Shale gas in China: Characteristics, challenges and prospects (II)

This paper mainly discusses the industrialization progress, sweet spot evaluation criterion, E&P technologies, success experiences, challenges and prospects of China's shale gas. Based on the geologic and engineering parameters of the Fuling, Changning and Weiyuan shale gas fields in the Sichuan Basin, this paper points out that China's shale gas has its particularity. The discoveries of super-giant marine shale gas fields with high evolution degree (R-o=2.0%-3.5%) and ultrahigh pressure (pressure coefficient=1.3-2.1) in southern China is of important scientific significance and practical value to ancient marine shale gas exploration and development to China and even the world. It's proposed that shale gas sweet spots must be characterized by high gas content, excellent frackability and good economy etc. The key indicators to determine the shale gas enrichment interval and trajectory of horizontal wells include four highs, that is high TOC (>3.0%), high porosity (>3.0%), high gas content (>3.0 m(3)/t) and high formation pressure (pressure coefficient>1.3), and two well-developed (well-developed beddings and well-developed micro-fractures). It's suggested that horizontal well laneway be designed in the middle of high pressure compartment between the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation. The mode of forming artificial shale gas reservoir by fracturing micro-reservoir group is proposed and the mechanisms of closing-in after fracturing, limiting production through pressure control are revealed. Several key technologies (such as three-dimensional seismic survey and micro-seismic monitoring of fracturing, horizontal wells, factory-like or industrialized production mode, etc.) were formed. Some successful experiences (such as sweet spot selection, horizontal well laneway control, horizontal length optimization and factory-like production mode, etc.) were obtained. The four main challenges to realize large-scale production of shale gas in China include uncertainty of shale gas resources, breakthroughs in key technologies and equipment of shale gas exploration and development below 3 500 m, lower cost of production, as well as water resources and environment protection. It is predicted that the recoverable resources of the Lower Paleozoic marine shale gas in southern China are approximately 8.8 x 10(12) m(3), among which the recoverable resources in the Sichuan Basin are 4.5 x 10(12) m(3) in the favorable area of 4.0x10(4) km(2). The productivity of (200-300) x 10(8) m(3)/a is predicted to be realized by 2020 when the integrated revolution of theory, technology, production and cost is realized in Chinese shale gas exploration and development. It is expected in the future to be built Southwest Daqing Oilfield (Gas Daqing) in Sichuan Basin with conventional and unconventional natural gas production.