In dealing with subsurface engineering problems concerning rock mechanics, the determination of insitu physical properties of rocks has assumed great importance. In recent years, various techniques have been applied in correlating the log-derived parameters with mechanical behavior of the rocks. These techniques have found applications in the petroleum, mining and construction industries.
Specific in-situ elastic properties of the formations must be estimated for resolving problems of rock mechanics. Mechanical behavior of porous rocks depends upon a number of factors:
elastic modulus values,
variation in elastic moduli with stress conditions,
overburden weight gradients,
stresses caused by geological conditions,
strength of cementation between grains,
rates of flow, and
fracture pressure gradients.
Elastic parameters are the key to pressure gradients. Elastic parameters are the key to predicting the mechanical behavior of rocks. predicting the mechanical behavior of rocks. Theoretical relations based upon the theory of elasticity are generally found to be valid for the calculation of elastic parameters in subsurface formations. Quantitative determination of rock elastic constants from well log measurements has recently assumed importance. This is because of improved technology of these measurements and greater computing capability available for field processing of borehole logging data.
In terms of well logging parameters and in practical units, the elastic constants for the rock formations are expressed as:
With sedimentary rocks, a difference is found to exist between the elastic moduli measured in a conventional testing machine and those determined by acoustic (dynamic) methods. The difference has been attributed to the slipping and frictional effects occurring at the contacts between grains during acoustic measurements. This is explained through a set of equations by Gassman. The dynamic and static bulk modulus values have recently been correlated from data measured on sandstones as shown in Figure 1.
The correlation is noted to be a function of porosity. Dynamic values of elastic moduli of porous rocks porosity. Dynamic values of elastic moduli of porous rocks are observed to be consistently higher than static values. However, the comparison between elastic constants determined by full-wave train acoustic logs under in-situ conditions show good agreement with dynamic laboratory measurements on rock samples under simulated formation pressures. From the practical standpoint of predicting rock mechanical practical standpoint of predicting rock mechanical behavior, dynamic measurements from well logs are considered more valid.