Log-Derived Stress in Anisotropic Formations
William P. Iverson Subsurface Engineering, Seattle, Washington

Abstract: The prediction of hydraulic fracture growth is controlled in part by the subsurface confining stress. Previous work is based upon a simple relationship between vertical overburden stress and horizontal stress as a function of Poisson’s ratio. Field measurements of closure stress have failed to consistently verify the relationship between vertical and horizontal stress, and therefore additional calibrations are required. Alternatively, consideration of anisotropic Poisson’s ratio results in a different relationship between vertical and horizontal stress. A portion of the observed difference between theoretical and calculated closure stresses is probably due to such an anisotropic effect. The azimuthal direction of induced hydraulic fracture growth is controlled by anisotropy in both tensile strength and Poisson’s ratio. Field measurements of closure stress do not necessarily determine the minimum horizontal stress in anisotropic rocks. Geophysical measurements of anisotropy can greatly affect the engineering of hydraulic fracture treatments.

Electrical Conductivities in Oil-Bearing Shaly Sand Accurately Described with the SATORI Saturation Model
A. de Kuijper, R. K. J. Sandor, J. P. Hofman, J. M. V. A. Koelman, P. Hofstra, and J. A. de Waal Shell International Exploration and Production B.V., Research and Technology Services, Rijswijk (ZH), The Netherlands

Abstract: We present a new class of saturation models based on a symmetrical anisotropic theory of resistivity interpretation (SATORI) that will lead to more accurate hydrocarbon saturation predictions. With SATORI models, the bulk electrical properties of rock can be accurately calculated from the volume fraction, shape, conductivity, and connectivity of the minerals and fluids in the rock. The models allow components to remain connected down to low volume fractions and can handle anisotropy in shape and conductivity of the various constituents. There are no ad hoc parameters, and the calculated conductivity is independent of mixing order. As a first implementation, we narrowed the general SATORI framework to describe the low-frequency electrical behaviour of isotropic shaly sands. Honouring the underlying electrochemistry, we obtained a shaly-sand model with three independent parameters: shale conductivity, shale volume, and pore-space connectivity. We tested this SATORI shaly-sand model against available core data. In all cases, the electrical conductivity and membrane potentials were calculated with a maximum error of a few percent. This is a major improvement, in particular at low salinities and for high shale fractions. Present computations suggest that hydrocarbon saturations calculated on the basis of SATORI can differ by as much as 5 to 10 saturation percent from those computed on the basis of existing empirical models. SATORI can be incorporated into a saturation model of which the input consists of basic petrophysical measurements; additional data are not required. The parameters in the model are derived from inversion on the measured data.

Rw Catalogs

Editors Note: This listing of Rw catalogs is an update to the listing in the 1996 January-February issue of The Log Analyst (v. 37, no. 5, p. 1). One new catalog has been added. No attempt has been made to verify these are all still in print. If you know of any more catalogs, please let me know and we will add them to the list.