Minimizing Problematic Depletion Effects in Unconventional Reservoir Recompletions: An Engineered Approach to Help Ensure Refracturing Results in Complex Environments
Authors: Sierra Leopoldo , Martysevich Vladimir , Bai Jie 
Published: October 2018

Fracture stimulation of horizontal wells in unconventional gas- or oil-producing reservoirs by placing a large number of transverse or pseudo transverse fractures in reference to the wellbore orientation is usually necessary to help maximize hydrocarbon recovery in complex environments.

Although economic completion and multi-stage fracture stimulation of unconventional reservoir using horizontal wells dates back to mid-2002, particularly in North America, it has been gradually modified and improved through extensive trial and error processes to improve the stimulation effectiveness in unconventional reservoir productivity. However, the trial and error process is not often effective nor a recommended practice in the refracturing processes where differential areal depletion is present. This paper demonstrates the effect of differential depletion normally present in existing unconventional producing reservoirs and how to optimize additional fracture(s) placement during refracturing processes or infill well placement to help maximize hydrocarbon recovery.

In line with the economic considerations and the massive implementation of completion activities, the industry has often applied the trial and error process in the unconventional reservoir. Initial completions involve variations of (but not limited to): pumping rate, total lateral length, spacing and number of perforation clusters, perforations per cluster, lateral length of treated stages, length of fractures generated, proppant and stimulation fluid volumes per stage, and lateral well spacing.

The initial completion strategy was often intended to prevent or minimize the negative effect of interferences or non-effective completion techniques which could result in the use of longer perforation clusters, longer fracture stage spacing, or conservative completions. Additionally, if the completion design is not effective in maximizing the fracture initiation points per stimulation stage, a significant bypass of hydrocarbon reserves between the fractures or well laterals can happen.

To produce the additional bypassed hydrocarbon reserves, an engineered process to refracture existing wells should be implemented if they are economically justified. Once the bypassed reserves are located between laterals, a properly located, completed, and stimulated infill well could help maximize hydrocarbon recovery in unconventional reservoirs. However, the fracturing of infill wells in areas where earlier areal depletion exists due to production can affect the stimulation process.

In general, the effective refracturing of existing partially drained long horizontal wells initially stimulated with multiple fractures is not an easy process. If not properly managed, localized differential depletion can result in the preferential refracturing of the depleted areas and the corresponding economic failure of the stimulation process.

Considering these challenges, this paper proposes an engineered methodology when refracturing wells in unconventional reservoirs as well as completion plans for placing and fracturing infill wells. In contrast to the actual more manual type methodology used to combine fracture simulators and reservoir simulators capabilities to consider the areal pressure depletion effect, the use of a new complex fracture stimulator having both capabilities in one package was used to evaluate the fracture stimulation, production estimation and its effect, and the different alternatives to minimize the areal depletion in refracturing or child wells placement and fracturing opportunities.