Comparison and Translation of Cap Models in Rock Mechanics

Abstract

Cap models describe non-linear compaction and decompaction of porous materials under three-dimensional loading and are frequently used for clays, sands, shales, mudstones and sandstones: Camclay probably being the most frequently used. However, many different cap models are in use in finite element applications, which appear to behave similarly under compressive loading. As such, it is not very relevant which model is used, if the main objective is the simulation of compaction. For dilatant (failure) behaviour the differences between the constitutive models are larger. Of much more relevance in such an analysis might be the proper determination of material parameters, since they determine the local compaction and stress evolution during pore pressure depletion or thermal loading. Cap models of three different finite element packages (Abaqus, Diana and Elfen) have been compared and fitted to each other to determine their relative parameter translation. These models are presently being used within Shell. The five models compared in this study are Diana–Camclay, Elfen–Camclay, Abaqus–Camclay, Elfen-SR3 and Abaqus Drucker–Prager Cap-model . A translation to standard Mohr–Coulomb parameters is also made. From the parameters, best-fit uniaxial (zero-lateral-strain) compaction constants have been derived, that can be applied for reservoir engineering or drilling purposes as a first order geomechanical approach. These constants are the uniaxial compressibility (Cm), the horizontal stress build up ratio (K0) and depletion constants ((h). This paper will be useful for the translation of laboratory or literature data, which are commonly written towards a specific model description or in-house format, to other model implementations or to packages that apply linearised or uniaxial plastic parameters.