The influence of pore pressure on rock-fluid interaction

Abstract

The experimental work in this thesis was performed at the chalk laboratory in University of Stavanger. The main objective is to determine the overall influence of pore pressure on the mechanical strength of chalk in the presence simple brines, mainly MgCl2 and NaCl with the same ionic strength as in seawater. The tests were carried out at both high and low pore pressures of 40MPa and 0.7MPa respectively. The temperature used during the performance of all the tests was 130oC. The chalk samples used in the experiments were from Stevns Klint near Copenhagen in Denmark. Generally the chalk has very high porosity and low content of silica, which confers on it a low mechanical strength. Each of the samples tested at high pore pressures undergoes four main stages while those carried out at low pressures have three phases. The first phase involves initial building of confining and pore pressures to 1.4MPa and 0.7MPa respectively while cleaning the samples with distilled water. Ramping of confining and pore pressures simultaneously to 41MPa and 40MPa respectively, followed by brine injection is carried out in the second phase. The third phase is hydrostatic loading of the samples to an effective stress of 12MPa, followed by the creep phase and sampling of effluents of the flooded brine. The last stage entails chemical analyses of the fractioned effluents using Ion Chromatography machine. Tests performed at low pressures does not involve the second phase of pressure ramping. Several problems were encountered during the tests because of the high pressure and high temperature conditions of the tests. Among the tests performed, 6 were accepted as successful, 2 were partially successful while 11 were adjudged as unsuccessful. Results from the experiments reveal that chalk cores flooded with NaCl are mechanically weaker than those injected with MgCl2 at both high and low pressures. Suggested possible reason for the difference in mechanical strength was that there was dissolution of the chalk and a subsequent precipitation of minerals for cores flooded with MgCl2. The precipitates tend to increase cementation and friction between the chalk grains, making them mechanically stronger. For cores injected with NaCl it was believed that precipitates were not formed which resulted in enhanced compaction taking place. In addition, “accelerating-like” creep was observed on chalk cores exposed to MgCl2 at high pressures but no such creep trend was observed on the rest samples subjected to other test conditions. Accelerating creep has not been reported in previous experiments on chalk.