Static and Dynamic Elastic Properties of Two Sandstones at Permafrost Temperatures

Abstract

In tests on water-saturated samples of Berea and Boise sandstone there were large increases in static elastic moduli and in ultrasonic velocities when the samples were cooled below 32F. At the same time there were also small increases in pore volume. Introduction Permafrost, or permanently frozen ground, covers Permafrost, or permanently frozen ground, covers about 20 percent of the earth's dry land and extends over large areas of Canada and Alaska. As these areas become more and more important for their vast natural resources, the study of the physical properties and behavior of frozen ground becomes very important in exploration for petroleum and other minerals, in the design of mine shafts and foundations for large structures, and in the construction of roads. The freezing process has little or no effect on such physical properties as density, magnetism, or physical properties as density, magnetism, or radioactivity, but it radically changes the elastic moduli and electrical properties of saturated rocks. Acoustic-wave propagation, therefore, provides a potentially important technique in studying the physical properties of frozen rocks and soils. Acoustic-wave properties of frozen rocks and soils. Acoustic-wave velocities in frozen rocks have been measured in the field by acoustic well-logging techniques and seismic surveys, and in the laboratory by the ultrasonic-pulse technique. Strain-gauge measurements and the ultrasonic-pulse technique have been used for the experiments described here to obtain the static and dynamic elastic properties of two sandstones as an initial basis for comparing them with an interpretation of available field data. Apart from test on sand and clay carried out by Miller, the only published results of laboratory measurements of the ultrasonic-wave velocities at permafrost temperatures are by Desai and Moore and permafrost temperatures are by Desai and Moore and Timur. Desai and Moore performed laboratory tests and reported measurements of compressional-wave velocities in Berea sandstone at permafrost temperatures. Testing this sandstone fully saturated with 15,000 ppm brine and subjected to zero axial stress, they observed an 83 percent increase in velocity due to freezing, but only a very slight increase in velocity in dry and oil-saturated samples. They attributed most of this increase to the pressure of ice in the pore space expanding upon the rock matrix. Similar results were reported by Timur who measured the compressional-wave velocities at permafrost temperatures on nine different rock types, varying from Boise and Berea sandstones to black siliceous shale and porous porcelain. His tests showed only a very slight increase of I percent in the velocity of compressional waves in dry Berea sandstone on freezing; however, for fully water-saturated Berea and Boise sandstones, the tests showed very sharp increases of 33 percent and 51 percent at axial stresses of 4,600 psi and 1,000 psi, respectively. He concluded that the increase in velocity of compressional waves takes place when, after the temperature is decreased below the freezing point, a temperature is reached where the surface-to-volume ratio in the remaining pore space is large enough to affect the freezing process and hence to change the shape of the velocity-vs-temperature curve. JPT P. 495