Geomechanical aspects of fracture growth in a poroelastic, chemically reactive environment




Ji, Li, active 2013

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Natural hydraulic fractures (NHFs) are fractures whose growths are driven by fluid loading. The fluid flow properties of the host rock have a primary, but hitherto little appreciated control on the NHF propagation rates. This study focuses on investigating the impacts of host rock fluid flow on the propagation and pattern development of multiple NHF in a poroelastic media. A realistic geomechanical model is developed to combine both the fluid flow and mechanical interactions between multiple fractures. The natural hydraulic fracture propagation is observed to consist of a series of crack-seal processes indicating incremental stop-start growth. Growth timing is on the scale of millions of years based on recent natural fracture growth reconstructions. These time scales are compatible with some model scenarios. My newly developed numerical model captures the crack-seal process for multiple NHF propagation. A sensitivity study conducted to investigate the impacts of different fluid flow properties on NHF propagation shows that permeability is a predominate influence on the timescale of NHF development. In low-permeability rocks, fractures have more stable initiation and much longer propagation timing compared to those in high-permeability rocks. Another aspect of great interest is the influence of fluid flow on fracture spacing and pattern development for multiple NHFs propagation in a poroelastic environment. My new poroelastic geomechanial model combines the natural hydraulic fracturing mechanism with the mechanical interactions between fractures. The numerical results show that as host rock permeability decreases, more fractures can propagate and a much smaller spacing is reached for a given fracture set. The low permeability slows down the propagation of long fractures and prevents them from dominating the fracture pattern. As a result, more fractures are able to grow at a similar speed and a more closely spaced fracture pattern is achieved for either regularly spaced or randomly distributed multiple fractures in low-permeability rocks. Investigation is also conducted in analyzing the distributions of fracture attributes (length, aperture and spacing) in low- and high-permeability rocks. For shales with high subcritical index, low permeability helps the fractures propagate more closely spaced instead of clustering. Meanwhile, in low-permeability rocks, factures have relatively smaller apertures, which lead to a slower fracture opening rate. The competition between the slow fracture opening rate and quartz precipitation rate will affect the effective permeability and porosity of the naturally fractured reservoir. However, the competition is trivial in high-permeability rocks. Other factors, such as reservoir boundary condition, layer thickness, subcritical index and pattern development stage, all have considerable impact on fracture pattern development and attribute distribution in a poroelastic media.



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