Rapid and accurate evaluation of reserves in different types of shale-gas wells: Production-decline analysis

The prediction of production levels and estimated ultimate recovery (EUR) with high accuracy is necessary for shale-gas development. Empirical decline methods are widely applied in the oil and gas industry owing to their simplicity and effectiveness; however, none of them can accurately predict the results for typical wells with fluctuating production or in transient flow (TF). To address the urgent issue, four empirical methods, namely the Arps' decline method, stretch exponential production-decline (SEPD) method, and two different patterns of rate decline for fracture-dominated reservoirs (the traditional Duong's method and Wks method), are compared in terms of their principles and characteristics. The results show that the traditional Duong's method is the most reliable, therefore, improvements are proposed (the improved Duong's method) to overcome the influence of production fluctuations and multiple solutions when considering wells that have achieved boundary-dominated flow (BDF). For reasonable production and EUR forecasting in wells with a rapid and unstable decline in TF production, a hybrid method is proposed using a combination of Yu's modified method based on SEPD (YM-SEPD) and the improved Duong's method. The accuracy and universality of the improved Duong's method and the rationality and effectiveness of the hybrid method are evaluated through field examples in the Weiyuan Block of the Sichuan Basin.