Principles and influencing factors for shale formation
SHI Can, LIN Botao
1 State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum-Beijing, Beijing 102249, China 2 College of Petroleum Engineering, China University of Petroleum-Beijing, Beijing 102249, China
It is an inevitable trend for the oil and gas industry to transform the exploration & development domain from conventional hydrocarbon accumulations to unconventional hydrocarbon ones. The global "shale gas revolution" has promoted shale gas exploration and development technology, and hydraulic fracturing has become one of the critical technologies for efficient shale gas & oil development. Field research on the morphology and propagation of complex fracture-networks in fracturing shale oil & gas reservoirs is still a fundamental problem, which seriously restricts rational development of shale gas resources. This article summarizes the current standard research methods into fracture propagation and analyzes the advantages and disadvantages of different methods. In addition, based on existing experiments and mathematical models, this paper analyzed the influence on hydraulic fracture propagation from geological and engineering factors. It systematically summarized fracture propagation under the influence of various factors. The following understandings have been obtained: (1) The physical and mechanical properties of shale affect the propagation of fractures, and highly brittle and heterogeneous formations are prone to form complex fracture
networks; (2) In-situ stress is the most critical factor influencing fracture propagation, which determines the morphology and propagation of fracture; (3) Weak-side surfaces (bedding and natural fractures, etc.) in shale reservoirs are important causes of complex fractures, and the properties of the surface, appearance, and in-situ stress difference determine whether the fracture can propagate through the weak-side surface; (4) High displacement and high viscosity can increase fracturing reconstruction range, but the complexity of fractures is low; (5) The shape of the crack obtained by spiral perforation is the most complicated, and the form of the planar perforation is the simplest. Although current experimental and numerical simulation research can describe the fracture propagation under the influence of specific formation and construction conditions to a certain extent, it still cannot satisfy the research on the fracture propagation of complex fracture networks under natural formation situations. In future, research into the fracture propagation in shale reservoirs will continue to improve the experimental and digital simulation methods to simulate complex fracture networks. Simultaneously, it is important to develop research on new hydraulic fracture monitoring
technologies to describe fracture morphology more accurately. At the same time, we should actively explore other methods to
better understand unconventional shale in China. The exploration and development of shale oil and gas resources provides a
robust theoretical guarantee for reservoir reconstruction.
