This thesis presents the methods and results of a systematic study of the
effect of clay, pressure and temperature on the electrical and hydraulic properties
of natural and synthetic rocks. A better understanding of the mechanisms of the
effect of pressure and temperature on rock petrophysical characteristics has been
achieved in this study. An improved shaley sand interpretation technique based on
the experimental data and the numerical simulation results has been developed.
A novel multi-sample high pressure and high temperature rock testing system
connected to an automatic data acquisition unit has been developed to measure
the electrical and hydraulic properties of 5 core plugs of 1.5" in diameter simultaneously.
The use of this novel experimental system increases the speed of testing
and, since all the samples are placed in identical conditions, eliminates experimental
comparison errors caused by the fluctuations of pressure and temperature.
A new technique of making synthetic shaley rock samples with desired clay
type, content and distribution mode has been developed in order to study systematically
the effect of clay minerals on the electrical properties of shaley sands at
both room and reservoir conditions. The synthetic shaley samples made are tested
in the multi-sample high pressure and high temperature experimental rig.
Two 3D pore space network models (NETSIM and CLAY) have been developed
to interpret and predict the effect of pressure, temperature and clay on the
electrical and hydraulic properties of porous rocks. NETSIM relates the electrical
resistivity and permeability of porous rocks to their microscopic pore structures
and, therefore, provides an insight into the mechanisms of the effect of pressure
and temperature. CLAY simulates the effect of clay content and clay distribution
on shaley sand conductivities, and provides a means for quantifying the clay
distribution coefficient defined in this study.
Significant effect of pressure and temperature on both the electrical resistivity
and the absolute permeability of sandstone rocks has been observed. This effect is
more pronounced for less porous and permeable rocks compared with more porous
and permeable rocks. The combined effect of pressure and temperature on the
electrical properties of sandstone rocks is found to be approximately the sum of
their individual effect. In addition to the actual effect of pressure and temperature,
a significant hysteresis effect has been observed in both pressure and temperature
tests. In order to ensure the accuracy of the estimation of in situ rock porosity
and hydrocarbon saturation from electrical logs, the Archie cementation factor and
saturation exponent need to be quantified in the laboratory at simulated reservoir
conditions accounting for the effect of coupled pressure and temperature and the
hysteresis.
An improved Weixman-Smits model is proposed by including a temperature
coefficient (w) for the equivalent clay counterion conductivity, which is a function
of temperature and clay content, and a clay distribution coefficient (r) for the clay
content [Qv). The values for w as a function of temperature and clay content
have been determined experimentally. The clay distribution coefficient (r) as a
function of clay distribution modes has been quantified for identified clay distributions
based on the numerical model (CLAY). The use of this modified W-S model
enables an improvement in the accuracy of the estimation of in situ porosity and
hydrocarbon saturation of shaley formations.