Analysis and Numerical Modeling of Hydraulic Fracturing During Cyclic Steam Stimulation in Oil Sands

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Summary Cyclic steam stimulation in oil sands above fracturing pressure is analyzed by numerical modeling. numerical model is formulated that simultaneously describes the fracturing process and reservoir behavior for different types of fracture geometry. The model is used to study the differences in performance expected for different fracture types. performance expected for different fracture types. The comparison of model results with the data from a first-cycle cyclic steam operation shows good agreement for single vertical fracture configuration. Introduction Because of declining conventional oil reserves in recent years, there has been increased activity in technology development related to exploitation of tar sands and heavy-oil deposits. Huge deposits are located mainly in Alberta and the Orinoco heavy oil belt in Venezuela. Table 1 gives a perspective of the oil in place in the major oil sand deposits in Alberta. The Athabasca deposit is the world's largest accumulation of hydrocarbons and is approximately four times larger than the largest conventional oil field, located in Saudi Arabia. Although about 74 billion bbl (11.7 × 109 m3) of the Athabasca reserve is in areas shallow enough to be recovered by openpit techniques such as those employed by Suncor Inc. (Great Canadian Oil Sands Ltd.) and Syncrude Canada Ltd., it generally is agreed that the remaining deeper reserves must be recovered by in-situ methods. One fundamental objective in applying any in-situ recovery process in an oil-sand or heavy-oil setting is the reduction of oil viscosity. Virtually all in-situ recovery techniques rely on application of heat to achieve this objective. A second objective of many in-situ recovery processes is to induce injectivity artificially. Both objectives commonly can be satisfied in many cases by the steam stimulation process. Despite the fact that numerous steam process. Despite the fact that numerous steam injection operations have been carried out above formation breakdown pressure, very little is known about the details of process mechanisms, fracture orientation, and fracture dimensions. Following an examination of the pertinent physical characteristics of oil sands in Alberta and a brief review of oil-sand fracturing experience, we discuss the philosophy of modeling the process and compare results from a cyclic-steam-injection field operation with predictions from a numerical model that takes into account heat transfer from the fracture and considers two-phase fluid flow during injection and production. Three types of fracture geometry are production. Three types of fracture geometry are studied and compared. Oil-Sand Properties The reservoirs consist of sands of fluvial, deltaic, and marine origin, and the complex sedimentary nature results in high variability of reservoir parameters. Nevertheless, for the present purposes we can make the following generalizations. 1. The main components of oil sands are sand, oil, fines, and water. The sand is in the form of rounded or subangular particles, and each particle is wet with a film of water. The balance of the pore volume is filled with heavy oil and water plus, in some instances, a small volume of gas. In addition, there are fines (clay and silt) present. The impact of fine particles can be very significant in the recovery particles can be very significant in the recovery process since they tend to break loose and plug the process since they tend to break loose and plug the pore throats, preventing oil recovery. pore throats, preventing oil recovery. JPT P. 2201