Abstract

Conoco's FAST (Fracture Assisted Steamflood Technology) process utilizes high rates of steam injection through a parted horizontal fracture to achieve efficient recovery of heavy oil or tar. This paper describes the numerical simulation of hydrocarbon solvent injection with steam in a FAST tar sands steamflood. The numerical model was the same used to achieve an accurate history match of the Saner Ranch South Texas tar sand pilot of the FAST process. The effects of solvent concentration in the injected steam, solvent composition, and total injected solvent volume on FAST process tar recovery and thermal efficiency are described. A cursory economic analysis of this technology is provided.

The numerical results show that injection of small volumes of a volatile solvent with steam in a FAST process tar sand steamflood can nearly halve the steam-to-oil ratio. Most of the injected solvent is recovered with the produced tar. The incremental tar recovery is at least ~ barrels per barrel of solvent lost to the reservoir. In specific cases, the benefits of lower steam-to-oil ratio far outweigh the cost of solvent retained in the reservoir.

Studies of solvent co-injection in conventional steamfloods have previously been reported in the literature. These studies have generally shown solvent injection to have a relatively modest effect on steamflood performance. The results presented here show that solvent co-injection has a pronounced effect on FAST process performance. This 1s explained 1n terms or the recovery mechanisms at work in the FAST process.

Introduction

The co-injection of a volatile or gaseous solvent with steam in conventional steamflood, steam stimulation, and steam slug heavy oil recovery processes has been the subject of both numerical modeling and laboratory studies.1,2,3,4,.5 The effect of solvent co-injection on thermal recovery process performance is dependent on the dominant oil recovery mechanism. For example, solvent injection generally has a modest effect on cyclic steam stimulation performance, but was shown to significantly improve heavy oil production in a steam slug process. In the present study, numerical simulation was used to determine the result of hydrocarbon solvent injection with steam in a FAST (Fracture Assisted Steamflood Technology) process tar sands steamflood. The objective was to provide an order-of-magnitude estimate of the potential incremental benefits of solvent injection and to elucidate the basic recovery mechanisms at work in the solvent enhanced FAST process. The effects of solvent concentration in the injected steam, solvent composition, end total injected solvent volume on FAST process tar recovery are described. A cursory economic analysis is provided to indicate the conditions under which solvent co-injection may be profitable.

THE FAST PROCESS

The FAST process6,7 for heavy oil and tar recovery was developed and 'tested by Conoco in two pilot operations in south Texas tar sands beginning in 1977. 8,10 Both FAST process pilots were technical successes. At the Street Ranch pilot, 66 percent of the QTIP was recovered at a steam-to-oil ratio of 10.9.

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