Asymptotic pattern formation in second and higher order quasilinear parabolic equations

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

A Theoretical Investigation on Formation Damage Mechanisms During Hydraulic Fracturing

The interactions between hydraulic fracturing fluid at lower temperature and salinity with the formation a cause non-uniform stress field. The formation experiences heat transfer to the fracture and local change in temperature, which in turn, leads to dynamically change in stress near the fracture. The changes in stress affects the formation porosity and permeability. Secondly, the chemical imbalance between the fresh fracturing water and saline formation water cause osmosis through semi-permeable clays in shale. Osmosis transfers the fresh water molecules, hence increases the net effective stress, which in turn leads to clay swelling. As a consequence formation porosity and permeability near the fracture could change. Finally, capillary end effect develops in tight gas and shale formations near hydraulic fractures as an additional formation damage mechanism during the flow-back of the treatment water and extends into the natural gas production period. It creates a high water saturation region inside the formation near the fracture. This thesis investigates how the formation properties are sensitive to the nonuniform stress field changes during the shut-in period of hydraulic fracturing and during the flow-back, looking into the mechanisms of stress change and water saturation buildup near the hydraulic fracture. A new multi-phase flow numerical simulator coupled with chemo-thermo-poro-elastic geomechanical model is developed, which includes the thermo-elastic, chemo-elastic and capillary end effects, to understand the role of they play on the non-uniform stress field and saturation field development. The reservoir model has a matrix pore structure mainly consisting of a network of micro-fractures and cracks under stress. The simulation model yields high resolution water-gas flow in this network with a capillary discontinuity at the hydraulic fracture-matrix interface and predicts changes in formation porosity and permeability. The simulation results show that the temperature difference and salinity imbalance between the formation and the fracture impacts the formation properties. The temperature difference reduces the mean normal stress of temperature which leads to lower porosity and permeability. Clay swelling has a reduction effect on the impact of temperature. The capillary end effect causes significant formation damage during the production period by holding the water saturation near the fracture at levels higher than that due to water imbibition only. The effect makes water in the formation less mobile, or trapped, during the flow-back and tends to block gas flow during the production. The capillary end effect during the gas production is more important compared to the changing stress. The capillary end effect cannot be removed completely but can be reduced significantly by controlling the production rates