Abstract
I. Abstract
Hydraulic fracturing operations, carried out by injecting large volumes of water, cause invasion of the injected water into the formation and trapping due to capillarity. This invasion, also termed as water blocking, can cause a reduction in the gas well productivity by reducing the relative permeability of gas with respect to water. The flow of gas toward the wellbore/fracture during production well will result in the removal of water block through viscous displacement as well as evaporation, which occurs primarily due to gas expansion over long period of time. However, recent observations from field show that the productivity of hydraulically fractured wells improves after a period of shut-in leading to a speculation as to whether capillary suction is responsible for the clean-up of water block which eventually leads to productivity improvement. In this work, we solve for the gas relative permeability improvement of the invaded zone of a fractured well accounting for both evaporation and capillary suction. The conservation equations, for both water and gas, are solved numerically to obtain water saturations and gas relative permeability in the damaged zone. Simulations, of gas relative permeability with time, show that when water invasion depth is small, capillary imbibition is the dominant clean up mechanism. For larger depth of invasion, both capillary imbibition and evaporation contribute to the clean-up of water-block leading to the improvement of gas relative permeability in the invaded region. However, when the viscosity of invaded liquid is high, and the liquid mobility is reduced, capillary imbibition may be negligible and evaporation becomes the dominant clean up mechanism. Our results show that the inclusion of capillary driven flows is important in the prediction of gas well deliverability calculations when water blocking occurs. This study presents a model, combining both viscous removal and evaporation, to calculate the evolution of fracture face skin due to progressive removal of waterblocks and hence well productivity under field conditions.
II. Introduction:
Liquid blocking is a condition caused by an increase in liquid saturation in the near wellbore area. The main causes for liquid blocking in gas wells are the invasion of water base drilling muds, fracturing fluids invasion during well completion, workover or stimulation operations. The invaded liquids are generally trapped in the rock due to the capillary effects. This leads to the persistence of high liquid saturations, which reduce the relative permeability to gas hence results in poor well performance. The water block can be removed by simply flowing well back to displace the invaded water. The displacement process may not be effective especially if the capillary pressures are comparable to the reservoir pressure.