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GEOMECHANICAL ASPECTS OF FRACTURE GROWTH IN A POROELASTIC, CHEMICALLY REACTIVE ENVIRONMENT

Supervisor: Jon E. Olson

Co-Supervisor: Matthew T. Balhoff

Natural hydraulic fractures (NHFs) are fractures whose growths are driven by fluid loading. The fluid flow properties of the host rock control the NHF propagation rates. This study focuses on investigating the impacts of fluid flow on multiple NHF propagation and pattern development 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 and the growth timing is on the scale of millions of years in both field and lab observations. The newly developed numerical model captures the crack-seal process for multiple NHF propagation. A sensitivity study is conducted to investigate the impacts of different fluid flow properties on NHF propagation. Permeability shows 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 influences of fluid flow on fracture spacing and pattern development for multiple NHFs in a poroelastic environment. The new poroelastic geomechanial model combined 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 more closely spaced instead of clustering. Meanwhile, in low-permeability rocks, fractures 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 impacts on fracture pattern development and attribute distribution in a poroelastic media.