No abstract

In trying to understand the affect of electrokinetics on the spontaneous potential (SP) log, the focus has generally been on the solid‐brine streaming potential. Within the accuracy of the measurements, the streaming‐potential coupling coefficient is shown to be independent of the permeability of the rock. The solid‐brine streaming potential is of much smaller magnitude than the electrostatic potentials from gas‐liquid and liquid‐liquid flow. Air bubbles were found to increase the streaming potential coupling coefficient by more than two orders of magnitude over the value for single‐phase brine flow. Thus, two‐phase gas‐liquid flow is more likely to have a significant impact on the SP log than is single phase liquid flow. Two‐phase oil‐brine flow may also produce a larger electrokinetic potential than single‐phase flow. The magnitude of the electrokinetic potential caused by oil‐brine flow will depend on the composition of the oil and the brine. Trace materials can have a major impact on the electrokinetic potential of hydrocarbons. In a system with multiphase flow, the solid‐liquid interaction is probably the smallest component of the electrokinetic potential.

The concept of pressure solution, that a stressed portion of a solid is more soluble than an unstressed portion, is not new, but the relationship between stress and solution has not been experimentally established. The solution rate was examined around a fluid-filled circular hole in a slab loaded by ao perpendicular to the axis of the hole. The known circumferential stress around the hole varied from a tension of -a0 to a compression of +3a0. A variety of geologic materials, including single-crystal halite, limestone, dolomitic limestone, marble, quartzite, and novaculite, was used. Although the details of the solution kinetics for these materials differed considerably, and the pore fluid, sample material, axial load, and temperature differed from experiment to experiment, the same result was obtained whenever the solution rate was limited by the dissociation of the mineral at the solid-liquid interface. The shape of the hole after solution occurred was approximately an ellipse and differed markedly from the shape that it would have attained if the rate of solution were proportional to strain energy density. The solution rate at a point seems to be directly proportional to local stress.

Quartz grains in granitic rocks usually have blue cathodoluminescence (CL). Within the blue-luminescing grains, there are often red-luminescing domains which are frequently impossible to detect without CL contrast. This finding suggests that the red-luminescing quartz is sealing preexisting microcracks. The presence of these now-healed microcracks has important implications with respect to the role of pore fluid pressure and fluid transfer in metamorphism, the origin of granites, long-period crustal deformation, earthquake mechanics, physical properties of rocks, and deep-seated geothermal energy.

We present a data-analysis approach to determine Archie parameters m and n from standard resistivity measurements on core samples. The analysis method, core Archie-parameter estimation (CAPE), results in computed water saturations that agree well with core-measured saturations. CAPE determines m and n by minimizing the error between computed and measured water saturations. The conventional method minimizes the error in nonphysical quantities. Also, CAPE provides a natural, physically meaningful method of "averaging" Archie parameters, and with an error statistic, aids in zonation of a well or reservoir into different sets of Archie parameters. Finally, we show that the Archie constant a is a weak-fitting parameter, with no physical significance, that can generally be set to unity.