Steaming and Operating Strategies at a Midlife CSS Operation

Abstract

Abstract Imperial Oil Resources Limited began pilot experimentation with Cyclic Steam Stimulation (CSS) at Cold Lake in 1964 and followed this with commercial application in 1985. Since that time an extensive amount of experimental data and operating experience has been acquired which has resulted in the continual evolution of steaming and operating strategies for CSS. Optimization of the many process variables involved in CSS is complex due to their interdependency. The optimization of steaming and operating strategies has advanced significantly with commercial development and continues to evolve as the CSS process matures and the reservoir reaches more advanced stages of depletion. This evolution has focussed on steam strategies to manage interwell communication and increase areal conformance, and operating strategies to maximize the efficiency of producing mobilized bitumen which lies at ever increasing distances from the wellbore. The continued success of CSS in Cold Lake is dependent on the continued improvement of these steaming and operating strategies. Introduction From research begun in the early 1960's, it was determined that some type of steam stimulation was the best suited recovery method for Cold Lake. The CSS process, in which steam is cyclically injected into the reservoir to reduce the viscosity of the bitumen, has proved to be the most successful and this process has undergone extensive piloting at Cold Lake since 1964. Many different well spacings and configurations, completion strategies and operating parameters were tested. This effort led to the commercial application of this technology in 1985. Despite the extensive piloting work, there was still much to be learned regarding CSS strategies in a large scale commercial operation and there has been significant development since then. Operating CSS in Cold Lake offers some particularly difficult challenges. In order to inject steam, pressures high enough to mechanically fail the formation must be achieved, resulting in complex fracturing and reservoir deformation behaviour. A high degree of interwell communication occurs when steam is injected, further complicating steam conformance and well behaviour. Complex reservoir heterogeneities limit fluid flow in the reservoir and equally complex geochemistry leads to reactions whose products can further inhibit fluid flow. The initial reservoir condition is a relatively simple two phase system of water and methane saturated bitumen. However, as mean formation temperature increases during successive steam cycles and as repeated changes from high to low pressures occur during each production cycle, the reservoir phase behaviour becomes increasingly more complicated. Both water and hydrocarbon gas continuously transfer between the liquid and vapour phases and fluid densities are constantly changing in response to temperature and pressure changes. The chemical reactions between injected steam and the reservoir rock generate significant volumes of carbon dioxide in the reservoir. The flow of fluids through the reservoir is further complicated by the dynamics of water, oil and gas emulsification and the inherent vagaries of interwell communication. P. 183^