Jeff Jones, Michael McWilliams, David Sturm
Santa Fe Energy Resources Inc.
Bakersfield, Calif.
Santa Fe Energy Resources Inc.'s aggressive redevelopment of it's Kern River operations increased oil production from 1,600 b/d to over 4,000 b/d, reduced operating costs from over $8 to about $4/bbl, and significantly extended the field's economic life.
Total capital expenditures were about $5 million over 5-years.
FIELD HISTORY
Kern River field's history closely parallels that of the U.S. oil industry. Discovered by Roe Elwood and others in 1899, the Kern River field began as a hand dug, redwood-cased well with a 70 ft TD. Since then, the field has been a major contributor to California and U.S. oil supply.
The field currently produces about 125,000 bo/d from about 7,000 wells. If ranked by its cumulative production of over 1.4 billion bbl, the field is the third largest in contiguous U.S.
Santa Fe Energy Resources (SFER) has operated leases in the field since the early part of this century. In fact, the discovery well is located on SFER's Elwood Fee property. The approximately 240 acres operated by SFER is in a strip along the north bank of the Kern River that includes its Rasmussen and Elwood Fee properties and its Kern County Land Co. (KCL) lease (Fig. 1) (29857 bytes).
The Kern River formation, at depths of 80-1,300 ft, is the main producing interval. Fig. 2 (64775 bytes) shows a typical well log with stacked Kern River formation reservoirs composed of sands and conglomerates derived from the Sierra Nevada mountains nearby to the east.
Clay and silt intervals between the reservoirs serve as local barriers to vertical fluid flow. The entire sequence dips 4 to the southwest.
On SFER's property at depths less than 900 ft, oil gravity ranges from 12 to 14 API. Reservoir pressure is very low, with the static fluid level several hundred feet below the top of oil-saturated sands. Permeability ranges from 0.5 to 5 Darcys. Porosity is over 30%.
Gravity drainage is the primary producing mechanism. Viscosity-vs.-temperature characteristics are ideal for low temperature thermal enhanced oil recovery (TEOR). Although these reservoir characteristics indicate steam flood TEOR as the most effective process, cyclic steam stimulation ("huff and puff") has also been used very successfully.
From discovery through the mid-1960s, the field produced under primary recovery. The field followed the industry through many boom and bust economic cycles and as of 1966 cumulative production was estimated to be 400 million bbl or 10% original oil-in-place.'
Use of TEOR methods can be traced back to the mid1950s with bottom hole heaters that improved oil pump-ability. In 1962, Tidewater Oil Co. (later Getty Oil Co., now Texaco Inc.) began injecting hot water into four pilot wells.
Most operators, including SFER, had started cyclic steam projects and continuous steam flood pilots by 1965. While Santa Fe Energy ran steam flood pilots on all of its properties, the cyclic technique proved more cost effective until the oil price increases in the 1970s.
In 1978, on the Rasmussen and KCL properties, SFER started continuous injection in the lowermost H zone. Eight 50 MMBTU/hr steam generators converted about 20,000 bw/d into 80% quality (80% vapor and 20% liquid) steam for injection into 65 wells. Line agreements with offset operators allowed steam injectors on common lease lines.
The steam flood program peaked in the early 1980s at about 4,000 bo/d and had an attractive cumulative steam/oil ratio of 6:1 (Fig. 3) (41553 bytes).
In 1986, the combination of very low oil prices and declining oil production due to the maturity of the flood forced SFER to terminate high-quality steam injection. The subsequent low-quality steam/hot water flood reduced fuel expenses and improved vertical sweep. Field expenses were dramatically reduced, but oil production continued to decline.
Low-quality steam injection continued until 1990.
THE 1990s
While cogeneration has helped other operators at Kern River to defray steam flood costs, Santa Fe Energy does not have enough properties to justify a cogeneration plant.
Therefore, to regain profitability, SFER initiated innovative operating techniques. After an intensive engineering evaluation, SFER decided to step back one rung on the TEOR ladder and institute an aggressive cyclic steam project.
In preparation for the new project, facilities were optimized to improve efficiency. Steam generators were equipped to run on either lease produced crude oil or pipeline natural gas, whichever is cheapest.
To ensure consistent delivery of quality steam to all wells, SFER improved the steam distribution system by installing vibrating densitometer flow monitors and modified steam diversion tees.
The tee design was developed by SFER2 for these retrofit applications and has been very successful.
Steam distribution headers were split so that multiple wells could be steamed simultaneously and each was fitted with an adjustable choke and measurement device.
While preparing the facilities for the cyclic project, SFER addressed the issue of refitting the existing producing wells for cyclic operations and identifying new well locations to fully develop the properties.
The several generations of existing wells in the field each needed different attention.
The first group consisted of seventy 2/2 acre, five-spot pattern producers drilled in 1977-78 for the steam flood. These wells had 7 in., 23 lb/ft casing cemented to TD. The casing had four/2-in. jet perforated holes/ft across the original H steam flood zone.
SFER's new program opened all the overlying oil sands through the C zone (as shallow as 80 ft) for production. This was expected not to hinder future steam flood potential because coring after the original flood indicated that limited entry perforating in producers had minimal effect on directing steam into a desired interval. Local lithology was identified as being more important in defining steam confinement.
The second group had 47, nine-spot, infill and off-pattern wells that were drilled since 1980 and identically completed to the five-spot pattern wells. These were also recompleted across all productive sands.
In 1991, SFER began drilling new infill wells in the remaining nine-spot locations and along lease lines. This segment of the project is about 70% complete with 47 wells added during 1993 and 1994.
A fourth group involved 14 steam injectors on lease lines that had been previously under lease-line agreements. These wells had been idle since the continuous steam program terminated in 1986.
Because offset operators had adopted different strategies, these wells could no longer be operated as injection wells. Therefore, each party agreed to convert their respective injection wells to producers.
Other than allowing for the 5/2 in., 17 lb/ft casing, these former injection wells were recompleted exactly the same as the pattern producers.
The next group had 65 old wells drilled prior to 1975. When these wells were originally completed, it was believed that Kern River wells required slotted liners for sand control. Unfortunately, this completion method proved susceptible to severe plugging upon exposure to steam. Thus, these wells typically have produced poorly.
Pressure washing, slot scratching, and liner replacement became common practices. But because of low penetration into the equally plugged sand face and then the rapid replugging of cleaned slots, all of these techniques proved to have limited benefits. Most wells in this group produced at much lower rates than the immediate offsets that were cased and perforated.
The response to new high-pressure jet washing suggested that proper techniques could increase production from these wells. SFER is now jet perforating these old wells as though they had been completed with cemented casing.
An early concern was that this perforating would split the liners. But by using conventional 4-in. hollow carriers containing much of the explosive energy, SFER has found minimal damage. Caliper surveys have indicated minor random splitting between rows of slots below the fluid level and no damage above. Sanding problems were not encountered and the wells came back producing at rates comparable to new wells.
An aggressive competitive bidding program for the recompletion contracts has resulted in low cost work that provides SFER with an attractive alternative to drilling new wells.
The final group had 48 pattern steam injectors. Two were jet perforated as a test at 4 shots/ft in the old drive zone and placed on production. This zone was still hot, over 200 F., and therefore steam cycling was not needed.
One of the two wells responded with production of 20 bo/d and the other with about 10 bo/d. Similar responses from the 46 remaining wells proved economically attractive.
Idle equipment and low cost tie-ins to gathering lines made these recompletions profitable even with low oil prices. While this zone will be abandoned when the drive is redeployed in the next shallower zone, the technique will effectively scavenge heat and produce oil from areas that otherwise would not have been exploited.
PROJECT RESULTS
The cyclic steam program was re-instituted in 1989. As wells were added to the project, steam was increased to the current rate of 9,000 b/d from three generators. This rate is adequate to provide each producer two, 7,000-bbl steam cycles/year.
Rubber cups on the tubing string have helped concentrate steam in selected zones and improved response. The cups proved so successful that they are now a standard part of the program.
Inter-well response to injection was observed. To take advantage of this stimulation, a "sequential steam"3 program will steam wells in an adjacent pattern so that oil is "driven" immediately after having been steamed. This program has produced oil from cyclic steaming at rates comparable to the peak production of the earlier steam flood, but using only 45% as much steam (Fig. 4) (30285 bytes).
SFER engineers expect this project to be attractive for another 4 years when the next interval will be steam flooded. At that time only 10% of the oil-in-place will have been produced so that adequate oil reserves will remain. In fact with the preheating afforded by this cyclic project, future steam flood efficiency should be significantly improved.
ACKNOWLEDGMENT
The authors thank Santa Fe Energy Resources Inc. for permission to publish this article.
REFERENCES
- Bursell, C.G., Taggart, H.J., and DeMirjian, "Thermal Displacement Tests and Results-Kern River Field, California," Petroleum Industry Conference on Thermal Recovery, Los Angeles, June 1966, pp. 45-49.
- Jones, J.A., and Williams, R.L., "A Two-Phase Flow Splitting Device That Works," SPE Production & Facilities Journal, August 1993, pp. 197-202.
- Jones, J., and Cawthon, G.J., "Sequential Steam: An Engineered Cyclic Steaming Method," JPT, July 1990, pp. 848-901.
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