In-Situ Combustion Appraisal and Status

Abstract

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleum engineering. Introduction At the end of World War II, the oil industry started to look for new methodsof producing a higher percentage of discovered oil. Before then, percentage ofdiscovered oil. Before then, waterflooding and gas repressuring were the onlysecondary recovery methods in widespread use. Also, for the first time, the oilcompanies initiated a major effort to research petroleum production byestablishing research laboratories devoted to production rather than refining. production rather than refining. In-situ combustion was the first EOR processto be developed. Laboratory work was initiated in 1947, and four major field tests of the process were performed by 1958. The first full-scale commercial performed by 1958. The first full-scale commercial application of the process was started in 1959, and two of the original pilot projects were soon expandedto commercial scale. These early field tests demonstrated the wide range of applicability for the combustion process. Mobil Oil Co.'s work was aimed at the unrecoverable heavy oil (18 degrees API [946 mg/cm3]), while Sinclair targeted light oil (37 degrees API [840 mg/cm3]). Both were successful in recovering additional oil. This rapid development and acceptance of the process-12 years from initial research to commercial operation-has not continued. In fact, general acceptance and use of the process have been very slow. This slow acceptance can be traced to two main sources: the advent of cyclicsteam stimulation with its low initial investment and immediate oil production response and the high level of manpower production response and the high levelof manpower required to engineer and to operate a combustion project properly. These two factors, combined with project properly. These two factors, combined with the discovery and development of oil in Alaska, have retarded thewidespread use of the combustion process. However, the favorable energy ratiowith process. However, the favorable energy ratio with which the process can be operated, no technical depth limitation, and applicability to a wide range ofreservoir and fluid characteristics will eventually lead to widespread use ofthe combustion process in one of its many configurations. Factors Influencing the Combustion Process The combustion process, with its associated zones and temperatures, isdepicted in Fig. 1. While most descriptions of the process show the variouszones present in terms of a frontal advance model (i.e., full present in termsof a frontal advance model (i.e., full vertical sweep), Fig. 1 tries to show amore realistic picture of the bypass model. Gravitational segregation pictureof the bypass model. Gravitational segregation results in the override of the injected air and lessthan-full vertical sweep because of density differences inthe fluids. In a heavy-oil reservoir (viscosity less than 3,000 cp [greaterthan 3 Pa.s]), a frontal advance project probably would fail as a result ofhigh flow project probably would fail as a result of high flow resistance. Anoil bank would form downstream of the heat bank. Because of its high viscosity, this cold oil bank would require a very high pressure gradient to allow thefront to move at economical rates or even at air-injection rates necessary tosustain combustion. In the bypass model with reduced vertical sweep, the heat front moves at ahigher velocity for a given air-injection rate and, therefore, is closer to theproduction well. The average temperature in the production well. The averagetemperature in the unswept portion of the reservoir is higher because heat islost to the zone. not to the overburden, so the viscosity is lower. The oilzone ahead of the heat front has a larger surface area and, therefore, is thinner and offers less flow resistance. These three factors combine to reducethe flow resistance and to allow the front to move at an adequate rate, butthey result in a lower initial vertical-sweep efficiency. In the bypass model, burning continues on the trailing edge of the burning front, if vertical permeability exists, as the front moves through thepermeability exists, as the front moves through the reservoir. JPT P. 1943