Geologic characteristics, development technologies and prospects of unconventional natural gas in China

Based on China’s latest exploration and development achievements, production performance data of over 7 000 horizontal wells, and the Unconventional Oil & Gas Digital-Intelligent Platform (UOG), and by integrating statistical analysis and machine learning prediction techniques, this study systematically compares four types of unconventional natural gas (tight gas, shale gas, shallow coalbed methane and medium–deep coal-rock gas) in the country, from the aspects of resource characteristics, key technologies, development indicators and prospects. China holds a substantial quantity of unconventional natural gas, especially shale gas and medium–deep coal-rock gas which boast prominent resource advantages and present a large-scale “continuous” spatial distribution. More than 75% of high-quality resources are concentrated in the Ordos and Sichuan basins. A type-adaptive key technical system has been established, incorporating extensive recovery of tight gas by virtue of “well pattern optimization + low-cost fracturing”, commercial development of shale gas relying on “geological-engineering dual sweet spot evaluation + super fracture network fracturing”, stable production of shallow–medium coalbed methane through “precision drainage and depressurization”, and breakthroughs in pilot technologies such as pressure-controlled development and energy-gathered fracturing for horizontal wells of medium–deep coal-rock gas. The four types of unconventional natural gas vary significantly in development indicators. Tight gas, shale gas and medium–deep coal-rock gas reach peak production 10–30 days after gas breakthrough, showing the characteristics of high initial production followed by rapid decline (with a first-year decline rate of 30%–51%). Specifically, shale gas horizontal wells have the highest average daily production in the first year (7.28×10 4 m 3 /d on average) and single-well estimated ultimate recovery (EUR) (8 255×10 4 m 3 on average). Shallow coalbed methane reaches peak production about 240 days after gas breakthrough, presenting a trend of slow rise–gentle decline, with the lowest single-well indicators. At present, the development of unconventional natural gas is faced with four major constraints including complex geology, technical bottlenecks, environmental restrictions and imperfect policies. It is necessary to address the predicament through multi-dimensional coordination in terms of resources, technology, environmental protection and policies.