Dilute Surfactant IOR - Design Improvement for Massive, Fractured Carbonate Applications

Abstract

Abstract Surfactants may be one of the best options to improve recovery from geologically challenging reservoirs. For over forty years, concepts and results of surfactant applications have been published. During recent years, depressed oil prices have limited surfactant consideration. However, surfactant recovery can be economically attractive for reservoirs where recovery is dominated by gravity and imbibition processes. Massive and/or fractured formations of moderate 30–50% recovery may warrant consideration for surfactant processing. Applications must meet laboratory performance hurdles for high incremental recovery at relatively low surfactant concentrations (quantified wett ability and bond number). Placement of chemical in the reservoir at optimum concentration is the next challenge. This paper presents analytical and simulation assessment of chemical placement within a massive fractured carbonate formation undergoing surfactant EOR field testing. Placement methods can range from continuous injection to interrupted "huff-n-puff" application. The gravity stabilized reservoir management process currently being applied in the reservoir avoids some of the chemical flood conformance challenges of classic applications. Oil mobilization by surfactant huff-n-puff in a naturally fractured reservoir is a complex process. The success of the application depends not only on surfactant chemical and physical properties and surfactant concentration injected, but also on fracture surface area treated by chemical solution and penetration depth of surfactant into matrix rock. In order to optimize the design of a dilute surfactant treatment in the Yates Field, an analytical model was developed to investigate effects of various parameters on the fracture surface area treated and penetration depth of surfactant solution into matrix block. These parameters include surfactant injection rate, injection volume, chemical diffusion, convection and fracture properties. Results of the analytical application are compared to simulator solutions. Laboratory testing, formation target screening, and field application are also discussed. Introduction Massive fractured carbonates provide both a great opportunity and challenge for surfactant EOR application. These fractured formations provide an opportunity because they often suffer low recovery of the original oil in place, leaving an EOR target that may exceed fifty-percent of oil saturation in most water-invaded pore types. This "bypassed" oil presents a challenge for chemical recovery since much of the oil remains furthest from the best connected flow paths. The challenge is split into four segments;circulating optimum concentration surfactant in water into the fractures,transfer of the surfactant from the fractures into both the less-directly-connected fractures and formation matrix poresthe chemical EOR mobilization of oil from the matrix into the fractures, andthe capture of oil from the fracture network. Items one, two, and four will be addressed in this discussion of field test design and results from the Yates Field Unit while the laboratory study of surfactant oil mobilization is reported by Chen 1. Related studies include Snell's 2 quantification of chemical tracer flow testing along a fracture network by applying discrete feature network (DFN) modeling La Pointe 3, to quantify effective permeability in X, Y, and Z directions for application in dual porosity simulation. Also Button 4 reports on vertical and horizontal completion connection to a depleting and moving fracture oil column.