The potential for large errors in the inferred minimum earth stress when using incomplete hydraulic fracturing results

Abstract The reassessment of a broad range of hydraulic fracturing data associated with the repeated injection and flow testing of a deep region of the earth—a 0.3-km3 volume of stimulated crystalline rock at a mean depth of 4 km—has shown that the pressure-opening of pre-existing joints can completely mask the minimum earth stress. This situation is further aggravated when the unjointed intervals exposed in the wellbore are limited, and the unflawed rock has a significant tensile strength—the most common in situ condition at depth. This obscuring of the minimum earth stress during hydraulic fracturing tests is particularly evident when the continuous sets of joints are inclined to the principal stress directions, as was the case in our situation, and led to an initial overestimate of the minimum effective earth stress by about a factor of three.