Successful Well Stimulation Program Has Revitalized a California Oil Field

Abstract

Complete plugging of bullet- and jet-perforations by distintegrated formation material and asphaltenes caused a severe well productivity decline in the Topic Zone of a California oilfield. Suction washing followed by circulation washing with acid/solvent dispersion cured the condition. Introduction The subject of this paper is the recent stimulation success in the Topic Zone, one of the producing zones in a California oil field. Six of the seven jobs on producers were successful. Oil gains were as high as 320 B/D in two wells, and averaged 135 B/D over all. Four of the wells are producing at rates in excess of initial rates of 8 to 10 years ago. Success has been in bullet- or jet-perforated wells; the only failure was in a gravel-packed well. Treatment of two water-injection wells was only partly, successful. Development of the Topic Zone began in late 1965. Fig. 1 shows the production history and the number of wells completed. The original reservoir pressure was near hydrostatic. From 1965 through 1968 production was by solution gas drive. During this period reservoir pressure declined to about one-half the discovery pressure declined to about one-half the discovery pressure. A pilot waterflood was started in 1967 and was pressure. A pilot waterflood was started in 1967 and was expanded in early 1969. Fig. 2 shows the injection history for the Topic Zone. All the Topic Zone wells have been produced by gas lift, except for a short flowing period early in the life of the field. As can be seen from Fig. 1, there was an initial response to waterflood, with production increasing to near 4.000 BOPD in 1970. However, oil production began a 30-percent/year decline. Through 1973, numerous generally unsuccessful attempts were made to recover productivity in the Topic Zone wells. productivity in the Topic Zone wells. In mid-1974 a review of the Topic Zone wells and pool performance strongly suggested that productivity performance strongly suggested that productivity impairment had occurred from unknown causes. A renewed stimulation effort was initiated. Details of the successful stimulation technique, suction wash/acid-circulation wash (SAC wash), are presented later. Results suggest that severe perforation plugging caused by alteration and disintegration of the formation in the vicinity, of the wellbore caused the productivity-capacity decline. productivity-capacity decline. The Rock Petrographic analyses of core samples of the Topic Zone Petrographic analyses of core samples of the Topic Zone sand showed the formation rock to be a poorly sorted, poorly consolidated, fine- to coarse-grained arkose or poorly consolidated, fine- to coarse-grained arkose or feldspathic-graywacke), with angular to subrounded grains of quartz, feldspar, highly altered biotite, and rock fragments in a micaceous matrix. X-ray diffraction analyses of core samples are listed in Table 1. These samples were selected to include obvious "shales" and obvious "sands" within the Topic Zone. Based on available routine core analysis, zonal porosity was about 29 1/2 percent and initial water saturation was about 32 1/2 percent and initial water saturation was about 32 1/2 percent. Numerous pressure buildup analyses indicated percent. Numerous pressure buildup analyses indicated initial permeability to oil in the reservoir was about 20 md. The fluids Oil The Topic Zone crude gravity is about 28 degrees to 29 degrees API. Asphaltene content is slightly more than 5 weight percent. percent. JPT P. 1420