Thermal Alteration of Sandstones

Abstract

Abstract With the advent o underground heating operations, interest has developed in the alteration of rock properties by high-temperature treatment. In the present work a number of sandstones were heated to temperatures in the range of 400 to 800C under both atmospheric and simulated reservoir pressures. Permeabilities increased by at least 50 per cent and sonic velocities and breaking strengths decreased by an equivalent amount. Differential thermal expansion and other reactions of constituent mineral grains are the causes of these alterations. Introduction In the underground combustion of petroleum reservoirs. Temperatures of the order of 600C are reported to have been reached in the combustion zone. At this temperature rocks are subject to extensive thermal alteration. Temperatures of this magnitude and higher may also occur in subsurface formations when subjected to bottom-hole heating, thermal drilling operations, and underground nuclear explosions. Temperatures of this magnitude might also be generated by conventional rock drilling methods at points of bit-tooth contact. In earlier work, the permanent deformation of rocks resulting from heating was reported. Major structural damage of rocks occurs due to differential thermal expansion of mineral constituents. A number of mineral alterations including crystal versions, loss of water of crystallization and dissociation, may also contribute to changes in physical structure and properties of rock. In the present work, samples of three typical sandstones were had to several temperatures up to a maximum of 800C and then allowed to cool to room temperature. Heating was done under both atmospheric pressure and simulated reservoir pressure conditions. Physical properties of the samples were measured before and after healing and comparisons made. Measured properties included permeability, sonic velocity, breaking strength and fracture index. Changes in physical properties were compared to changes in mineralogical characteristics as determined by thin-section. X-ray diffraction and chemical tests. EQUIPMENT AND PROCEDURE Two outcrop sandstones (Bandera and Berea) and one sub-surface sandstone (St. Peter) were selected for the tests. These samples have a wide range in composition and physical properties as shown in Table 1.The first series of tests was made on 2-in. diameter by 5-in. long test specimens. Test specimens used in all later work were 3/4-in. diameter by 1 1/8-in. long. this being the specimen size required for heating at simulated reservoir pressures. After careful washing, the cores were oven dried at 100 +/- 5C for a minimum of 24 hours before the tests were run. Test specimens were heated in an electric furnace at a constant rate of temperature increase of 3C per minute. When maximum temperature of the run was reached, the sample was allowed to soak for one hour. The furnace was then cooled to room temperature at the same rate of 3C per minute. The entire heating operation was designed for reproducibility without subjecting the test specimens to excessive thermal shock. For samples heated under simulated reservoir pressures. a pressure cell designed by Dean was used (Fig. 1). The core sample was inserted into a thin-walled (0.006 to 0.01-in.) copper cup which was then mounted in a high- pressure cell. Provisions were made for the application of internal pore pressure as well as confining pressure. Tests showed that the thin-walled copper cup closed tightly around the core and satisfactorily transmitted confining pressure to the core. The core was heated by placing the entire cell into the electric furnace. The heating program was the same as that used in the atmospheric pressure runs: temperature rise of 3C per minute to maximum temperature of the test, soaking at maximum temperature for one hour. and cooling at a rate of approximately 3C per minute. The cell was designed to withstand 5,000 psi at 1,000C. However, since it was considered likely that repeated heating and cooling would in time weaken the steel, 2,000 psi at 850C was set as a working limit. In the present series of tests, the pore pressure was held constant at 750 psi and the confining pressure at 1,500 psi. The pressure source was a high-pressure nitrogen tank. The two pressures were controlled manually but are accurate well within +/- 50 psi. JPT P. 589ˆ