Acoustic and Quasistatic Laboratory Measurement and Calibration of the PorePressure Prediction Coefficient in the Poroelastic Theory

摘要

The rock mechanical poromechanics pressure property, orrnBiot's effective stress parameter, α , is an important rockrnmatrix and grain characteristic. The Biot's parameter relatesrnstress and pore pressure and weighs the effect of the porernpressure within the concept of the effective stress analysis, inrnthe geomechanics disciplines, and in particular when appliedrnto reservoir engineering and wellbore time-dependent drillingrnmechanics and stability. It measures the compressibility of thernskeletal framework of the rock with respect to the solidrnmaterial composing the rock. In addition, it also reflects therncompressibility of the rock structure which is one of the mostrnimportant parameters for predicting oil reserves.rnThe poroelastic constant, α , is a complex function of thernrock in-situ stress and porosity. The petroleum industry hasrnhistorically calibrated empirical pore pressure relationships tornthe effective stress assuming a value of α to be unity or arnconstant value that may change as the reservoir is beingrndepleted. A theoretical and experimental understanding of thernmeasurements of the poroelastic constant, α , should improvernthe existing models in the areas lacking the data necessary forrnan accurate calibration.rnThe paper discusses the various methods of measuring thernrock poroelastic parameter, α . These methods include quasi-staticrnand acoustic approaches. In the quasi-static approach,rntwo experimental set-ups, known as the direct and indirectrnmethods, were used in this study to determine α , comparedrnsimultaneously with the acoustic method which is based on therncompressional and shear wave velocities measurements underrnhydrostatic loading.rnThe direct method using the quasi-static approach utilizes the measurements of the change in pore volume and bulkrnvolume of the sample for the calculation of α while thernindirect method uses the bulk modulus of the fluid saturatedrnrock sample and solid grains in the computation of α . Thernacoustic approach of measuring α utilizes the measurements ofrnboth compressional and shear wave velocities to compute bulkrnmodulus which is then used to compute α ; thus, very similar tornthe indirect technique. The measurements of α using thesernapproaches were performed on fluid-saturated Berea cores,rnwith mineral oil as the saturating fluid. The results obtainedrnfrom these various techniques are discussed in this work.rnThe indirect method reveals a higher magnitude of thernporoelastic constant, α , compared with direct methodrnmeasurements. This difference was found to be large forrnsamples with high porosity, while for low porosity samples thernmagnitude for the poroelastic constant, α , measured using therntwo methods was comparable. This is due the fact that lowrnporosity samples have high bulk modulus which tends tornlower the magnitude of the poroelastic constant, α , when usingrnthe indirect method. The acoustic measurements showed thernsensitivity of α to the stress level at which it was measured.rnThe magnitude of the poroelastic constant, α , dropped by 20%rnfor some samples when the pressure increased from 1000 torn9000 psi. The results of this work also indicate that thernmeasurements of the poroelastic constant, α , using the directrnmethod and acoustic method at early pressure are quite similarrnfor the case of low porosity sample. For high porosityrnsamples the magnitude of the poroelastic constant, α ,rnmeasured from direct methods was found to fall in the rangernof acoustic measurements at high pressure. The quasi-staticrndirect method showed a good prediction of α in the absence ofrnthe jacketing effect.