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Petroleum Geoscience

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The use of leak-off tests as means of predicting minimum in-situ stress

¹ Department of Geological Sciences, University of Durham, South Road, Durham DH1 3LE, UK
² Present address: National Centre for Petroleum Geology & Geophysics, Thebarton Campus, Adelaide University, SA 5005, Australia (e-mail:ade_white{at}hotmail.com)

Leak-off tests (LOTs) or, preferably, extended leak-off tests (XLOTs), can be successfully used in minimum in-situ stress, S₃, estimations. Selecting a point on the leak-off graph that represents the best proxy for S₃ can reduce inaccuracies in the use of LOTs as a means of determining S₃. If the testing procedure is well conducted and recorded, picking the leak-off pressure (LOP) or instantaneous shut-in pressure (ISIP) gives equally valid estimates of S₃. During testing, most of the pressure applied in the deduction of S₃ is exerted by the static mud column, particularly in overpressured settings where higher drilling mud weights are used. Since the mud column contributes such a large proportion of the applied pressure, estimating S₃ from tests conducted at greater depth means the observed small difference between LOP and ISIP has even less of an effect on the deduced S₃ value. The data used in this study show that LOP closely matches ISIP when considering multiple cycle XLOTs. It can therefore be inferred that the LOP is the fracture re-opening pressure and hence S_h given that the assumptions made by the Kirsch equation for wellbore failure are upheld. This study also considers the implications for calculating the magnitude of S_H.

KEYWORDS: pump + testing, reservoir pressure, Mid-Norway, stress analysis, mud weight

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