The Effect Of Temperature And Pressure On Wave Velocities In Porous Rocks

The interpretation of sonic logs from deep, high temperature reservoirs is complicated by lack of data on the effects of temperature and pressure on sonic velocity and the relation of these effects to pore properties. In the present work, dilatational wave velocities were measured for several sandstones and were found to decrease by as much as ten percent when temperatures were raised from 68F to 392F. This could result in over-estimation of porosity by nearly one-third. A temperature and pressure correction chart for interpretation of sonic velocity data is proposed. Introduction Improvement in drilling techniques and the consequent drilling of more deep wells have given rise to problems associated with the high pressures and high temperatures encountered in deep wells. Temperatures approaching 400F and pressures exceeding 10,000 psi are becoming common, leading to problems with drilling fluids and with the running of logs. In addition, the behavior of subsurface formations becomes more complex at these extreme conditions. For correct interpretation of logs run under these conditions, knowledge is needed of the effect of temperature and pressure on the property being deduced from the log data. An example with which this paper deals is the effect of high pressures and temperatures on elastic wave velocities in rocks and the relation of these effects to sonic log interpretation. The literature contains theoretical analyses and test results which indicate that elastic wave velocities increase with increasing pressure. Test results show that the rate of increase in wave velocities is higher at lower pressures than at higher pressures. This increase is attributed to closing of microcracks within the rock and to increase in grain-to-grain contact area with pressure.