Abstract

In constrast to the experiments carried out on Pembina. Cardium cores discussed in Part I, these floods were conducted on Beaverhill Lake cores in both "circulating" and "once-through" modes. Beawrhill lake limestone cores were characterized by permeability gains and losses. Microscopically, carbonated brines followed am etched preferred paths, forming a braided network of variably sized tubules. Insoluble minerals such as quartz and illite bound to 'the matrix and mixed with drilling fines were released during the etching process. These migrated and, in some cases, became lodged in constrictions. This work suggests that carbonated. brines- can affect limestone reservoirs in a variety of ways which, from an EOR point of view, may or may not be beneficial.

Introduction

This paper is the second in a series dealing with alterations in reservoir rock properties that could occur during carbon dioxide flooding. Pembina Cardium sandstone cores were treated in the first paper,1 while the present report deals with Beaverhill Lake carbonate cores.

In the literature survey presented in Part 1 of this series,1 it was mentioned that carbonate minerals such as calcium, magnesium, and iron carbonates could dissolve in carbonated brines through the formation of soluble bicarbonates. Furthermore, a release in pressure could lead to the reprecipitation of the carbonates. Thus, it is expected that more extensive rock transformations will occur in the Beaverhill Lake cores treated in the present paper than in the Pembina Cardium cores discussed in Part 1.

The experimental apparatus and procedure were presented in detail in part 1. To briefly recapitulate, cores; were flooded with carbonated brine in two modes: once-through am circulating. In the once-through mode, fresh carbonated brine was continuously injected through the cores. This parallels the situation near the injection well in an oil reservoir. On the other hard, in the circulating mode the same brine was continuously recycled through the core. This procedure simulates what would occur in parts of the reservoir away the injector well wherein the brine flowing through the rock bas already been in contact with other portion of the reservoir.

Experimental conditions and results are presented in the next section, am conclusions drawn from the tests are then summarized. Additional details pertaining to this work may be found. in Reference 2.

Experimental Conditions

Eleven core flood were carried out in total on Beaverhill Lake cores. The brine used for these floods was a 5 wt % NaCl solution in deionized water, carbonated at about 14 MPa and the test temperature (25 or 45 °C). Information on these floods is summarized in Table 1. The first six runs (Cores BL1 to BL6) were carried out in the once-through mode, while the remaining five (Cores BL7 to BL11) were performed by continuously circulating carbonated brine through the core. The once-through floods were carried out at: 22 °C since the carbonated brine reservoir was placed outside the thermostated oven, hence was maintained at roan temperature.

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