Carbon Dioxide Well Stimulation: Part 1-A Parametric Study
Authors: Patton John T. , Coats Keith H. , Spence Ken 
Published: December 2007

Summary
Cyclic CO2 stimulation of a production well, especially in viscous oil reservoirs, is developing as a method of rapidly producing tertiary oil and obtaining valuable data. History matching provides confirmation of CO2 /crude interactions measured in the laboratory resulting in increased accuracy of CO2 flood predictions. Profitability of the process is enhanced by proper control of key operating parameters. The most important are CO2 injected per cycle, number of cycles, and backpressure during production. Reservoir parameters dictate the selection of commercial applications. The dominant factors are oil viscosity, oil swelling and viscosity reduction resulting from CO2 dissolving in the crude, trapped gas saturations, fluid saturation, permeability, and wettability. In the absence of published field data, this study utilized a numerical simulator to predict incremental oil recovery as a function of these operating and reservoir parameters. Multiple regression analysis was used to relate the efficacy (STB incremental oil/Mcf CO2 injected) of the CO2 cyclic stimulation process to six parameters. Under ideal conditions, one extra barrel of stock-tank oil is produced for each thousand cubic feet of CO2 injected. Efficacy decreases with both number of cycles and volume injected.

Introduction
Starting with the early discovery of reservoirs containing viscous oil, engineers have been striving to develop commercial techniques for stimulating the production of this resource. In many instances, viscous oil cannot be displaced efficiently by water and other flooding agents. Therefore, significant effort has been directed at cyclic single-well processes. The steam stimulation process developed independently by Jersey Production Research Co. and Shell Development Co. during the early 1960's was an early breakthrough in stimulation technology. Since that time, a variety of novel chemical additives has been evaluated for enhancing the flow of viscous oil. Until recently, hydraulic fracturing was the only commercially viable alternative to steam stimulation. The productivity problem stems from the retarding effect on oil flow imparted by the high viscosity of the fluid. Viscosity can be reduced effectively by heating, as in the steam process, or by diluting with proper solvents. The ideal solvent would possess these desirable characteristics:dissolve in the oil, thus reducing its viscosity;not break out as an immiscible, highly mobile phase produced preferentially to the oil; andif it does break out as in (2), it should remain trapped as an immobile phase, providing energy by expansion to promote stimulated oil flow.
During the past 20 years, many solvents that more or less meet those criteria have been evaluated for cyclic stimulation processes. Early solvents tested lacked the cost effectiveness to be commercial. The main problem with the organic solvents is their inability to reach deep into the reservoir, and, hence, most of the process is devoted to injecting and producing solvent, with little net gain in oil production. During 1977, a new dimension was added to the solvent concept-the use of supercritical CO2 to achieve solvent-reduced viscosity deep into the reservoir. Since that time, three companies have field tested the process on widely varying viscous oils, with good results. Because of today's more reasonable heavy-oil pricing and incentives incorporated in the windfall profits tax, several commercial applications have been designed, and one began operating in 1981.
JPT
P. 1798^