Abstract

The production or injection of fluids in reservoirs results in a redistribution of stresses. In this paper, the extent of stress reorientation has been calculated for fractured production and injection wells and the results have been analyzed for their impact on refracturing operations. Rules of thumb and charts have been provided to help candidate well selection for refracturing based on the study.

For previously fractured wells, it is possible to create a secondary fracture that is perpendicular to the first fracture. The secondary orthogonal fracture can be created only within a certain time-window that in turn depends on the reservoir properties. Conditions leading to orthogonal secondary fractures in different kinds of reservoirs (oil, gas and tight gas reservoirs) have been analyzed to establish some rules of thumb. The effects of the layers bounding the pay zone and of permeability heterogeneity and anisotropy on stress reorientation are also discussed.

Our results, for the first time, allow us to quantify the phenomenon of orthogonal secondary fracturing around fractured production wells by calculating the extent of the stress reversal region as a function of time. The results of our model are shown to agree qualitatively with field observations obtained from micro-seismic measurements. The model presented in the study helps to demystify the concept of refracturing and provides a quantitative estimate of the time-window for refracturing as a function of dimensionless parameters. The final result demonstrates the potential of the model to increase the reservoir sweep in unconventional reservoirs for which the optimum time-window for refracturing is of the order of months to years. The conclusions of this study are useful for the design of refracturing operations and candidate well selection.

Introduction

Refracture treatments are often applied in wells that have previously been fractured. The performance of these treatments has been observed to be highly variable with some wells underperforming while others are restored to initial production rates. A procedure for the selection of candidate wells that will improve the odds of a successful treatment is needed. This paper presents guidelines based on a poroelastic model that allow an operator to (a) select candidate wells (b) choose the timing of the refracture operation in the life of the well, and (c) suggest a design for the refracture treatment that will result in the best chance of success.

The varying stress state has a central role in many petroleum engineering problems such as borehole stability, formation sand control, wellbore casing damage, reservoir compaction and subsidence.

In this article, we investigate stress and hydraulic fracture reorientation for fractured wells. Hydraulic fracture orientation is critical to both primary and secondary oil recovery from low permeability reservoirs. In primary recovery, common production problems caused by hydraulic fracture reorientation are, incomplete and often overlapping drainage patterns, poor choice of well patterns and location of new wells. In secondary recovery, poor sweep and pre-mature breakthrough of water and steam can result from fracture propagation and reorientation.

The acknowledgement of the existence of stress changes in the reservoir due to production or injection from a previous fracture has resulted in the development of a new concept: oriented or altered-stress refracturing. Refracturing makes it possible to complete new intervals and improve the productivity of previously un-stimulated or under-stimulated zones. The quantitative evaluation of these effects is crucial in the design and the success of the refracturing of vertical wells.

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