On the Use of Influence Functions for Subsidence Evaluation

Fast models to estimate subsidence resulting from subsurface extraction processes usually employ a nucleus-of-strain solution based on a center of compression. The solution for such a strain nucleus is then employed as an influence function. This approach is based on the superposition principle that applies for linear elasticity of the subsurface. We argue that the use of influence functions based on a center of compression is incorrect in the case of a non-spherical reservoir of which the elastic modulus is significantly different from the surroundings. This is corroborated with numerical calculations. An influence function can still be used but it must be based on the actual geomechanical parameters and geometry. As a consequence the assessment of the geomechanical parameters becomes even more important. Two important relevant subsurface applications are caverns realized for salt solution mining, and laterally extending gas fields with properties different from the over- and underlying strata.