Scaled Model Experiments of Fireflooding in Tar Sands

Abstract

Summary Fireflooding of tar sand reservoirs following a preheating phase exploiting a reservoir heterogeneity was investigated in three dimensional (3D) physical models. The research strategy for applying the results and the experimental facility developed for this work are discussed along with the results from four tests. Three different types of reservoir heterogeneities were used: communicating and noncommunicating bottom-water zones and a thin, heated layer simulating a fracture. The preheating phase with the bottomwater zone was found to have a pronounced effect on fireflood performance. A short steam preheating phase did not provide sufficient oil saturation in the communicating bot-tomwater zone to ensure formation of a closed combustion front. A longer steamflood at a lower rate resulted in gravity override of the steam and the subsequent fireflood. A thin, electrically heated horizontal layer simulating a heated fracture yielded encouraging results with potential for further improvements. Introduction The great demand for crude oil and the decrease in conventional reserves have generated more interest in heavy oil and tar sands. Although an important fraction of the world crude supply comes from heavy oil, there is nocommercial in-situ production of the immobile oil con-tained in tar sands. However, tar sands have become an attractive alternative source for liquid hydrocarbons because of the size of the resource base. Canada and Venezuela have the largest tar sand/heavy oil deposits, each estimated at more than 1012 bbl (150×109 m3) of hydrocarbons in place.1x002D;3 Currently, most of this resource is too deep to be recovered economically by surface mining. Extensive work is being performed to develop in-situ methods for producing tar sands.2x002D;4 Although the sands are usually very permeable, the bitumen is immobile because of its high viscosity, often exceeding 106 cp (1 kPa·s). Most of the proposed recovery methods apply heat to the reservoir to increase the bitumen temperature, which drastically reduces its viscosity. Steam injection has been the most common method used for heating the reservoirs. However, fireflooding could be performed with more effective use of surface energy.5 In-situ combustion has been tested in more than 100 field applications, many of which have been reviewed and compared in the literature.6x002D;8 Screening guides have been developed for evaluating the suitability of reservoirs for forward combustion.7,9,10 An important criterion in each of these guides is that the reservoir must have sufficient fluid transmissibility. Since most tar sand reservoirs do not meet this criterion, they traditionally are not considered good candidates for fireflooding. Local heating of the reservoir near an injection well will not provide the communication between wellbores necessary for a displacement process. In-situ combustion in a tar sand requires some scheme to provide initial interweIl communication and mobility to a portion of the tar. A pretreatment might involveheating through a naturally occurring reservoir heterogeneity, such as an adjacent mobile water zone, or an induced fracture with steam injection or fireflooding. Combustion has been field tested in induced frac-tures11,12 and at oil/water interfaces.13 Alternatively, interwell communication might be achieved by reverse combustion14 or electrolinking. Preheating also could be performed by extensive steam stimulation or electrical techniques. The cost of the preheating phase might limit its duration to the minimum extent necessary to begin a displacement process.