Surfactant Flooding Carbonate Reservoirs

Abstract

Summary Some of the most difficult yet most attractive EOR targets are found in the west Texas carbonate reservoirs, where tertiary recovery is complicated by characteristically very low permeability coupled with high hardness and salinity of the reservoir brine. This paper presents data obtained in two well-pair surfactant floods undertaken in the San Andres formation, Bob Slaughter Block, Hockley County, Texas. To provide the most data in the least time, a close well spacing and relatively large production/injection ratios were used. For each well pair, a different surfactant system was developed, tailored to resist the local aquifer brine salinity and hardness. Consequently, neither system required a preflush. Each test consisted of a large slug of surfactant, followed by biopolymer, fresh water, and formation brine, together with appropriate nonadsorbent tracers. The large slug sizes were intended to allow subsequent designs to be made by interpolation rather than extrapolation. The data provided by these tests demonstrate that (1) low-permeability carbonate reservoirs can be chemically flooded, (2) polymer can be injected and propagated in these reservoirs without any biodegradation, (3) these systems can mobilize tertiary oil effectively at a chemical consumption near that observed in the laboratory, and (4) two-well tests can provide a quick field evaluation of surfactant systems proposed for large-scale use. Introduction The west Texas carbonate reservoirs contain vast amounts of residual hydrocarbons even though most of these reservoirs are in an advanced stage of depletion. The San Andres dolomite reservoir in the Texaco Bob Slaughter Block lease is a good example of such a reservoir. This lease was drilled in the late 1930's and initially produced a large volume of oil by primary production. The lease has been subjected to waterflooding since the 1960's, and despite 20 years of waterflooding, the lease still contains a large tertiary oil potential. Geologic Setting The field lies on the northeast flank of the Permian Basin. The reservoir rock occurs in the Upper San Andres formation. The underlying Lower San Andres formation was deposited during the maximum transgression of the Permian Sea and is dominantly a micritic limestone containing little porosity. The Upper San Andres formation contains intertidal and supratidal sediments deposited during withdrawal of the sea across the broad, shallow shelf. A hot, acid climate combined with the broad, flat shelf resulted in carbonate deposition, including algal mats, in what geologists call the sabkha environment. The carbonates were penecontemporaneously dolomitized by downward-seeping marine storm waters or terrestrial floodwaters alternating with times of moving capillary evaporative waters. Anhydrite beds developed, usually above or landward of the dolomites, and also precipitated as nodules and infillings in the dolomite. As regression of the sea continued, the anhydrite unit slowly covered the dolomites, thus sealing them. Several distinct dolomite facies were formed. These dolomite lithofacies reflect the original carbonate characteristics, such as grain size and origin of particles.