Predicting Pore Compressibility of Reservoir Rock

The effective pressure in a reservoir is characterized by two sets of stress conditionsthe stress condition at the time the reservoir is discovered and the stress condition existing during the pressure decline period due to production of reservoir fluids. Since correlations of pore compressibility presented in literature may have been obtained by several different types of laboratory testing procedures, a technique of converting these various procedures to a reference set of stress conditions is required for reservoir evaluation. With the method presented here we can quickly interpret results of various pore compressibility measurements regardless of the testing procedure used. The limits of effective stress at the time of discovery of the reservoir, according to van der Knapp, can be described by the equations: effective stress = = ( ) for "passive" earth pressure condition, and effective stress = for "active" earth pressure condition. In these equations:Pz = overburden pressure, psipi = initial pore fluid pressure, psi= pore fluid stress, psi= mean external stress, psiA, B = constants related to the Mohr failure diagram van der Knapp suggests that the "passive" stress condition exists at initial reservoir conditions and further chooses values of A and B that lead to ( - ) = 5/9 (Pz - Pi). However, on the basis of fracture gradient data, it seems more reasonable to assume values of A and B for consolidated sandstones, limestones and dolomites that give a value of ( - ) = (Pz, - Pi). P. 340