Gas-water relative permeability of unconventional reservoir rocks: Hysteresis and influence on production after shut-in

Abstract

Relative permeability is one of the most important parameters in reservoir modeling. However, measurements of relative permeability in unconventional reservoir rocks are rare, and the influence of relative permeability hysteresis on reservoir studies has not previously been addressed. This paper presents a systematic investigation of gas-water relative permeability and hysteresis based on a laboratory technique developed through our previous work. Gas relative permeability was measured using the modified gas expansion method under the scenario of drainage and subsequent imbibition. Water relative permeability was estimated based on Brooks-Corey (1956) equations. Results of gas-water relative permeability that cover a broader range of saturation than that in our previous work were obtained. Causes of relative permeability hysteresis are discussed in detail. The laboratory-based gas-water relative permeability of one of the samples is compared to history-matched results from the literature, and upscaling of the laboratory result is discussed on the basis of this comparison. Finally, the potential influence of relative permeability hysteresis on water and gas production after hydraulic fracturing and shut-in is illustrated through a conceptual model and quantitative analysis. Gas production rates can be overestimated after hydraulic fracturing when hysteresis is ignored. Shut-in can enhance gas production rates significantly. Enhancement of production rate is greater at the initial phase, but it diminishes later, after continuous expansion of the imbibition zone. This work presents the first known study of gas-water relative permeability hysteresis in unconventional reservoir rocks.