Quantifying Transient Effects in Altered-Stress Refracturing of Vertical Wells

SPE Journal
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.

Date Published
09/2010
Volume
15
Issue
03
Number of Pages
770-782
DOI
http://dx.doi.org/10.2118/119522-PA
Other Numbers
SPE 119522-PA