Cold Lake Cyclic Steam Stimulation: A Multiwell Process

Abstract

Summary Cyclic steam stimulation (CSS) typically is thought of as a single-well process. At Cold Lake, however, where steam injectivity is achieved by fracturing the formation, considerable interwell communication is observed. The result is usually the watering out of a producer by condensed steam from a neighboring injector. These interwell interactions greatly complicate sam-injection scheduling for commercial projects involving hundreds of wells but do not seem to reduce bitumen production in early cycles. Field experience indicates that steaming rows of wells sequentially with 50% overlap in injection time between adjacent rows significantly reduces water transfer between wells, increases the conformance of the injected heat, and reduces the field's tendency to form communicating well couplets. Exploratory numerical simulations show that the impact of steaming strategy on bitumen production is not significant until later cycles. Introduction CSS is a complex recovery process composed of a variety of recovery mechanisms whose relative importance changes with cycle number. For the Cold Lake reservoir, where steam injectivity is achieved by fracturing the formation, modeling the process is particularly challenging. The basis for investment decisions for commercial expansion, therefore, has been largely empirical. Even so, efforts to understand the process, and in particular to delineate the recovery mechanisms, are continuing. CSS generally has been modeled as a single-well process. The injected heat and fluids are envisioned to remain in the vicinity of the wellbore, lowering bitumen viscosity and increasing reservoir pressure. During the production phase, the increased reservoir pressure, along with gravity, drives heated bitumen to the wellbore. No-flow boundary conditions are assumed to encircle the spacing area of the well. Conceptually, such a model is correct for small steam-stimulation volumes. As the steam-stimulation volumes increase, however, the disturbances associated with heat injection become less localized; for very large volumes associated with continuous injection, the process evolves into a multiwell steamflood process. Thus, a continuum exists from single-well CSS to multi-well displacement. At the Leming pilot at Cold Lake, substantial single-well CSS behavior has been observed and successfully modeled for well patterns with large aspect ratios. CSS is not a single-well process. however, for commercial projects with steam-stimulation volumes of 8000 m3 [50,000 bbl] or more and wells on ha [4-acre] spacing with aspect ratios of 1.7 For example, a fraction of the fluids injected in a well may not be produced by the same well but rather by neighboring wells on production, indicating that elements of displacement also exist in the process. Interwell interactions during CSS at the Cold Lake area have been reported. This interwell interference or "communication" affects the production profiles dramatically. Thus, the total performance of the reservoir is not merely the summation of isolated wells whose performance is independent of the steaming sequence. This paper gives examples of the interwell communication observed with CSS at Cold Lake and introduces terminology useful for quantifying the process. Approaches to inferring the properties of the interwell communication path from surface measurements are outlined, and the practical limitations in their use noted. The paper then discusses a steaming strategy to reduce the degree of fluid breakthrough and to increase the reservoir conformance of the injected heat. Finally, results from numerical simulations of the multiwell CSS process are presented and compared with field experience.