Horizontal-well refracturing is important in the development of unconventional plays to improve the productivity of the refraced well and to protect parent wells from the fractures propagating from the nearby child wells. In the field, heel-biased fracture propagation is frequently observed in refracturing treatments, leaving a large fraction the wellbore understimulated. In this paper, we explain the mechanisms of heel-biased fracture propagation and suggest strategies to mitigate such behavior.
We model refracturing operations accounting for reservoir geomechanics, wellbore hydrodynamics, completion strategies, as well as near-wellbore effects. A coupled reservoir geomechanics simulator is used to calculate the poroelastic stress changes in the reservoir due to production. Then, a multi-fracture simulator with newly developed wellbore fluid and proppant transport model is applied to simulate the refracturing process. A field refrac case is simulated and the simulation results show good agreement with field diagnostics including proppant distribution among multiple fractures. The simulation results help us understand the mechanisms behind heel-biased fracture propagation in refractured wells and allow us to investigate strategies to improve refrac treatment efficiency.
The mechanisms affecting treatment distribution in refracs are investigated from two perspectives: the non-uniform pore pressure and stress profile in the reservoir, and the wellbore hydrodynamics. The latter perspective studies the fluid and proppant transport and separation in the wellbore. With the long wellbore length and the large number of open fractures, the impact of wellbore hydrodynamics on the refrac treatment distribution is very important. Wellbore friction is believed to be a minor reason for the heel-biased treatment distribution compared to proppant inertia, which could lead to the pre-mature screen out of the toe-side clusters.
Simulation results show that using small refrac stages with more frequent diverting agent application can help avoid over-stimulating the dominant fractures and promote more uniform fracture propagation. Strategies that moderate the proppant inertia could also help mitigate the heel-biased trend of refrac treatment distribution.