We present visualization studies of hot water and steam injection in micromodel geometries that mimic a matrix-fracture system. Emphasis is placed on the matrix-fracture interaction during steam injection. We visualize the movement of steam and determine the conditions under which matrix penetration by steam occurs. More generally, the effectiveness of steam injection in displacing oil from the matrix is assessed.
Displacement of oil was observed to occur by three mechanisms: a thermally-induced solution gas-drive, capillary imbibition of condensed water and displacement by steam, when the latter penetrated the matrix. Vaporization of light components at elevated temperatures generated an efficient solution-gas drive in the matrix, without the need for steam penetration. This also led to the formation of stable foam-like lamellae, which improved oil production by blocking the movement of steam and water in the fracture. This mechanism was also found to apply during hot water injection. Condensed water imbibed the matrix according to rules that govern imbibition of a fracture-matrix system. Steam penetration of the matrix occurred when the steam rate exceeded a critical value, determined from drainage considerations. The results should be useful in modeling of steam injection in naturally fractured reservoirs.
Steam injection is an effective method for the recovery of heavy oil worldwide (Prats, 1982). Steam injection can also be used to recover light crudes, with an efficiency that approaches under certain conditions that of waterflooding Naturally fractured reservoirs are estimated to contain 25-30% of the world supply of deposits (Saidi, 1988). Although of potentially high importance, the application of steam injection in fractured systems, however, is not widely applied.
There exist some obvious differences between displacements in homogeneous and in fractured media. Unlike homogeneous reservoirs, in fractured systems the injected steam does not necessarily sweep uniformly the porous media, but may preferentially flow through the high-conductivity fractures. Furthermore, the mechanism of heating in fractured reservoirs is mostly by conduction, while in a homogeneous system heat is transferred mainly by convection. Therefore, steam injection recovery mechanisms can be quite different in fractured media. Experimental and numerical studies to understand steam injection in fractured systems have been conducted by various investigators. Below, we summarize the most important findings.
Reis (1990) provided a comprehensive review of oil recovery mechanisms in steam injection in fractured reservoirs. Thermal expansion and gas generation were described as two key mechanisms. In the first, the expansion of fluid and rock minerals at high temperatures result in the expulsion of fluids from the matrix to the fracture. A differential thermal expansion of the order of 0.05%/ F was estimated by the combination of porosity reduction and fluid expansion. P. 67