Abstract

Over 20 billion barrels of oil are contained in the marginal heavy oil reservoirs of Saskatchewan and Alberta. These marginal reservoirs have high porosity (∼30%). high permeability (1–5 darcies), and the oil viscosity ranges between 1000 to 12000 cp; but these reservoirs are also thin (3–7 meters of pay) and often in communication with an underlying water zone. Primary recovery in these reservoirs typically amounts to less than 10% of the initial oil in place (IOIP). Cyclic steam stimulation and steam flooding operations using vertical wells have proven uneconomic in these reservoirs, mainly due to excessive channeling of steam. On the other hand, horizontal wells have been shown to be capable of improving steam flood recovery performance, due to their extended contact with the reservoir and their ability to improve sweep efficiency (due to a delay in steam override, and minimizing effect of heat loss to the cap/base rock).

Using a scaled physical model of the Aberfeldy reservoir On Saskatchewan), experiments were performed to investigate steam flood recovery performance, using horizontal wells, for several different types of reservoirs, including those with a 20% net pay bottom-water. The experimental study looks at how different recovery strategies such as shut-in, cross-pattern flooding, and reservoir factors such as thickness of bottom-water layer, and dip affect the recovery process. Results from these experiments are scaled up, with the use of scaling criteria, to predict field performance.

Experimental data for Runs 5 and 6 showed that long shut-in period, during steam flooding operations, is undesirable as the water oil ratio (WOR) increased by one order of magnitude. With an increasing WOR, more heat is required, upon re-injection, to promote the formation of the steam zone. The amount of heat accumulated in the reservoir, after 0.7 PV of steam had been injected and 1.0 PV of shut-in time, was approximately 47% of the heat injected prior to shut-in. Runs 9, 11 and 13 were completed to study the effect of steamflooding reservoirs having a 20% bottom-water layer. Experimental data showed that large amount of heat is stored in the bottom-water layer; hence, incremental oil recovery for such a reservoir depends on the management of heat contained in the fluids. Steamflooding strategies used in Run 9 increased the recovery efficiency as compared to the other two runs. In this process, attempts were made to first heat the bottom-water layer. As heat was conducted into the oil zone, the mobility became more favorable leading to higher oil recovery.

Introduction

Primary recovery for the marginal heavy oil reservoirs in Saskatchewan and Alberta typically amounts to less than 10% of the IOIP. In a few cases, the use of horizontal wells has increased primary recovery to 20% prior to water coning. This still leaves up to 80% of the 10lP unrecovered. Conventional steamflooding methods, such as cyclic steam stimulation, have proven uneconomic in these reservoirs; therefore, different recovery strategies must be developed to efficiently recover the amount of oil remaining in these reservoirs. Hence, a thorough understanding of various recovery mechanisms is required.

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