Abstract
Fundamental testing of cement-to-formation bonding under stress is needed to provide better input to models quantifying the risk of debonding as a function of evolving stress conditions. This is particularly important for long term CO2 sequestration, where sealing of the injection wells has to last for centuries and where stresses are probably going to evolve with time. A new experimental technique is here proposed, whereby composite rock/cement plugs are created under stress and temperature. Different interface angles permit testing the interface shear strength and friction angle under unconfined compression. Direct tension tests are also performed in order to get insight on tensile bond strength. Results so far show that even for competent Berea sandstone, tensile failure seems preferentially to occur in the rock part and not at the interface nor in the cement part of the tested plug. Similarly, in unconfined compression, failure seems to occur primarily in the sandstone part of the plug.
1. INTRODUCTION
Recent work has identified the well cement-to-rock interface as the weakest link, when it comes to leakage from wells [1, 2]. This leakage occurs for many older production wells, not necessarily from the target production reservoir, but often from higher, pressurized permeable layers [3]. Several experimental techniques have been used to investigate cement integrity: ring geometry under HPHT conditions [4], "dog bone" geometry cement specimens in direct uniaxial tensile tests [5], unconfined compression testing in a load frame equipped with axial and radial strain gauges to assess unconfined compressive strength (UCS) and elastic parameters [6] on a cement cylindrical specimen, cement bond shear strength testing where cement is pushed out of a rock borehole while measuring the applied force on the cement [7], direct shear testing (of concrete) in shear box [8], pure shear testing in load frame with modified, split anvils whereby only half of the tested specimen is compresses, as well as triaxial testing of concentric cylindrical specimens including casing, cement and rock [9].
