Surveillance of South Belridge Diatomite

Abstract

SPE Member Abstract This paper illustrates the surveillance methods used in Shell's 1-1/4 and 5/8 acre (0.5 and 0.25 ha) waterfloods in the South Belridge Diatomite field:computer-assisted monitoring of injection pressures and rates,online databases with well tests and allocated production and injection data,step pressure tests in numerous water injectors,a geological database with cycle markers and directional surveys,sonologs, andsalinity tests. Methods (1) – (4) require use of custom software to be practical. The geometry of waterflood patterns in the Diatomite (well spacing compared with length of hydrofractures, injectors in-line with or offset from producers, and pattern orientation relative to the direction of maximum in-situ stress) influences the rate and frequency of coupling between the injectors and producers. It is shown that wells in the "direct" 1-1/4 acre patterns are less prone to coupling than the "staggered" ones because a "linkage potential" (defined in the text) is higher. The proximity of hydrofractures in the 5/8 acre staggered patterns makes the injector-producer coupling unavoidable if the patterns do not follow the direction of maximum in-situ stress. The coupling develops in the N20 degrees +- 5 degrees E direction, probably along the cycle tops. The step pressure tests of many injectors in waterflood Phases I through III have shown that hydrofracture extensions are common and we are currently unable to predict the "correct" injection pressures for individual wells. It is concluded that to avoid reservoir damage, each injector must be controlled individually. Injection pressures can be increased with time by trial-and-error, but the injection rate must be kept below a safe limit to preclude excessive damage if the hydrofracture is extended. Introduction The South Belridge Field, Kern County, California, is located near the western margin of the San Joaquin Basin. Geologically, the field (Fig. 1) is a NW-SE trending anticline, approximately 7 miles (11 km) east of the San Andreas Fault. Oil production in the South Belridge Field comes from two major pay intervals,the shallow marine Pleistocene Tulare sands, andthe marine Miocene-Pliocene Diatomite/Brown Shale. The latter interval has several unusual rock properties that make it unique. The rock has high porosity (50-70%) that is composed of several different pore types (interparticle, intraparticle, moldic, and fractures). The particle and pore sizes are small (0.1–100 pm). The matrix is chemically unstable and the grain density varies from 2 to 2.5 g/cm3. Despite a high pore volume compressibility (100-300 usips (14.5–43.5×10-6 kPa-1)), the rock contains natural fractures, some of which may be open, i.e., not cemented. The lithology of the Diatomite is the end-effect of cyclic variations in depositional environments that yielded a series of stacked silica-rich layers (cycles D - N in Fig. 1), separated by low permeability clay barriers. A typical cycle consists of a low quality clay/silt-rich interval overlaid by an increasingly pure diatomite deposition. This trend continues until a subsequent terrigenous influx marks the beginning of the next cycle. A functional definition of the top of the Brown Shale is the point (Fig. 1) below which the matrix porosity falls beneath 60% across most of the interval. Because the Diatomite/Brown Shale contact depends on diagenesis, it cuts across stratigraphic markers. Initially, the Diatomite was developed on a 2-1/2 acre (1 ha) spacing, with each well hydrofractured in several (3–6) stages over most of the interval. P. 171^