A study of borehole stability and sand production in gas reservoirs is presented, 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 borehole stability gas flow, compared to oil flow, results from the higher strength of dry sandstone as opposed to irreducible water-saturated sandstone. Flow-through drying of the 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 rock. Flow-through drying involves the solution of axisymmetric simultaneous, two-phase flow equations for 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.
Borehole stability and sand production in gas reservoirs are regarded as a more critical problem than in oil reservoirs due to the higher fluid flow velocities in gas production compared to oil production. High fluid velocities coupled with sand production can cause severe erosion of completion and production facilities. High velocities are also associated with turbulent flow, which may have a greater erosion potential towards the reservoir rock. Finally, gas flow due to the compressibility of gas gives rise to higher pressure gradients near the wellbore during production, which further worsens the conditions for borehole stability and sand production.
