Abstract

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 Sh 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 SH.

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