Abstract
Hydration induced cracks could promote the complexity of hydraulic fractures in marine shale gas reservoir. But the evolution process and forming mechanism has not been fully investigated. In this paper, Longmaxi marine shale were collected and immersed in three types of fluids (distilled water, fracturing fluid, and mineral oil) for more than 10 days. The spatial-temporal evolution of soaking fractures was recorded and analyzed. A fracture mechanical model was established, considering the effects of in-situ stress, fluid pressure, hydration stress, and capillary force. The promotion mechanism of hydration cracks in forming complex fracking network was discussed. Results showed that hydration fractures were extremely developed and evenly distributed in a state of network for specimens immersed in distilled water. For specimens soaked in fracturing fluid, the hydration cracks were moderately developed for the addition of anti-swelling agent. Fractures were rarely developed for specimens treated in mineral oil. The hydration fractures were mainly formed in the first 5 h and showed strong anisotropy. Cracks parallel to the bedding planes accounted for the vast majority, with a small proportion developed in vertical direction. Theoretical calculations indicated that the stress intensity factor (SIF) caused by hydration stress and capillary force was greater than the measured fracture toughness. The micro crack would probably propagate along bedding planes and grow up into macro horizontal fractures, which promoted the formation of crisscrossing fracture network in shale gas formation.
Highlights
Marine organic-rich shales deposited in oceanic and continental margin basins are the most important source rocks of both conventional and unconventional oil and gas resources all over the world (Zou et al, 2019)
This paper presents a detailed study on the fracture evolution of Longmaxi marine shale, soaked in three different types of fluids
By performing shale soaking experiment in three different fluids, conclusions are as follows: 1) For specimens soaking in distilled water, hydration fractures were extremely developed and evenly distributed in a state of network, and their final average cumulative length were 299 mm
Summary
Marine organic-rich shales deposited in oceanic and continental margin basins are the most important source rocks of both conventional and unconventional oil and gas resources all over the world (Zou et al, 2019). Three kinds of soaking fluids (distilled water, fracturing fluid, and mineral oil) were selected to study the crack induced by hydration in shale. For specimens soaking in distilled water, four out of six end faces developed vertical fractures and the total fracture length were greater than those not forming vertical fractures. Heng et al (2015) got the fracture toughness of shale (0.566 MPa m1/2 along the bedding plane and 1.146 MPa m1/2 perpendicular the bedding plane) by performing three-point bending test By comparing this with previous study, it is apparently that the SIF caused by hydration stress and capillary force is greater than the measured fracture toughness and the micro-fracture would probably propagate
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
