Research on the Definition of Economic Limits for the Development of Unconventional Natural Gas under New Situations

The development of unconventional natural gas in China is facing three new situations: market-oriented pricing reform, intensified competition in state subsidies, and reduced production capacity growth, which may cause the development of unconventional natural gas to be on the edge of economic feasibility. Therefore, the definition of economic limit is even more important. In this paper, through analysis on the historical prices, outputs, and subsidy policies of three unconventional natural gases, methods to estimate future prices, subsidies, and outputs of unconventional natural gas are designed in this paper based on the mean reversion model and the generalized Weng model, respectively; fuzzy data are processed by using probability density function combined with a discounted cashflow method to improve the utilization of original data; the economic limit model for well depth, well spacing, and gas recovery rate is designed through break-even analysis with the subsection function of drilling cost to well depth, the modified Cher Card Geoff empirical formula of recovery ratio to well spacing, as well as the fitting formula of gas recovery rate and stable production time. This model is applied in the case of Deep CBM Block Ji 4&10. According to the estimation and case calculation, in future China, the subsidies for unconventional natural gas will gradually decrease and the gas output will significantly increase, with shale gas taking the leading position and CBM gradually declining; the economic limits of well depth, well spacing, and gas recovery rate of Ji 4&10 are 2,203.2 m, 300 m × 300 m and 469 m × 469 m, and 2.1% and 4.3%, respectively, under the economic infeasibility probability of 90%, and the overall economic infeasibility probability is 58%, indicating that the development of this block is subject to great risks and careful consideration needs to be given.