During acid fracturing, different fluids, including a reactive one, are injected. As a result certain complex phenomena are evident. The determination of the performance of the treating fluids from injectivity tests departs from techniques that are applicable in propped fracturing because of the drastic change in rheological and leak-off properties between the viscous pad and the acid. Also, the simulation of the treatments includes the mechanism of diffusion of the reactive fluid to the fracture walls, and the injection of fluids with a large contrast in viscosities. Mechanical instabilities such as wormhole growth and viscous fingering have been shown to develop during the injection.

A design model coupling the fracture geometry to the diffusion mechanisms, such as heat and mass transfer, is presented. The model allows the simulation of both multi-stage injection treatments, with alternating stages of pad and acid, and gelled acid treatments.

A comparison between the two types of stimulation is performed. The implications on fracture conductivity after closure are analyzed based on different models of closure on etched channels in elastic and elastoplastic media. Guidelines for designing treatments in those formations are given, as well as a comparison with proppant injection.

An important aspect of the design is the estimation of fluid leakoff. Current pressure decline analysis techniques typically assume that a single fluid is injected. A method is offered to interpret injectivity tests with fluids having large contrasts in rheological and leakoff properties. Predicted pressures are finally compared with observed treatment pressures in the case of large viscosity contrasts.

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