1. Introduction

Hydraulic tensile strength is a crucial value for planning reservoir stimulation and stress measurements. It is used in the classical breakdown pressure (Pb) relation by Hubbert & Willis[1], where Pb is a function of major and minor princicpal horizontal stresses SH and Sh, hydraulic tensile strength δT and pore pressure P0:

  • Pb=3Sh-SHT-P0

For hydraulic fracturing laboratory experiments (MiniFrac – MF) under isostatic confining pressure Pm this might be reduced to:

  • Pb=cPmT-P0

The coefficient c should be equal to two when porepressure is neglected. However, many laboratory experiments [2,3] resulted in values of about 1 for c, which might be explained by poroelastic effects. Thus, when poroelasticity is excluded in the experiments by taking dry samples and sealing off the central borehole by an impermeable membrane (like a polymer tube), one would expect that c equals two and δT will be in the range of the tensile strength as determined by other tensile strength tests.

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