Numerical simulation of production from tight gas reservoirs by advanced stimulation technologies

The present thesis focusses on two main issues: (i) the development of a multi-phase simulation tool for the characteristics of tight-gas reservoirs, and (ii) the investigation of advanced stimulation techniques. The latter mainly implies the analysis of certain damaging mechanisms, as well as the derivation of general modelling guidelines for fractured wells and underbalanced drilling. A special simulation tool is developed, realised in a Fortran-MATLAB coupling. The numerical model is based on the control-volume method with finite differences. It accounts for inertial non-Darcy effects, non-Newtonian fluid rheology and stress dependency of permeability via a simplified approach. The discretisation framework is fully unstructured, using the connection list approach and the common two-point flow stencil. Wells and boundary conditions can be handled very flexible in the code. Contrary to conventional treatment in simulators, wells are discretely included in the simulator. Inertial non-Darcy flow and stress dependency of reservoir permeability are shown to affect the accuracy of simulation models, despite low gas rates. Considering a realistic scenario, with nonDarcy flow and permeability (stress) dependent non-Darcy flow coefficients, stress dependency of reservoir permeability and fracture closure, a total reduction of 40% is possible in a 10 year production period under realistic conditions. New type-curves are presented for non-Darcy flow in fracture and reservoir, allowing for the determination of non-Darcy flow related parameters. The stress sensitivity of tight-gas rocks is crucial when simulating such reservoirs. The stress dependency of the reservoir permeability impacts the productivity to a much higher degree than the fracture closure. A two-phase model is presented for the simulation of cleanup processes in terms of load water recovery. The fracturing fluid is treated as the water phase. The load water, causing hydraulic damage, hardly curtails productivity. To get considerable reductions in productivity, permeability in the fracture vicinity needs to be severely impaired. Due to the flow pattern, fractured wells are generally less sensitive against near wellbore damage than radial wells. An enhanced three-phase cleanup model is presented for the investigations of the polymer gel cleanup, incorporating a yield power law rheology (the Herschel-Bulkley model). The combined occurrence of loadwater recovery including capillary forces and the gel cleanup, are investigated for the first time. First results indicate that both processes are only weakly coupled. A new simulation methodology is presented to investigate underbalanced drilling, taking into account multi-phase reservoir flow with capillary forces. A sensitivity analysis points out that the degree of water encroachment is the key factor for a successful UBD operation. Countercurrent imbibition, causing water encroachment is also analysed. Hydraulic damage turns out to be far more pronounced in tight-gas formations.