Abstract
Borehole stability and sand production in gas reservoirs is studied and a mechanism that explains the observed higher borehole strength and delayed sand onset is advanced. This mechanism takes into account the drying of the connate water in the reservoir rock due to gas decompression near the borehole. The increased strength during gas flow, as compared to oil flow, results from the higher strength of dry sandstone as opposed to irreducible water saturation sandstone. Flow-through drying of a near wellbore formation is coupled with a constitutive model with water saturation dependent strength and stiffness to analyse the coupled problem of flow-through drying and mechanical behaviour of a rock. Flow-through drying involves the solution of axisymmetric simultaneous, two-phase flow equations of two immiscible fluids, a single-component liquid (water) phase and a binary (air-vapour) gas phase. These comprise the conservation equations for the water and gas components, constitutive equations for the relative permeability and capillary pressure, and Raoul’s law for phase equilibrium. The water weakening model for sandstone is based on a phenomenological law that makes the material cohesion and stiffness a function of saturation. Results from finite element simulations are presented and compared with hollow cylinder test data on Red Wildmoor sandstone, a reservoir sandstone analogue with strong water sensitivity.