Electrical imaging and hydraulic testing for a complete stress determination

Abstract

A complete stress determination has been performed in Lodève (southern France) by conducting hydraulic tests in a 200 m deep borehole, with a probe combining electrical imaging with an inflatable straddle packer. Both classical hydraulic fracturing and hydraulic tests on pre-existing fractures have been conducted. The normal stress acting on the tested fracture planes was determined from the simultaneous analysis of both hydraulic and electrical signals observed during shut-in phases. Values derived from quasi-static reopening pressure tests are systematically larger than those derived from shut-in and have been discarded. Hydraulic fractures provide information on the principal stress directions orientation as well as on the minimum principal stress magnitude. HTPF tests provide information on the vertical and the maximum horizontal principal stress components magnitude and are unaffected by any pore pressure effect. At a depth of z=165 m , the vertical stress component is principal and is given by S v =4.0±0.5 MPa . The maximum horizontal principal component is oriented N155°±10° and is given by S H =10.0±0.2 MPa . It is almost twice the value found for S h , S h =5.8±0.1 MPa . These results are consistent with the results obtained nearby. They show that hydraulic properties of faults (discontinuities with length larger than 1 km and offsets larger than 100 m ) do not depend simply on the relative orientation of the fault with respect to the principal stress directions.