Determination of Mechanical and Petrophysical Properties of Sandstone Reservoirs Using Compressional and Shear Waves

Abstract

In the evaluation of a petroleum reserve, it is necessary to determine accurately certain petrophysical
properties such as porosity and permeability of the reservoir rocks under different compaction conditions. These
properties are affected by the relevant physical properties and such physical properties and mechanical properties affect
the drilling programs and the development plans for a reservoir. It is more convenient to use homogenous rock samples
with nearly constant initial permeability, obtaining such cores is very difficult. In this paper a simulated natural and
homogeneous compacted sandstone rock with known physical and petrophysical properties were used. The physical
properties include grain size, cementing material concentration, and compaction (confining) pressure. The effect of these
properties on the petrophysical properties of Rock such as permeability and porosity were also known. For the same
simulated natural sandstone rocks, Sound wave velocity was measured using an ultra sound tool. Good relationships
have been developed between sound wave velocity and other rock properties; porosity, permeability, cementing condition
and grain size distribution at different confining pressures. The sandstone cores have been grouped according grain size
to five groups ranged between 45 and 300 μm mixed with different concentrations of cementing material. The mixture
was compacted at three different compaction pressure ranges from 11000 to 23000 psi. These varying lithification
factors gave these sandstone rocks a wide range of petrophysical and physical properties. The results of this study were
presented as graphs of simulated lithification factors, porosity, and permeability versus sound wave velocity.
Sonic logging data; compressional wave travel time (∆tp) and shear travel time (∆ts) are tested to investigate the type of
reservoir fluids. Ratio (∆tp/∆ts) or VP/VS has shown higher sensitivity than relying only on shear wave or compressional
wave data to study change of reservoir fluids from water to oil and to gas. Reliable reservoir fluids identification
depending on the ratio (∆tp/∆ts) has been mainly attributed to the different physical response of compressional wave and
shear wave to change of fluids from oil to gas in the presence of water in the reservoir. Field examples from sandstone
reservoirs showed that sonic ratio (∆tp/∆ts) helped to identify the type of reservoir fluids gas, oil and water. Results of
well testing data for studied wells confirmed the capability of VP/VS cross plot technique in identifying types of reservoir
fluids.