Minimizing damage from negative “frac-hits” on parent/offset wells during infill/child-well fracturing remains a significant challenge for shale operators. Some of the available options include (a) changing the frac-job design of the child wells (b) pumping fluid into the parent well to re-pressurize the near wellbore area (c) refracturing the existing parent well before completing the child wells. In this work, we further explore the option of injecting fluid (water or gas) into the parent well to form a protective stress shield by increasing the pressure (stresses) and ward off incoming fractures from the child well. We developed a fully coupled geomechanical compositional reservoir simulator to model fluid injection (water and gas) for achieving parent well protection in any reservoir fluid type. The simulator solves component mass balance and pressure equations which are coupled with rock deformation and calculates stress changes due to both poroelastic (pressure changes) and mechanical (fracture opening) effects. The phase behavior of the injected gas with a specified composition (which is different from the in-situ reservoir fluid) is accounted for using phase stability and flash calculation algorithms.
We present results from simulations using representative rock and fluid data from an unconventional reservoir and observe the following trends using our simulation studies: (a) Different reservoir fluid types (black oil, volatile oil, dry gas) result in different pressure and stress buildup due to fluid injection. The highest stress increase is observed in the case of a black oil reservoir and the least in a gas reservoir. (b) Water and gas injection simulations show considerable differences in the observed pressures and stresses due to differences in compressibility, relative permeability and phase behavior of these fluids. Due to higher compressibility of gases, pressure and stress buildup is slower. Thus, gas injection may need to be continued for weeks or months as opposed to a few days in the case of water injection. However, gas injection also provides the added benefit of improving the recovery from the parent well. (c) A higher fracture surface area in the parent well, results in a slower pressure and stress buildup. This work illustrates how geomechanical compositional simulation can be used by an operator to provide significant insights on how to design a fluid injection process in a parent well. Specifically, several design questions can be answered such as (a) injection volumes (b) injection rates (c) injection fluid and composition (d) how far from the parent well the pressure and stresses will increase.
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