The rapid development of unconventional reservoirs in the US has led to the drilling and completion of a large number of fractured horizontal wells. In an attempt to hold leases, larger well spacing and sub-optimal completions were sometimes used. In most cases, this led to insufficient drainage of the targeted hydrocarbon reserves. Refracturing and drilling infill wells are strategies to recover more oil and gas from these reservoirs, however, both strategies present significant challenges that need to be addressed.

Refracturing partially depleted horizontal wells sometimes results in inefficient re-stimulation of depleted intervals. Diverting agents are used to divert fluid and proppant to un-depleted zones along the well. Field evidence suggests that biased stimulation of some sections of the well still occurs even after the use of these diverting agents. A novel strategy to reduce this depletion induced bias was tested in the field. This strategy suggested the injection of fluid at a very low rate for an extended duration before ramping up the rate and executing the refracturing job. “Slow-injection” of fluid prior to refracturing can help in increasing pore pressure and stress along the entire lateral by repressurizing depleted existing fractures and the rock matrix immediately around these fractures. This induced increase in pressure and local stress is temporary and may last only a few hours or days but can be helpful in distributing the refracturing fluid in a more uniform manner into all fractures (new and old) as well as improving the placement of diverters throughout the lateral.

The proposed low-rate repressurization strategy was simulated using a fully 3-D poro-elastic geo-mechanical model. The results helped in understanding the impact of this slow-injection phase on the pressures and stresses that develop during depletion and repressurization. This helps in designing the rate and the duration of the slow injection to achieve efficient refracturing of depleted wells.