduced pockets of the reservoir rock with residual oil saturations 10-30 saturation units higher than the adjacent rock with higher permeability. Some of the lower quality reservoir rock is already heated to 220 degrees (F) or greater and shows no sign of draining. Other areas have been noted with high oil saturation which appear not to be draining and are at lower temperatures than the surrounding rock. A study is underway to determine if the lower permeability (rock quality) is the sole reason for the pockets of high residual oil. 70 cores taken over the last 30 years are being reviewed, along with core photographs, wireline logs, and 3D models to determine the character of the targets and their extent. Two examples (Toltec lease and Mitchell lease) of by-passed oil are reviewed here.
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Toltec LeaseChanges in
reservoir development and associated changes in oil saturation are shown
by means of a Thin-sections reveal poorly sorted, angular arenite with significant biotite. Calcite and kaolinite may be the result of alteration by steamflood (Figure 6C). Laser particle-size analysis shows unimodal distributions with grain sizes ranging from granular to silt size. The particle size data correlate well with the capillary-pressure data (Figure 6D and E). Overburden centrifuge capillary-pressure data show Swir values ranging from 48% to 29% (Figure 6E). Integration of these multiple data sets correlates very well with each other and demonstrate the heterogeneity of KRF (Figure 7). Results of the steamflood analysis demonstrate the relationship between original water saturation of the reservoir and fractional oil recovery following steamflood. Residual oil saturations following steamflood range from 17.8% to 25.9% (Figure 8). Comparison to residual oil saturations measured from logs indicates that the potential for additional recovery may be limited in this area with current production methods.
Mitchell LeaseChanges in
oil saturation and in reservoir development are shown by means of a
Thin-
Results of the steamflood analysis indicate fluid recovery values ranging from 28.1 to 54.4% of the original oil in place (Figure 12).
ConclusionIn comparing the two leases presented here, from the Kern River Field, with production history that has extended across three centuries (Figure 13), we found that the reservoir rock on the Toltec lease exhibited alteration by-products from the steamflood process that resulted in a change in rock quality. This is probably inhibiting efforts to get the higher oil recovery percentages we see in other parts of the reservoir. The reservoir rock on the Mitchell lease exhibits no alteration of the reservoir rock, has a very good permeability, and consequently a greater potential for additional oil recovery. The fact that it has not produced much in previous attempts at cyclic steaming does not appear to be a (permeability) reservoir problem. The use of focused steamflood applications and/or horizontal drilling technology may be applicable in the Mitchell lease to distribute steam to the reservoir in a more efficient and effective manner. |