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Summary

Stress testing (micro-hydraulic fracturing) is recognized by the petroleum industry as the most direct method of determining the minimum in situ (closure) stress for a given reservoir rock and the surrounding formations. In general, it is variations of in situ stress between formations that dominates hydraulic fracture height growth and overall fracture geometry. Misleading interpretations of stress test data (cased or open hole) can lead to significant errors m the prediction of stress contrast between the producing and bounding rock layers as well as an erroneous estimation of closure stress in the productive interval. In either case, hydraulic fracture treatment designs based on this information may not be designed optimally and the subsequent interpretation of the fracturing treatment pressure response may not be correct. This paper presents an evolutionary approach in the analysis of stress test data which leads to more consistent results that relate directly to actual fracture treatment pressure responses. Although the emphasis in this paper is on cased hole stress test data interpretation, the methodology presented is also applicable to open hole stress testing and larger scale pump-in/shut-in (i.e. calibration or minifrac) pressure falloff responses.

Introduction

The interpretation of stress test data is generally considered the basis by which other stress interpretation techniques and pressure analysis methods are compared and/or calibrated. Unfortunately, the interpretation of stress data does not appear to be as straight forward as initially perceived. Many of the same phenomena observed during large scale pump-in/shut-in and pump-in/flow-back treatments (i.e. minifracs or calibration treatments) as discussed by Nolte also complicate the analysis of stress test data. Some of these phenomena include: 1) continued fracture tip extension; 2) pressure dependant leakoff; and 3) adjacent barrier effects. Other effects which tend to complicate analyses include: 1) near-wellbore closure; 2) multiple fractures; and 3) near-wellbore pressure drop (i.e. tortuosity). Generally, a simplistic approach to analyzing stress test data (especially in cased hole) is pursued without regard to any of the previously mentioned phenomena. Ironically, these phenomena may have a larger impact on the interpretation of stress test data than on "minifrac" data.

Standard modern well test pressure transient analysis techniques are the most commonly used methods for analyzing pressure falloff data from stress tests.