THE ELASTIC COEFFICIENTS OF DOUBLE-POROSITY MODELS FOR FLUID TRANSPORT IN JOINTED ROCK

Phenomenological equations (with coefficients to be determined by spec ified experiments) for the poroelastic behavior of a dual porosity med ium are formulated, and the coefficients in these Linear equations are identified. The generalization from the single-porosity case increase s the number of independent coefficients for volume deformation from t hree to six for an isotropic applied stress. The physical interpretati ons are based upon considerations of different temporal and spatial sc ales. For very short times, both matrix and fractures behave in an und rained fashion. For very long times, the double-porosity medium behave s like an equivalent single-porosity medium. At the macroscopic spatia l level, the pertinent parameters (such as the total compressibility) may be determined by appropriate field tests. At an intermediate or me soscopic scale, pertinent parameters of the rock matrix can be determi ned directly through laboratory measurements on core, and the compress ibility can be measured for a single fracture. All six coefficients ar e determined from the three poroelastic matrix coefficients and the fr acture compressibility from the single assumption that the solid grain modulus of the composite is approximately the same as that of the mat rix for a small fracture porosity. Under this assumption, the total co mpressibility and three-dimensional storage coefficient of the composi te are the volume averages of the matrix and fracture contributions.