A CYCLIC HYDRAULIC JACKING TEST TO DETERMINE THE IN-SITU STRESS NORMAL TO A FRACTURE

Hydraulic fracturing and cyclic hydraulic jacking (step-pressure) test s were used to determine the virgin stress normal to six natural joint s in granitic rocks. A numerical model was used to analyse the coupled problem of fluid flow and rock/fracture deformation during the field tests. Fracture closure and virgin normal stress was determined from t he zero-flow pressure in a well pressure-well flow diagram. The cyclic hydraulic jacking tests gave a zero-flow pressure in close agreement with both the shut-in pressure and the weight of the overburden rock f or four out of six tested joints. For the two remaining joints the vir gin normal stress could be estimated by a linear extrapolation to zero -flow rate. The zero-flow pressure is clearly defined and is insensiti ve to the transmissivity and non-linear normal stiffness of the fractu re near the wellbore. Cyclic hydraulic jacking is therefore an alterna tive to traditional shut-in analysis when shut-in pressure is difficul t to determine. Copyright (C) 1996 Elsevier Science Ltd