Performance Of Horizontal-Vertical Combinations For Steamflooding Bottom Water Formations

Abstract

Abstract In Alberta and Saskatchewan, many heavy oil formations have a communicating high water saturation zone, which makes the use of conventional oil recovery methods unattractive. Often, attempts to produce such marginal oil reservoirs result in excessive water production due to coning and steam migration into the bottom water zone. Waterflooding is ineffective because of the high mobility ratio of water to heavy oil. This paper discusses recovery strategies and reservoir situations under which a steamflood can be conducted efficiently. A heavy oil reservoir in Saskatchewan was selected as the prototype for the scaled model steamflooding experiments. Four combinations of vertical and horizontal wells (injectors and producers) and two bottom water thicknesses were used. In one series of runs, the use of a small volume of solvent with steam was examined, with a view to divert steam early from the water zone. The results showed that the horizontal injector-horizontal producer combination recovered 68% of the oil in the presence of 10% thickness of high water saturation. In the absence of bottom water, the horizontal combination was only slightly better than vertical wells. With thicker bottom water zones (50% of model thickness), the oil recovery fell about 20% in all four cases. A 10% PV solvent slug injected ahead of steam improved oil recovery by only 5%. Guidelines are offered for steamflooding similar heavy oil reservoirs. Introduction Many heavy oil formations in the Lloydminster area of Saskatchewan and in eastern Alberta, classified as "marginal reservoirs" are becoming the focus of the current technology. Marginal reservoirs are characterized by a relatively thin pay (6 m to 13 m) with a high water saturation zone beneath the oil zone. The water zone, which is often in communication with the oil zone, can be either a transition zone or else a zone of high water saturation that can approach 100%. It may even be an active water aquifer. While low recoveries and poor sweep are attributed to the tendency of injected fluid to migrate through the path of least resistance via the bottom water zone, it is believed that the bottom water zone can also serve as a means for providing initial injectivity for a very viscous oil. A scaled physical model of the Aberfeldy heavy oil reservoir, in Lloydminster, was constructed according to the low-pressure scaling criteria developed by Stegemeier, Laumbach and Volek(1). These criteria were extended to include the case of solvent injection(2). Steamfloods incorporating various vertical and horizontal well combinations were conducted in the low-pressure model in order to determine optimum production strategies for bottom water reservoirs. Homogeneous (no bottom water), thin bottom water (10%), and thick bottom water (50%) cases were examined. In addition, the effect of solvent injection prior to steam injection in a thin bottom water formation was also investigated as an adjunct to the study. This paper discusses the results of the above experiments, suggesting strategies which may permit the economic production of a marginal heavy oil reservoir.