Abstract:

Conventional microseismic analysis is often inadequate to determine whether refracture treatments stimulate undepleted areas or simply re-stimulate already-depleted areas. The Eagle Ford refracture treatment case study in this paper demonstrates the application of microseismic geomechanics to assess the effectiveness of diversion and estimate the distribution of fluid between previously stimulated and unstimulated areas.

The case study shows that depletion-induced changes in stress can enhance diversion, and the distribution of fluid between fractures in depleted and undepleted areas evolves during the treatment. The geomechanical model is used to identify the characteristic pressure signature and microseismic patterns associated with different hydraulic fracture geometries. A practical diagnostic of diversion effectiveness based on microseismic moment is derived from advanced microseismic analysis of the geomechanical response, and some options for completion optimization are suggested.

Introduction

Due to the low permeability of shale reservoirs, it is generally assumed that production occurs only from those parts of the reservoir that are close to the hydraulic f racture or to natural fractures activated by the stimulation process. Many older horizontal wells were stimulated with 10 or fewer stages, with the stages separated by some distance, due to limitations of completion hardware technology. This resulted in sub-optimal stimulation and some areas of the laterals were not stimulated at all. Refracturing these older wells has been seen as an opportunity for oil and gas operators to increase production at a lower cost than drilling new wells.

In some cases, diverters are used to block access to open fractures and redirect the refracture treatment to stimulate new areas around the wellbore that were not accessed by the initial treatment. It is important to know whether the diversion is successful or if the initial fractures are being reopened.

Microseismic data is often used as a tool to visually assess the geometry of hydraulic fractures, and the related stimulation. This paper shows that this simplistic approach (assuming that microseismic activity is associated with fluid flow and stimulation) is inadequate for refracturing. A more complete workflow and modeling methodology is demonstrated using an Eagle Ford case study.

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