A Low-Tension Waterflooding Process

Abstract

A field test to examine some aspects of surfactant behavior, and a later polymer injection study, led to the conclusion that a tertiary oil bank can polymer injection study, led to the conclusion that a tertiary oil bank can be formed in a reservoir using low-tension surfactants. Another conclusion is that it is essential to control the mobility immediately behind the bank to insure that a significant fraction of the mobilized oil will be driven to the producing wells. Introduction The work summarized here represents a part of the effort by Mobil Research and Development Corp. to develop an economic low-tension waterflooding process. Various aspects of displacement at low tension process. Various aspects of displacement at low tension are discussed in general terms. Details of the extensive background effort supporting the necessary laboratory and field experiments, an interpretation of results, and the development of an adequate transport theory are deferred to later publications. Not all aspects of this process have been field tested yet. However, Mobil has carried out a field test in South Texas to examine factors relating primarily to surfactant behavior. A polymer injection study was conducted at the same site some time later. As a result of these field studies and supporting theory we conducted that a tertiary oil bank can be fanned in a reservoir using low-tension surfactants and that mobility control immediately behind the bank is essential to insure that a significant fraction of the "mobilized oil" will be driven to producing wells. A surfactant waterflooding process capable of producing a tertiary oil bank has also been described by producing a tertiary oil bank has also been described by Gogarty and Tosch. One significant difference between their "Maraflood" process and the process described here is the manner in which the surfactant is used. Maraflood employs a surfactant slug that is miscible with the reservoir crude. Miscibility implies zero interfacial tension between this slug and the reservoir crude oil. Achieving and, particularly, maintaining this miscibility condition places rigorous requirements on the composition of the slug. Our process, on the other hand, does not depend upon process, on the other hand, does not depend upon miscibility between crude oil and water, but relies on very low interfacial tension between a water solution/ dispersion of a surfactant and the reservoir crude oil. Also, the compositional requirements that must be met in order to achieve and maintain a condition of very low tension are somewhat different from those needed in the Maraflood process. Description of the Process In what follows it is assumed that the process is started in a sandstone reservoir that is nearly or completely watered out. The water phase present in the reservoir at this stage is assumed to be a typical oilfield brine, high in total dissolved solids and in divalent cations, particularly calcium and magnesium. A regular pattern from the existing injectors and producers is chosen, with high areal sweep as an producers is chosen, with high areal sweep as an important design criterion. The process consists of injecting three slugs of water with different chemical compositions. These will be denoted as the protective slug, the surfactant slug, and the mobility-control slug, or as Slugs 1, 2, and 3, respectively. The protective slug is an aqueous solution of sodium chloride, Within limits, its volume is somewhat arbitrary, in the range of 0.1 of the pattern pore volume. JPT P. 205