Pre-Loading Depleted Parent Wells to Avoid Frac-Hits: Some Important Design Considerations
Authors: Zheng Shuang , Manchanda Ripudaman , Gala Deepen , Sharma Mukul , Hwang Jongsoo 
Published: September 2019

Mitigating the negative impact of frac-hits on production from parent and child wells is challenging. In this work, we show the impact of parent well depletion and repressurization on the child well fracture propagation and parent well productivity in different US shale reservoirs. By repressurizing the parent well, we do not imply repressurization of the entire depleted reservoir. By repressurizing the parent well, we imply pressurization of only the near fracture regions. Our goal is to develop a method to better manage production/injection in the parent well and stimulation operations in the child well to minimize frac-hits and improve oil and gas recovery.

We have developed a fully implicit, 3-D, parallelized, poroelastic, compositional, reservoir-fracture simulator to seamlessly model fluid production/injection (water or gas) in the parent well and model propagation of multiple fractures from the child well (Zheng et al., 2019a; Manchanda et al., 2019a). This simulator implicitly solves for the reservoir deformation and pressure, fracture pressure and injection/production rate to quantify the stress changes due to production/injection, and also the propagation of child well fractures resulting in parent-child well interactions. Component mass balance equations and equation of state-based flash calculations are coupled with the implicit solver to account for the phase behavior in different reservoir fluids and also during the injection process.

We have analyzed the effects of drawdown rate and production time in three shale plays: Permian (oil), Eagle Ford (volatile oil/gas condensate) and Haynesville (dry gas) reservoirs. The results show that different reservoir fluids and drawdown strategies for the parent wells result in different stress distributions in the depleted zone and this affects the child well fracture propagation. We studied different strategies to repressurize the parent well by varying the injected fluids (gas vs. water), pre-load fluid volumes, etc. It was found that frac-hits can be avoided if the fluid injection strategy is designed appropriately. In some poorly designed pre-loading strategies, frac-hits are still observed. Lastly, we analyzed the impact of pre-loading on the parent well productivity. When water was used for pre-loading, we observed water blocking in the reservoir that caused damage to the parent well. However, when gas was injected for pre-loading, the oil recovery of the parent well was observed to increase.

We present, for the first time, fully compositional geomechanical simulations of child well fracture propagation around depleted parent wells. We study the impact of parent well production reservoir fluid, etc. on child well fracture propagation. Fluid injection (pre-loading) strategy in the parent well and subsequent avoidance of frac-hits is also modeled. Such simulations of parent-child well interactions provide much-needed quantification to predict and mitigate the damage caused by depletion and frac-hits.