Application of a New Shaly Sand Model for Interpreting Resistivity
and Dielectric Log Measurements
Olivar A. L. de Lima and Carmen L. R. Dalcin: Federal University of
Bahia, Brazil
Abstract: The use of resistivity and high-frequency dielectric logs for the petrophysical interpretation of shaly sands is discussed on the basis of a simplified membrane polarization model presented by Lima and Sharma (1991, 1992). According to this model, the clays were treated as spherical charged wet particles agglomerated as continuous coatings over the sand grains. The electrical behavior of shaly sands is described using simple total current conductivity functions for clay and clay-coated particles, in connection with self-similar mixture equations. The effective bulk conductivity and permittivity of granular rocks containing clays are shown to be dependent on a clay parameter b1/a (b1 being the charge density on the clay particle of radius a), on the clay or shale volume content p, on the salinity and amount of the clay water and on that of the free pore electrolyte, and on the water saturation level in the rock. Here, we derive the limits of these electrical properties at very low and very high frequencies and demonstrate they can be used to compute from resistivity, dielectric, and porosity log measurements, both the clay type and its relative content, as well as the water saturation within shaly sand oil reservoirs. The resulting logging interpretation procedures are satisfactorily applied to log data from oil wells in the Rec"ncavo and Potiguar basins, northeastern Brazil.
The Characterization of Reservoir Rocks Using Nuclear Logging Tools:
Evaluation of Mineral Transform Techniques in the Laboratory and Log Environments
J. C. Lofts, P. K. Harvey, M. A. Lovell: University of Leicester, United
Kingdom
Abstract: New techniques transform elemental oxides, derived from nuclear logging measurements, into a set of geologically viable mineral phases (mineral modes). Some of these different mineral transform methods are evaluated using a set of well-constrained laboratory measurements on North Sea reservoir core and synthetically produced rock samples. Mineral proportions derived from each transform model are compared with those from conventional petrographic techniques to evaluate and constrain each transform method.
Emphasis has been placed on evaluating transform techniques using well constrained laboratory data before their application to logging tool measurements (such as elemental data derived from spectral logging tools). Applying correct mineral transforms to the logging environment will lead towards a continuous, accurate measurement of mineralogy downhole, especially with the introduction of higher resolution logging tools.
The resulting mineralogy logs are potentially valuable on their own, especially for the quantification of clay minerals, but they can also be used to determine other formation descriptors such as grain density, porosity, and cation exchange capacity (CEC). In wells where core recovery is poor, they can help extend formation evaluation across these poorly defined zones, thus increasing the reservoir characterization available for exploration and appraisal.
Particle Filtration in Sandstone Cores: A Novel Application of Chemical
Shift Magnetic Resonance Imaging Techniques
Christian Straley, Dan Rossini, and Lawrence M. Schwartz: Schlumberger-Doll
Research Michael E. Stromski, Mirko Hrovat, and Samuel Patz: Radiology
Department, Brigham and Womens Hospital, Harvard Medical School
Abstract: Recent developments have led to increased interest in the application of borehole nuclear magnetic resonance (NMR) as a probe of petrophysical properties. Of particular importance in this connection is the measurement of the longitudinal relaxation time, T1. As T1 is to a large extent controlled by the effective pore surface area, its value may be strongly influenced by the invasion of submicron clay particles found in drilling muds. We study this effect by applying magnetic resonance imaging (MRI) techniques. The resolution of conventional readout gradient MRI is limited in sandstone cores by very broad natural linewidths associated with internal magnetic field gradients. This problem can be overcome by applying a phase-encode (i.e., chemical shift) imaging sequence that decouples the phase shifts caused by internal field gradients from those caused by imaging gradients. The utility of this technique is illustrated by a study of the effect of bentonite invasion on T1 values in Berea sandstones. Our experimental results indicate that the extent to which T1 values are affected by particulate invasion depends on several characteristics of the drilling mud. In thinned spud muds (i.e., dispersed bentonite slurries), we see a region deep within the core where T1 values are significantly reduced because of an initial spurt of clay particles. In better formulated muds this effect is essentially eliminated. Our imaging studies are supported by measurements made using a laboratory "inside-out" NMR spectrometer that measures a T1 value at single depth, roughly 2 cm, below the in-flow core end.