Abstract
Relative permeability is one of the most critical parameters for reservoir modeling. However, reliable measurements of relative permeability are lacking, and the influence of relative permeability hysteresis has not yet been addressed for unconventional reservoir rocks. This paper will present a systematic investigation of gas-water relative permeability and hysteresis using a laboratory measurement technique developed through our previous work. A procedure of drainage and imbibition is adopted. After each step of drainage or imbibition, gas relative permeability was measured using the modified gas expansion method. Water relative permeability was estimated using modified Brooks-Corey Equation. Results of gas-water relative permeability that cover broader range of saturation than that in our previous work are obtained. Causes of relative permeability hysteresis will be discussed in details based on measured water saturation data and pressure decay curves. The measured gas-water relative permeability is compared to history-matched results based production and bottomhole pressure data from literature, and close results are shown. Scale-up of the laboratory measurement results to reservoir scale is also discussed based on the comparison. Finally, the influence of relative permeability hysteresis on water and gas production after shut-in is illustrated through a conceptual model and quantitative analysis based on measured relative permeability curves in primary drainage and subsequent imbibition. Shut-in can enhance gas production rate at the initial phase, but the enhancement will diminish later with continuous expansion of the imbibition zone. Shut-in and hysteresis has minor impact on water production. This work presents the first known such study in gas-water relative permeability and hysteresis in unconventional reservoir rocks. The methods and results can provide contribution to improve the reservoir modeling and production prediction with more accuracy and less uncertainties in unconventional reservoirs. They can also provide guidance on optimal design of time-length of shut-in for a well.
While the importance of relative permeability (kr) for reservoir studies is well known, understanding of kr is still very limited for unconventional reservoir rocks. This limited understanding is mainly caused by the lack of directly measured kr data, which in turn is because of the difficulties related to measurements of these types of rocks that have extremely low permeability at sub-micron-Darcy levels. In a previous study (Peng, 2019), a method of laboratory measurement of gas relative permeability (krg) was established and applied to several mudrock and tight formation samples. Gas permeability under different water saturations in the scenario of imbibition was measured using a modified gas expansion (MGE) method (Peng et al., 2019). Water relative permeability (krw) was estimated based on the measured data, a conceptual model of water-gas two-phase distribution, and a modified Brooks-Corey equation. Substantial new insights on water-gas two-phase flow, water block effect, and the influence of wettability and pore structure during the process of water imbibition and water-redistribution have been gained based on this investigation.
