Earth Resources Laboratory :: Abstracts

Elastic Properties of sedimentary Anisotropic Rocks
(Measurements and Applications)

Franklin J. Ruiz Peña

Submitted to the Department of Earth, Atmospheric, and Planetary Sciences on October 9, 1998 in partial fulfillment of the requirements for the degree of Doctor of Philosophy

Abstract

In multidisciplinary studies carried out in the Budare Oil Field of the Great Oficina Oil Field, there was difficulty in matching well log synthetic seismograms with 2D and 3D seismic data. In addition, the seismically determined depths of reservoir horizons are greater than the well sonic log depths. To examine this discrepancy we conducted an experimental study of dynamic elastic parameters of the rocks in the oil field. We chose core representative samples of the lower Oficina Formation, the main reservoir of the field. The rocks selected were sandstones, sandy shales and dolomite shales.

For the velocity measurements, we used the ultrasonic transmission method to measure P-, SH- and Sv-wave traveltimes as function of orientation, and pore and confining pressures to 60 and 65 Mpa, respectively. We found that, in room dry condition, most of the rocks studied are transversely isotropic. The stiffness constants, Young's moduli, Poisson's ratios, and bulk moduli of these rocks were also calculated.

The velocity anisotropies, together with the behavior of the elastic constants for dry rocks, indicate that: 1) the elastic anisotropy of the sandstones and sandy shales is due to the combined effects of pores, cracks, mineral grain orientation, lamination and foliation. The velocity anisotropies caused by the preferred oriented cracks decrease with increasing confining pressure. 2) For the dolomitized shales, the elastic anisotropy is due to mineral orientation and microlamination. In these cases the very high intrinsic anisotropy does not decrease with increasing confining pressure. 3) The velocities of the compressional waves are greater in sandstones saturated with water than in the dry specimens, but the opposite behavior was found for shear waves. 4) The P-wave velocity anisotropy decreases after saturation; the magnitude of the decrease depends on the crack density and on the abundance and distribution of clay. 5) The Vsh-anisotropy does not show a pronounced change after saturation, and it is only slightly affected by confining pressure. Visual description, petrography and mineralogical analysis from thin sections and x-ray diffractions revealed the vertical and lateral heterogeneous nature of sandstones and sandy shales, whereas the dolomitized shale specimens looked homogeneous.

The results of laboratory measurements are consistent with an elastic model, using the equivalent medium theory for fine-layered isotropic and anisotropic media. However, in order to do reliable seismic migration and solve the problem of thickness calculations and time-to-depth conversion of surface seismic data, the ultrasonic data need to be extrapolated to low frequencies. Determining rock mechanical properties in situ is important in many applications in the oil industry such as reservoir production, hydraulic fracturing, estimation of recoverable reserves, and subsidence. Direct measurement of mechanical properties in situ is difficult. Nevertheless, experimental methods exist to obtain these properties, such as measurements of the stress-strain relationships (static) and elastic wave velocities (dynamic).

We investigate the static and dynamic elastic behavior of sedimentary, anisotropic rock specimens over a range of confining and pore pressures up to 70 Mpa, the original reservoir conditions. The static and dynamic properties are simultaneously measured for room dry shales, room dry sandstones, and brine saturated sandstones. We found that (1) All the ratios of dynamic to static velocities and of dynamic to static elastic parameters in all directions, Rm(O) {O = Theta}, decrease with increasing confining pressure. However, the rate of decrease is greater in the vertical direction than in the horizontal direction. 2) After saturation, all the ratios of dynamic to static velocities, Rm(O), decrease, except the bulk compressibility ratio, Rkb, which increases. 3) All the ratios of dynamic to static moduli, Rm(O), decreases when the pore pressure is raised, except Rkb which increases. 4) The magnitude of the ratio of dynamic to static velocities or moduli, Rm(O), depends on the direction of the measurements. Not all the ratios Rm(O) are equally affected. The ratio of dynamic to static P-wave velocity, Rp(O), is greater in the vertical direction than in the horizontal direction. On the other hand, the ratio of dynamic to static SH-wave velocity, Rsh(O), does not depend on the direction of propagation. 5) The modulus determined from: uniaxial stresses, hydrostatic compression or any other stress system yields different values. This is because of the rock porosity. 6) All the static and dynamic velocities and elastic parameters decrease with increasing confining pressure. 7) The static velocity anisotropies and static modulus anistropies are always greater than the corresponding dynamic anisotropies, over the entire range of confining pressure and directions. 8) After saturation, the dynamic Vp-anisotropy, Ed, decrease, while the dynamic Vsh-anisotropy, (gamma)d, is affected much less. The static anisotropy also decreases after saturation. 9) Both Vp(dyn)and Vp(stat) increase after saturation and with increasing pore pressure. However, the increase is more pronounced in the Vp(stat). 10) Vs(dyn) decreases after saturation and with increasing pore pressure. On the other hand, Vs(stat) increases both after saturation and with increasing pore pressure. 11) The increase of elastic moduli with confining pressure is much larger than the increase in the corresponding dynamic ones.