Reported in this paper is an experimental study on Biot's effective stress coefficient of downhole cores by both static and dynamic approaches. Triaxial tests were conducted on more than 30 sandstone samples with ultrasonic wave velocity measurement under a series of confining pressure conditions. It has been shown that the Biot's effective stress coefficient derived from static measurements can be correlated with that obtained from dynamic measurements. Furthermore, the Biot's effective stress coefficient obtained from static measurements appears to be correlated with core porosity which increases with increasing in porosity. These correlations allow the Biot's effective stress coefficient to be estimated from sonic wave velocities and core porosity, which can be obtained from routine well logs.
It is well known in the petroleum industry that production of oil and gas from a reservoir induces variations in stresses and pore pressure in the rocks surrounding the well bore and in the reservoir. Wellbore instability and sand production occur when the variations result in failure of the rock (Bradley, 1979 and Morita, 1994). The mechanical response of the rock formation to the stress and pore pressure variation is in general assumed to be governed by Biot's effective stress (б¹) in poroelastic theory. Where cr and Ch are grain and bulk compressibilities respectively. They may be determined by static or dynamic approaches (Yew & Jogi, 1978 and Yew et al., 1979). For effective stress calcula- tion in a reservoir or wellbore wall, the value of a determined by static methods is required. In the context of petroleum industry, obtaining downhole cores may not be always possible or economic. Even with downhole cores available, laboratory measurement can only yield values for a at discrete depths in a reservoir. On the other hand, well logs for sonic velocity and core porosity are conducted routinely. If correlations can be established between Biot's effective stress coefficients obtained by static and dynamic approaches, these well logs can be utilised to evaluate the Biot's effective stress coefficient. Reported in this paper represents an effort to compare the Biot's effective stress coefficient obtained by static approach with that by dynamic (ultrasonic) approach. Triaxial tests were conducted on more than 30 downhole sandstone samples under a series of confining and pore pressure conditions. The static and dynamic measurements were conducted simultaneously. The primary objective of this paper is to establish a correlation between Biot's effective stress coefficients obtained statically and dynamically. Experimental results showed that the Biot's effective stress coefficient measured by static and dynamic methods could be correlated with each other. It was also observed that the Biot's effective stress coefficient obtained by static approach could be correlated with porosity of the core materials. It increased with increasing in porosity. Furthermore, the experimental result showed that a for low porosity rocks, in contrast to the Terzaghi's effective stress. This could have significant implications in well bore stability and sand production analyses.could be significantly less than one, particularly