Experimental and Modeling Study of Water Imbibition and Flowback in Shale: Prediction of Relative Permeability and Capillary Pressure Curves

Understandingthe invasion and flowback processes of water, asthe main source of hydraulic fracking fluid, is crucial for the properevaluation and development of complex gas shale formations. Directmeasurements of full gas-water relative permeability and capillarypressure curves by steady-state or unsteady-state techniques are notfeasible in ultralow permeability shales. Therefore, the objectiveof this study is to investigate the dynamic rock-fluid propertiesof a Mancos shale sample by mimicking water imbibition and flowbackexperiments. To do so, a water injection experiment was done and triedto measure the flowback. After that, history matching on the experimentaldata of the water injection and flowback process was done to predictgas-water relative permeability data and capillary pressurecurves. Low water flowback was observed, which insists on water entrapmentin shale rocks. The results show a high threshold pressure and meaningfulrelative permeability of water, which can reduce the gas productionrate dramatically. The saturation profiles show a high percentageof water in the invaded zone (>75%); however, the continuous imbibitionof water with time (increase in shut-in time) helps the front of theinvasion zone to move further deep into the shale, which enhancesgas production. The S-shape gas relative permeability curve most likelyrepresents the actual trend of the curve. An optimum shut-in timewas predicted, which enables us to avoid delays in starting productionoperations.