Characteristics of Time-Lapse Well Test and Evaluation Method of Dynamic Reserves During Water Influx in High-Pressure Gas Reservoirs

For weak water invasion gas reservoirs with lagging dynamic response to water invasion and high-pressure variable volume, traditional material balance methods have difficulties in identifying water influx in the early and middle stages and have low accuracy in evaluating dynamic reserves. It is necessary to solve the problems of characterizing the time-lapse parameters of weak water invasion in gas reservoirs, calculating the water influx under high-pressure variable volume, and evaluating the dynamic reserves. Based on the dynamic response of weak water invasion and the theory of transient flow, the typical characteristics of the unsteady well test curves with time shift during water invasion were clarified, and the time-lapse parameters of the gas-water contact are quantitatively characterized. Combining the analytical methods of high-pressure material balance and pseudo-steady-state water influx, a method for evaluating the water influx and dynamic reserves in high-pressure weakly water-invaded gas reservoirs is established. The research shows that with the increase of time, the unsteady well test curve of weak water invasion under high pressure exhibits the characteristic of “time lapse of the gas-water composite boundary”. The gas-water contact advances towards the wellbore, and the water saturation increases. The volume of the aquifer is the main controlling factor affecting the water influx and the dynamic reserves. The example shows that the evaluation of dynamic reserves takes into account the characteristics of time-lapse of water influx and high-pressure variable volume, which is more scientific and reasonable, and the accuracy of the calculation results is improved by 20%. The research results provide a scientific basis for the identification of weak water invasion in high-pressure gas reservoirs and the evaluation of dynamic reserves, and also offer a basis for the subsequent reconstruction of gas storage and the design of gas storage space.