Gas Recovery From Tight Sands: Impact of Capillarity

Abstract

Summary Hydraulic-fracturing operations carried out by injecting large volumes of water cause invasion of the injected water into the formation and create a water block. The flow of gas toward the wellbore/fracture during production will result in the removal of the water block through viscous displacement, as well as evaporation, that occurs because of gas expansion over a long period of time. However, some observations from the field show that the productivity of hydraulically fractured tight gas wells improves after a period of shut-in, leading to a speculation as to whether capillary suction is responsible for the cleanup of water block that eventually leads to productivity improvement. In this work, we use laboratory-scale experiments and modeling to find that capillary-driven transport is an important mechanism that helps redistribute water within the tight gas rock sample. Without capillarity, the model underpredicts the effective gas relative permeability recovery in the laboratory sample. We also find, using simulations, that capillary transport has the effect of enhancing the overall evaporation rate of water from the rock core. The model for calculating saturation changes and the effective gas relative permeability is complete with regard to all the mechanisms, such as displacement and evaporation. This is unlike previous studies, which did not include one or the other. Field-scale-simulation study of gas flowback using the new integrated model shows that the effective gas relative permeability of the invaded zone is significantly affected by capillary suction. In the absence of capillary suction, displacement and evaporation proceed as usual, but the invaded-zone water saturation does not dissipate quickly enough. The fracture-face skin, which is a function of the effective gas relative permeability, decreases faster as the invaded zone water is redistributed because of capillary suction. The simulations show that the evaporation of water from the invaded zone is very slow because of the low gas-flow rates in the tight rock matrix. In comparison to evaporative removal of water from the invaded zone, capillary-suction removal is significantly higher and faster. A sensitivity study on fracture-face skin shows that capillary suction has a significant effect on the cleanup at low drawdowns and smaller invasion depths. At complete shut-in conditions, the invaded-zone saturation continues to dissipate because of capillary suction. This confirms the general observation and anecdotal evidence that tight-sandstone wells produce at greater gas-flow rates after a period of shut-in. The methods described in this study can be adapted to perhaps determine the duration of such shut-in periods. Additionally, the models can be used to rigorously predict gas-production rates from a fractured well, including capillary effects, without resorting to averaging concepts such as fracture-face skin.