Thermo-Mechanical Behavior Of Porous Rocks

Abstract

Publication Rights Reserved Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and considered for publication in one of the two SPE magazines with the paper. Abstract Earlier work on the thermal expansion of rocks has shown that a large amount of permanent deformation may exist upon cooling the test samples to room temperature. In the present paper, the changes in flow and mechanical properties of porous rocks as a result of heating, are presented and discussed. Three typical sandstones were heated to several different temperatures, cooled to room temperature, and then changes in physical properties were measured. Increases in bulk volume were comparatively small [of the order of 3 per cent], but the resultant changes in porosity and permeability were substantial. Porosities were increased by as much as 20 per cent and permeabilities by over 60 per cent, depending upon the maximum heating temperature. Ultimate compressive strengths were decreased to one-half of original values on heating to 1,000 degrees C. Similar reductions in Young's moduli and sonic velocities were also noted. Changes in physical properties of rocks subjected to high temperatures may be explained as due to differential thermal expansion of constituent mineral grains, dehydration of clays, and dissociation of certain minerals such as dolomite and calcite. The present heating tests were run at atmospheric pressure; under reservoir pressure conditions, the latter two reactions would be suppressed. Nevertheless, changes in properties were considered to be of sufficient magnitude to warrant additional tests under simulated reservoir conditions. Introduction The application of high temperatures to subsurface formations has become important in bottomhole heating and in-situ combustion of petroleum reservoirs. In planned applications of underground nuclear explosions, rocks will be subjected to extreme temperatures locally and to moderately high temperatures at considerable distances from the explosion center. Rock drilling methods utilizing high temperatures have been proposed. It is probable that localized high temperatures are generated by conventional rock drilling methods. Physical properties of rocks may be altered considerably by heating. In earlier work the permanent deformation of rocks resulting from application of high temperatures were reported. Major structural damage of rocks may occur due to differential thermal expansion of mineral constituents. Mineral alterations may also contribute to changes in the physical structure of the rock. In the present work, samples of three typical sandstones were heated to several temperatures up to a maximum of 1,000 degrees C. After cooling, physical properties of the samples were measured and compared with values obtained on identical tests before heating. Measurements included linear expansion, bulk-volume expansion, porosity and permeability.