In recent years, it has become evident that the depletion of legacy wells creates a pressure sink that attracts fractures from new infill wells nearby. This causes asymmetric growth of fractures from infill "child" wells more toward legacy "parent" wells. The asymmetric growth ends in fracture interference and subsequently results in an ineffective stimulation of the reservoir. To avoid the interference, operators have been exercising pressurization of the depleted region by means of shut-in parent wells while fracturing infill wells or by refracturing the parent well before fracturing of infill wells; a practice commonly known as "protection refrac". The objective in this work is to study the effectiveness of protection refracs in re-pressurizing depleted reservoirs for the purpose of mitigating asymmetric fracture growth.
A transient poroelastic displacement discontinuity model is developed to study the problem of undesirable hydraulic fracture propagation toward depleted zones nearby an example parent well. The model is able to solve for pore pressure distribution and changes in principal stresses direction and magnitude around both parent and child wells. Pore pressure calculation due to production and so depletion of parent wells, and the resultant change in stress magnitudes are critical to understand and predict the extent of fractures from new infill wells. The model is capable of handling dynamic changes over time and can predict the optimum refracturing timing and volume.
Pore pressure and stress changes while production are calculated, and an estimated timing for protection refrac is suggested. Different refrac scenarios are examined throughout the life of the well and optimum refracturing volume and well spacing for infill wells are recommended. It is shown that there is a critical time in the life of the well that protection refrac could help pressurizing the formation directly by increasing pore pressure through fluid injection and indirectly by mechanical dilation of existing fractures. Beyond this critical time, any attempt in increasing pore pressure effectively may not be practically feasible due to the nature of the severe depletion.
The paper presents a novel approach in calculating stress changes and dynamic fracture propagation into depleted region over time. Asymmetric fracture growth is uniquely shown in depleted formations along with mitigation strategies to overcome this undesirable scenario.