ABSTRACT:

Pressuremeter testing (PMT) is a formation test that consists of inflating a cylindrical packer inside a borehole, while measuring radial deformation or injected volume as a function of packer pressure. Measurement of packer pressure and injected volume provides the required data to infer formation shear modulus (G), if the packer stiffness is known. PMT is a well-established technique in soil mechanics and has been extensively used in mining and civil engineering. However, its application to rock mechanics has so far remained limited to small diameter packers and very shallow boreholes, mainly due to a lack of sufficiently stiff packer systems. PMT could provide an in-situ measurement of G on the meter length scale, for typical field scale packers. Moreover, if the Poisson ratio is known or estimated a priori, then the Young Modulus (E) can be obtained without resorting to sample coring and laboratory testing. To gain insight into PMT and its potential applicability to deep downhole formations, we explore PMT at the laboratory scale. We present results of experiments that use a stiff, custom-built packer to infer G from the applied pressure and injected fluid volume. Packer calibration was performed in stainless steel cylinder with known elastic properties and geometry. And, a variety of analog samples, such as Lexan and Plexiglass were tested. We explored the influence of packer aspect ratio L/D (by varying packer length, L while maintaining packer diameter D) on the inferred shear moduli. Our lab-scale PMT measurements and the interpretation framework provide precise estimates of G, while the accuracy increases with increasing packer length. Further parameter space exploration, including rock samples, would provide additional insight into the approaches of static elastic properties assessment using PMT. Our experimental results support the feasibility of PMT for inferring G and E. Additionally, the experimental results provide an indication of possible challenges that field scale trials might encounter.

1 Introduction

Pressuremeter testing (PMT) is a formation test that consists of inflating a cylindrical packer inside a borehole, while measuring the radial deformation or injected volume as a function of packer pressure. Knowing the stiffness of the packer, PMT provides in situ static shear modulus, G, on the meter length scale, for typical field scale packers (Figure 1). Furthermore, if the Poisson ratio is known or estimated a priori, then the Young Modulus can also be obtained, without resorting to sample coring and laboratory tests.

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