July-August 1999
Volume 40 No. 4

Ali A. Garrouch

ABSTRACT
For matching LWD and wireline resistivity logs, it has been customary to include the effects of several factors such as invasion, dip, anisotropy and dielectric constant. Experimental results were obtained by measuring the electrical resistivity of Berea sandstone, tight-gas-sand rocks and Ottawa sand-bentonite mixtures saturated with NaCl brine solutions at frequencies between 10 Hz to 10 MHz. These results indicate that rock resistivity becomes dispersive at frequencies above 0.1 MHz for the various conditions of salinity, wettability, clay content, and degree of rock consolidation investigated. Therefore, an additional correction for frequency dependence has to be added for matching LWD and wireline resistivity logs.
Similar conclusions have been reached by simulating shaly-sand resistivity using a generalized Hanai-Bruggeman model, and carbonate-rock resistivity using the Complex Refractive Index model. Supporting well log examples were obtained in impermeable non-dipping shales in two Conoco test wells. A systematic trend of resistivity decrease with increasing frequency has been observed in these test wells with all the electromagnetic instruments operating in the 20 KHz to 1 GHz frequency band.

T. S. Ramakrishnan, L. M. Schwartz, E. J. Fordham, W. E. Kenyon, and D. J. Wilkinson
 

ABSTRACT
In the conventional approach to the interpretation of nuclear magnetic resonance (NMR) measurements on water-saturated reservoir rocks, it is assumed that the T2 distribution and the pore size distribution are directly related. However, both laboratory and log data show that this relationship breaks down in many pore systems, especially carbonates, which consist of micro (intragranular) and macro (intergranular) porosity. This breakdown limits our ability to predict permeability and movable fluid fractions. Physically, it is due to the diffusion of magnetization between the intra- and intergranular pores.
We present here three geometrical models that help clarify the relationship between NMR measurements and the underlying pore geometry. All of the models characterize this geometry in terms of four parameters:

(1) the volume fraction of total porosity, f

(2) the volume fraction of intergranular porosity,  ¦m

(3) the pore volume to surface area ratio for the micropores, VSm

(4) the pore volume to surface area ratio for the macropores, VSm

In the first model, we apply random walk numerical simulations to an ordered cubic packing of consolidated microporous grains. For given values of the above parameters, the T2 distribution is evaluated as a function of surface relaxation parameter, r. In the second and third models, the microporous grains are treated as a continuum. For the r values of greatest interest, roughly, 1.50 (r) 7.50 mm/s, essentially identical results can be derived from a three-dimensional (3D) analytical model. In addition, for all values of r, many features of the T2 distribution can be represented in terms of a one-dimensional (1D) model pore space.