Abstract

The application of miscible CO2 flooding for enhanced oil recovery in a vuggy/fractured carbonate formation has found commercial success in the Weyburn reservoir (Saskatchewan). The Weyburn waterflood performance indicated that flow could be classified as matrix flow, with certain sections of the reservoir dominated by fracture flow. The physical mechanisms that lead to improved miscible flood recovery in fracture-dominated flow are only partially understood. To highlight the different recovery mechanisms, coreflood tests in homogeneous, matrix cores and artificially fractured limestone cores were conducted. For some of these miscible CO2 displacement tests, the in situ oil saturations were continuously monitored using a magnetic resonance imaging (MRI) technique. Image analysis demonstrated how channeling, gravity segregation, and partial displacement led to contrasting recoveries in matrix and fractured cores. Additional improvement in oil recovery was obtained by implementing conformance control methods such as foams, gels, and gel-foams. Injecting blocking and diverting gels into the fractured cores proved to be the most effective means of conformance control, providing improved sweep efficiency and resulting in accelerated oil production during subsequent CO2 injection.

Introduction

In the Weyburn, Midale reservoir, two oil-bearing intervals, the Vuggy and the Marly, have different fracture characteristics, extents of water invasion and degrees of heterogeneity1,2. Since this reservoir is characterized by trending fractures and alternating layers of high and low permeability, conformance control of the injected CO2 is crucial in optimizing the oil recovery and enhancing the storage capacity of the CO2. Once miscibility has been achieved, in situ, between the injected CO2 and the displaced oil then conformance control is the key factor in conducting an economic miscible displacement flood.

Two forms of conformance control have already been implemented in the Weyburn reservoir. Injecting the CO2 in water alternating gas (WAG3,4) mode controls the mobility of the gas between more viscous slugs of water. A mechanical form of conformance control has been achieved by placing the horizontal injection and production wells such that the overall fluid flow is in the off-trend fracture direction. But even in the off-trend direction, fractures with generally smaller apertures can be found. Thus CO2 channeling between injector and producer could occur. Additional conformance control measures for the CO2 injection project need to be implemented in order to achieve maximum oil recovery efficiency. The overall scope of this study was to evaluate the performance of commercially available conformance control technologies and move the technology from laboratory tests to field application. These technologies were all based on chemical formulations, which can be categorized into mobility control agents (foams5,6,7), blocking and diverting agents (gel8,9,10), and hybrids thereof (gel-foams11,12,13).

The first objective in this research project was to screen and optimize conformance control formulations for the three chemical conformance technologies. The impact of the optimized formulations for CO2 conformance control was then tested in coreflood experiments. Homogeneous carbonate packs were used as the porous medium to conduct early coreflood studies. More rigorous testing of the conformance products was conducted in artificia

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