An Overview of Recent Logging Research at The University of Texas Petroleum Engineering Department

Logging research at The University of Texas has been carried out in several areas. We have studied how rock resistivity varies with water saturation when other variables, such as rock wettability, stress, saturation history, and shale content are varied. Both experimental and theoretical work have been done. Rock wettability (oil or water wet) has by far the largest effect. Shale content and saturation history are also important. Rock stress is the least important, at least in the Berea sandstones and glass bed packs we have studied. We have published several papers and theses which describe this work in detail. We have also studied the effect of certain trace elements (boron, mainly) on the neutron log. Boron has a very high thermal neutron capture cross Section. Analyses of a number of Frio formation cores from the Texas Gulf Coast area show that boron occurs frequently in these rocks in amounts (up to 100 ppm or more) that would seriously affect several procedures in neutron log interpretation. It could, for example, reduce or even eliminate the neutron log--density log porosity reversal that is commonly used as a gas indicator. A recent paper reports details of our work in the Frio. We are currently extending our trace element studies to the West Texas area. The lower porosities occurring there make a given boron content in the rock even more important than in the higher porosity Gulf Coast area. Another effort has been the application of logging data to obtain better estimates of rock type, and inferentially, rock permeability. The work is semi-empirical, and results are usually limited in application geographically. These results can be very important, however; for example, when extrapolating core data to parts of a field where no cores were taken. Several papers have reported details of this work, which is continuing. Finally, we have advised the Department of Energy on logging programs and problems in the Geothermal-Geopressured research effort. In the early phases of the work we studied why water resistivities calculated from the self potential log were so often wrong. and showed that most of the error came from use of incorrect mud filtrate resistivities in the calculation. Several papers and theses give details of this research. A recent problem has been the estimation of free gas saturation in a deep, thick, geopressured sand in the Hulin Well in South Louisiana. A pulsed neutron log run recently in this well supports free gas indications from open hole resistivity logs run in this well earlier by Superior Oil Company. The presence of free gas here is important, since the gas/water ratio from this zone will be much larger if free gas is present. Future logging research under consideration or being started at The University of Texas includes (1) petrophysical problems in tight gas sands (bimodal porosity systems, trace element effects, etc.), (2) resistivity versus water saturation relations at high desaturation pressures (1500 psi), and (3) improved theoretical and computer modeling of rock resistivity as affected by water saturation, rock wettability, saturation history, etc. Support of logging research at The University of Texas is currently from the Department of Energy and from a consortium of companies and organizations (Gas Research Institute, logging companies, and oil companies)