Sea-Water and Subsurface-Water Injection In West Delta Block 73 Waterflood Operations

Abstract

Experience with sea-water injection in an offshore Louisiana waterflood is presented and compared with the use of subsurface formation water. presented and compared with the use of subsurface formation water. Subsurface water proved to be highly satisfactory in this waterflood. Introduction The West Delta Block 73 field is located 27 miles south-east of Grand Isle, La., and 17 miles west of the mouth of the Mississippi River's Southwest Pass. Water depth is 175 ft. The field was discovered in 1962 and developed during 1963-66. At the end of 1975, cumulative oil production was 128 million bbl and daily rate was 19,600 production was 128 million bbl and daily rate was 19,600 BOPD. Waterflood operations were initiated in 1967 at three small West Delta 73 waterflood plants using filtered and deaerated Gulf of Mexico water. There were no conductors available to drill source-water wells and economic incentive existed for using sea water if feasible. Waterflood planning was based on satisfactory injection at a small waterflood in another offshore field plus analysis of core and water samples from West Delta 73 wells. By 1969, injectivity problems required higher injection pressures. Also, allowable increases that necessitated greater injection capacity resulted in installation of enlarged waterflood plants on four platforms in West Delta 73. Consistently satisfactory injection water was not obtained, and a 1972 study resulted in changing from sea water to a higher-salinity subsurface water. This paper presents the data supporting the change to subsurface water and summarizes 1974-75 experience and costs resulting from the change. Waterflood System Implementation History The West Delta Block 73 field is a low-dip anticlinal structure containing 12 oil reservoirs overlying each other between 8,000 and 9,500 ft with no significant faulting. Shaly intervals are interspersed throughout most reservoirs. Porosity averages 26 percent and typical permeabilities to air are 500 to 800 md in sand members. permeabilities to air are 500 to 800 md in sand members. Early in its life, declining reservoir pressures signaled the need for pressure maintenance, and water injection began in July 1967. As shown by Fig. 1, injection rate was low until an expansion project was completed in 1970. Mechanical problems with injection pumps hampered injection rates problems with injection pumps hampered injection rates during late 1970; as these problems were solved and injection increased, injectivity problems began to dominate. Fifty workovers were completed in 1971 alone in an effort to sustain satisfactory injection rates, but sharp declines rapidly erased much of the initial increases. Sea-Water Treating System The sea-water system at West Delta 73 actually consisted of four separate systems on different platforms. Fig. 2 is a flow schematic of a typical sea-water injection system. Sea water is lifted from -20 ft with a shaft-driven turbine pump powered by a gas engine or electric motor. pump powered by a gas engine or electric motor. Chlorine is injected into the sea water for bacteria control and a polyelectrolyte solution is added for solids flocculation before entering the primary filters. For this service, upflow-type filters are preferred from size-capacity considerations. A second sea-water lift pump is used to backwash the filters. JPT P. 623