Diagnosis of induced hydraulic fractures during polymer injection
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Polymer transport with complex fluid rheology was implemented into an existing hydraulic fracturing simulator (CFRAC), and the implementation was extensively validated. Shear thinning viscosity was included for polymer fluid flow in both porous media and fractures. Shear thickening viscosity was implemented for flow in the matrix. Polymer injections were simulated to investigate the effect of polymer rheology, including both shear thinning and shear thickening behaviors, on polymer injectivity and on the possibility of induced hydraulic fracturing. The results indicated that shear thickening decreases injectivity and can induce initiation of hydraulic fractures. The hydraulic fractures substantially enhance injectivity, and eliminate the reduction in injectivity due to shear thickening. Pressure fall off tests were simulated to study the effect of polymer rheology on the identification of hydraulic fractures from the shut-in transient after shut-in. The results showed that standard methods from pressure transient analysis can be applied to diagnose the presence hydraulic fracturing by identifying a linear flow regime and fracture closure on a Bourdet derivative plot and a square root of time plot. It was found that these methods are effective regardless of shear thickening and shear thinning rheology. However, the results suggested that if the fracture is small and closes quickly, this could cause difficulty for the diagnosis.