Borregos Field Surfactant Pilot Test

Abstract

A pilot test of petroleum sulfonate flooding was conducted during the mid-1960's in the Borregosfield, Kleberg County, Tex. The test was run in a heterogeneous Frio sand containing considerable montmorillonite. Oil recovery was limited by excessive fractionation of the injected petroleum sulfonate and by poor sweep efficiency. Introduction A tertiary surfactant pilot test was conducted during 1965 and 1966 in the Borregos field, Kleberg County, Tex. Results of a more recent pilot test have been published. Aqueous surfactant flooding uses a preferentially water-soluble surfactant capable of drastically reducing water-oil interfacial tensions and, therefore, of reducing capillary forces trapping discontinuous residual oil. Although success with dilute petroleum sulfonate solutions had been demonstrated in laboratory core floods, a field test was needed to define the status of the process and to identify possible problem areas. Surfactant adsorption and process sweep efficiency were two major concerns. Description of Pilot Area Zone F-4 in the Borregos field is a thin Frio sand. Structurally, the field is a broad anticline. In the pilot area (Fig. 1), the F-4 sand has a gross thickness of about 8 ft. Elevation of the top of the sand at its highest point in the pilot (Well 238) is 4,998 ft subsea. Fluid contacts were pilot (Well 238) is 4,998 ft subsea. Fluid contacts were far removed from the pilot area. The oil-water contact was at 5,089 ft subsea and the gas-oil contact was at 4,972 ft subsea. At the time pilot operations were initiated in 1965, Zone F-4 was producing by gas-cap expansion and solution-gas drive. Reservoir pressure had dropped to 2,000 psi from a discovery pressure of 2,500 psi. psi. The pilot pattern was a 1.25-acre, isolated five-spot. An existing well, Well 238, was converted to an injector and four new wells were drilled. These wells were conventionally cored with natural mud to avoid alteration of rock wettability. One-hundred-percent core recovery was obtained in Wells 411, 412, and 414. Core recovery was very low in Well 420. Hole deviations during drilling caused the down-hole pattern to be somewhat skewed. Bottom-hole locations (Fig. 2) were determined by multishot directional surveys during drilling, except for Well 238, where a cased-hole survey was performed.A pressure coring operation conducted upon termination of the surfactant flood provided additional reservoir description. Core Hole 441-1 and Deviations 441-2 and 441-3 were drilled to determine residual oil saturations. The bottom-hole locations of these postflood core holes are also shown in Fig. 2. Geologic Study A detailed environmental-facies study was undertaken to provide a geologic framework for the pilot area. The provide a geologic framework for the pilot area. The external form of the F-4 sand is a tabular sheet; internally, it consists of a number of elongated, individual channel deposits that coalesce and cross one another. Each channel consists of an impermeable basal clay-clast bed, fine-to medium-grain sandstone, and another impermeable bed of siltstone. Channel-to-channel communication occurs through the sides of the channels and across erosional boundaries. The clay minerals are chiefly montmorillonite. Matrix (less than 0.03 mm) clay content is unusually high, averaging 8.5 weight percent. Porosity and permeability are also influenced by local zones of calcite cementation. Calcite cement was most prevalent in Well 414 and, to a somewhat lesser extent, in Well 412. JPT P. 695