| Popis: |
ABSTRACT: Pressuremeter testing (PMT) is a formation test that consists of inflating a packer downhole, while measuring packer pressure exerted on the borehole wall as a function of injected fluid volume. Provided the stiffness of the packer is known and large enough compared to that of the formation, changes in packer pressure associated with changes in injected fluid volume provide a direct measurement of formation stiffness and, along with the relevant length-scales, produces an in situ static formation shear modulus, a very important parameter for Mechanical Earth Model calibration. Here, we report on the first field-scale campaign of PMTs in deep boreholes performed using a wireline formation tester (WFT) tool. These PMT measurements were carried out alongside a stress measurement campaign, to fully characterize potential sites for a deep geological repository for radioactive waste in Switzerland. The initial field trials highlighted challenges in calibrating packer stiffness, key in inferring formation stiffness. We overcame the challenges in packer characterization by adopting a true single packer configuration and by performing reference PMT measurements in casing and formations of known stiffness. Then, we performed PMT inflation and deflation cycles to infer in situ static shear moduli at every station tested. PMT-derived static shear moduli results are consistent with static shear moduli derived from sonic logs using independent dynamic-to-static elastic moduli transformations. The precision of the in situ PMT static shear moduli increases as the stiffness of the measuring system, WFT, increases with respect to formation stiffness. These first results show the viability of in situ PMT in deep boreholes with a wireline formation tester tool as it can be performed at multiple depths in a single run, in a time-efficient manner, and in combination with micro-hydraulic and sleeve fracturing stress tests for an integral approach to in situ geomechanical assessment. |
