Production Optimization of a Mature Gas Field

Abstract

SPE Members Abstract This paper summarizes production optimization of a mature gas field, located in the Sacramento Valley near Stockton, California. A substantial increase in gas production was obtained at a very low cost. It involves the various applications of reservoir and production engineering: Nodal analysis, recompletion, and acid stimulation. To keep the cost down and minimize formation damage, all the work was performed through-tubing without pulling the well. Introduction The Union Island Gas Field was discovered by Union Oil Company of California in 1972 and the first commercial production started in 1976. It is located in an area of prolific gas production near Stockton, 20 miles southeast of Rio Vista Field. It is one of the largest dry gas fields in California (Figure 1). The Union Island gas reservoir is a Winters formation comprised of six gas sand lobes and a common aquifer which supplies a weak to moderate edge water drive (Figure 2). Listed in Table 1 are the reservoir parameters. The peak production from a total of fourteen wells was 110 MMCF/Day in 1977. By early 1988, production had dropped to 50 MMCF/Day from twelve wells with a severe decline rate of nearly thirty percent per year. The initial depletion strategy was based on having only one sand at a time open in each wellbore. As a well watered out or production declined to an uneconomic rate, it was recompleted to the next higher sand. This strategy worked well in the early life of the field, but tended to restrict individual well and field deliverability. A new depletion strategy was developed and initiated in 1990. The basis for the new strategy was to maximize deliverability and try to out-run the water influx. Due to cash-flow constraints, it was necessary to minimize initial investments. A concerted effort was made to do as much through-tubing well work as possible. PRODUCTION OPTIMIZATION RECOMPLETIONS In 1990 the production optimization was started with a substantial increase in production (Figure 3). A total of 337 feet of gas sands in eight upstructure wells were perforated with through-tubing gun, six holes per foot. The wells were completed with 5-1/2" and 7" casing and 2-3/8" and 2-7/8" tubing. Prior to recompletions, cased hole logs were run through-tubing to determine the saturations of the different sands behind pipe. No wet sand was opened during the recompletions. The total cost of the recompletion was one hundred sixty thousand dollars and the estimated incremental gas production was 15 BCF through the middle of 1993. COMPRESSION AND NODAL SYSTEM ANALYSIS In 1992, a compression study and well performance system analysis (Nodal Analysis) were performed. The well performance analysis showed that production could be substantially increased if gas was produced both up the tubing and casing-tubing annulus. When compared to compression, a modified well completion not only had greater deliverability, but also had lower initial investment and operating costs. P. 393^