Effects of Foamy Oil and Geomechanics on Cold Production

Abstract Foamy oil flow is known to give unexpectedly high recovery rates in heavy oil reservoirs, and has been subject to extensive experimental study in the lab. Once gas exsolution is initiated, the generated tiny gas bubbles are dispersed along with heavy oil flow, thereby providing driving force for oil production. Superimposed onto foamy oil flow are other mechanisms such as the failure of the solid skeleton and concomitant sand production. This paper investigates the intertwined effects of failure, sand production and foamy oil flow on the oil recovery process using a finite element based numerical model developed by the authors. The effects of material failure and foamy oil flow and sand production are studied in a perforated oilwell undergoing cold production (non-thermal recovery process). In particular, reference is made to geomechanical issues such as shear dilation and strain localization. It is found that sand production is a result of two distinct mechanisms: one which involves the material dilating in shear mode under deviatoric stresses, and another one which involves an increase in porosity of the solid matrix under hydrodynamic and erosion actions with foamy oil flow. Some interesting results are presented in which wormhole formation and propagation are captured without any numerical difficulty or instability, due to a proper formulation and discretization of the governing equations. Introduction The idea of using a non-thermal process for the primary recovery of oil when the viscosity and permeability are less than 10, 000 cp and 5 Darcy respectively has matured over the past several years. This process coined as "cold production", or more elaborately Cold Heavy Oil Production with Sand (CHOPS), has been tried in unconsolidated or weakly consolidated sands as a non-thermal stimulation process in which both sand and oil are produced together in order to enhance oil recovery. The oil production process is also typified by the formation of a socalled foam (Maini, 1996)[1] as a result of gas exsolution and dispersion of tiny gas bubbles with limited growth in size. The intrigue is that the resulting foamy oil flow seems to greatly enhance oil production rates with high primary recovery factors, despite of the high oil viscosity. There have been many explanations put forward for interpreting such a phenomenon, namely: sand production, retardation of reservoir pressure decline, low GOR's (Gas to Oil Ratio), enhancement of absolute permeability, and high critical gas saturation. This paper looks into some of the above-mentioned issues by exploring numerically the inter-relationship between sand production, sand failure and foamy oil flow during the enhancement of oil production in a non-thermal process such as CHOPS. The Model The first author has written a series of papers on the topic of sand production modelling[2,3,4] in conjunction with geomechanical issues, and recently foamy-oil flow[5,6,7,8] with the second author. These form the basis and frame of reference for the modelling effort reported in this paper. The avid reader seeking for details of formulation and computer implementation is thus directed to the above-mentioned references.