Abstract

When carbon dioxide is injected into petroleum reservoirs it forms carbonic acid in the brine phase and interacts with reservoir rock. Flow tests were performed by continuously circulating CO2 saturated 'brines through Cardium Formation cores. All the cores showed initially a large drop in permeability, after which it rose steadily but did not regain its initial value. Microscopic examination of the cores indicated that fines had been released which had migrated towards pore throats reducing permeability. In addition, mineral transformations had taken place, including the dissolution of calcite and siderite, which may account for the gradual rise 1n permeabilities noted in the experimental.

Introduction

When carbon dioxide is injected into petroleum reservoirs it partitions between the 0il, brine, and gas phase, thereby forming carbonic acid in the brine phrase. As a result of this, the brine's pH is lowered, and it will contain carbonate and bicarbonate ions, as well as undissociated CO2. The work presented in this paper is primarily concerned with the effects of this mechanism an the properties of reservoir rocks and consequently an. the enhanced oil recovery process.

This report is subdivided into four sections. The next section contains a survey of previous relevant works presented in the literature. Following that is a description of the experimental procedure used to flood Pembina Cardium cores with carbonated brines. The results of these tests are then presented, and finally, conclusions drawn from this work are discussed.

A campanion paper1 contains a description of a similar study an Beaverhill Lake carbonate cores. Additional details of the Pembina cardium and Beaverhill Lake core studies may be found in Reference 2.

Literature Review

Sarcistone reservoir rocks, in general, contain siliceous material, clays, and various carbonates. The latter are predominantly of calcium, although iron and magnesium carbonates are common too. These minerals are present in various proportions in different rocks. and react differently to the charging environment brought about by the injection of CO2. Thus, the net effect of this process depends on the type of rock, the fluids being injected, the injection rates and the reservoir conditions.

Silica. is quite inert to CO2 or to carbonated brines at normal reservoir temperature, as a very strong acid such as hydrofluoric acid is required to dissove quartz-rich sandstone. However, some siliceous minerals such as iron chlorite are unstable in acidic environments and become water soluble. On the other hand, alkaline materials at pH > 9 react readily with silica3 For example, sodium hydroxide reacts with silica to form sodium silicate, which is a water soluble compound, and which forms a gel if the medium is acidified. At higher temperatures, typical of steam flood conditions, silica can react with water to form the soluble silicic acid (H4 SIO4) as discussed by Stone et al.4

Carbonate minerals, such as calcium and magnesium carbonate, are the major mineral constituents of carbonate reservoirs, but are less dominant constituents of sandstone reservoirs. It is well established that these carbonates react readily with carbonated brines.

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