The water-alternating-steam process (WASP) uses alternating slugs of steam and water injected over more than one cycle to recover oil. The process was applied to the vertical expansion (VE) sands in the pilot area of Section 13D, West Coalinga Field, to stop wasteful steam production and to improve vertical conformance of injected steam. By replacing a part of steam with unheated water, the process also reduced fuel consumption and improved the steamflood economics.
Prior to the WASP application, steam breakthrough in the VE sands caused well sanding, cutting of downhole tubulars, and high temperature fluid handling problems. To alleviate these problems, pumps had to be raised in five wells and one well had to be shut in reducing oil production from both the VE sands and the lower waterflooded zones. A WASP field test, based on a numerical simulation study, was implemented in July of 1988 with alternating lugs of water and steam, each injected over four months.The WASP eliminated steam production, allowing pumps to be lowered in the five producing wells and one shut-in well to be returned to production. The oil production remained constant through the first cycle and increased during the second cycle of WASP. The sales oil (total production minus oil used to generate steam) increased due to the saving of generator fuel during the water leg of each WASP cycle.
Encouraged by the test results, the WASP is continuing in the pilot area. It is presently applied to another steamflood area in West Coalinga Field and considered for two other areas in the San Joaquin Valley.
References and Illustrations at end of paper.
Steam injection was first used in the late 50's to recover high viscosity oils. This EOR method has become increasingly popular during the past 30 years and now accounts for over one million barrels per day of oil production worldwide, about half coming from heavy oil fields of the U.S. 1
Despite the success of steam in improving the recovery of viscous oils, there are characteristic problems associated with its use: steam gravity override and steam channeling. Steam gravity override results from the large density difference between the injected steam and in-situ fluids. It reduces the reservoir vertical conformance and leads to early steam breakthrough at production wells. If high permeability channels are present in the reservoir, steam can propagate through them, also leading to early steam breakthrough and poor vertical conformance. These are undesirable events because they reduce the ultimate steamflood oil recovery and cause such well production problems as sanding, cutting of downhole tubular, and high temperature fluids in the wellbore.
Several methods are currently in use or being field tested to combat the steam breakthrough and reservoir conformance problems. One method involves the use of surfactants to form steam-foams In situ that reduce steam mobility and controlinjection profiles.2 While field tests of this method reported technical success in improving the vertical conformance, very few claimed economic success. Additional research and field tests are needed before this method can be used on a commercial scale.