Hydraulic fracturing is the most effective technique used to stimulate very low permeability reservoirs. A significant body of literature on hydraulic fracturing has been primarily devoted to optimization of fracture propagation and proppant placement. However, the chemical interactions between fracturing fluid and rock matrix have not received much attention. Clay swelling is one of the well-known undesirable interactions of this kind. If clay swelling occurs on the surface of hydraulic fractures, it can cause a major damage to the conductivity of the fractures. Thus, a detailed understanding of clay stability issue is essential for fracturing fluid selection and operation planning.

In this study, a mechanistic approach was presented to model clay swelling as a function of fracturing fluid salinity, formation brine composition, and clay mineralogy. UTCOMP_IPhreeqc, a coupled multiphase reactive-transport simulator developed at The University of Texas at Austin, was used to comprehensively model this process. Double diffusive layer mechanism was applied to capture clay volume expansion. In each time step of the simulation, the volume expansion of clay materials exposed on the fracture surface was used to modify the fracture aperture. Then the permeability of hydraulic fracture cells was re-calculated using a desired function. In order to determine the production loss due to the conductivity damage, the cumulative production was measured at a constant pressure drop along the hydraulic fracture. To illustrate the application of this approach, a simplified numerical simulation example was presented. The example model represented a typical lab-scale core flood experiment with one induced fracture. A sensitivity study was performed on fracturing fluid concentration. The damaged conductivity was measured when fracturing fluid salinity was reduced to zero (fresh water). The simulation results indicated a substantial conductivity damage when fresh water was injected. In this example, a maximum of 30% production loss was observed due to fracture blockage. Thus, for various fluid compositions and shale mineralogies separate analyses are required. The presented approach provides a capability to study clay stability in more details. Such analysis can further improve the chemical design of the fracturing fluid to prevent the undesirable rock-water interactions.

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