Simulation of flow behaviour through fractured unconventional gas reservoirs considering the formation damage caused by water-based fracturing fluids

摘要

Hydraulic fracturing is essential for commercial-scale gas production from many unconventional gas reservoirs. While the effectiveness of the fractures created is associated with the stress created during the fracturing process, the use of water in the hydraulic fracturing process has been found to significantly reduce fracturing efficiency. In particular, the formation damage caused by water imbibition may have a significant negative impact on the flow capacity through both created fractures and rock matrix. These effects can be minimised by using non-water based fracking fluid such as CO2. The intention of this study is to investigate the formation damage caused by water invasion and multi-cycle confinement on the gas production of fractured reservoirs. A laboratory-scale discrete fracture model (DFM) was developed based on the experimental results of a series of permeability tests conducted on intact and fractured siltstone samples under steady-state conditions at room temperature using gaseous CO2and water as the injection fluids. The developed model shows the ability to simulate the flow behaviour of fractured samples. Based on the laboratory-scale model, an expanded DFM model of an assumed fractured reservoir with a horizontal well was then built to quantitatively investigate the influences of multi-cycle confinement and water invasion damage on gas production from gas reservoir.Based on the results of the expanded scale simulation, when the effect of water invasion damage on matrix permeability is not considered and only the change of fracture aperture is considered, the ratio of the rate of gas production from fractured formation without any formation damage, fractured formation suffering from the effect of multi-cycle confinement, fractured formation suffering from the combination effect of multi-cycle confinement and water invasion damage, and unfractured formation is around 15.15, 5.14, 2.23, 1 respectively, and the corresponding ratio of total gas production is around 17.62, 4.86, 2.13, 1 respectively, over a 10-year period. If the effects of water invasion damage on matrix permeability and fracture permeability are considered at the same time, the ratio of the rate of gas production rate from fractured formation without any damage on matrix permeability in the damage zone, fractured formation with 30% of initial permeability in the damage zone, fractured formation with 3% of initial permeability in the damage zone, and unfractured formation is around 2.23, 1.69, 0.94, 1 respectively, and the ratio of total gas production is 2.13, 1.54, 0.92, 1 respectively. This indicates that formation damage caused by multi-cycle confinement and fracturing water invasion can greatly impair gas production from unconventional gas reservoirs.