Rock strength under true triaxial loading, seismotectonics of northern South America and geomechanics and coal bed methane production in the Powder River basin

In this thesis, I present three research projects in the areas of rock mechanics, seismotectonics and geomechanics. In the first study, I examined seven failure criteria by comparing them to published polyaxial test data for five rock types at a variety of stress states. I demonstrated that the way a failure criterion fits the data would depend on the type of failure criterion and on the intermediate principal stress (sigma2 ) dependence of the rock in question. Additionally, if only data from triaxial tests are available, it is possible to incorporate the influence of sigma2 on failure by using a polyaxial failure criterion.; In the second study, the combination of in-situ stress, neotectonic, and GPS data was used to generate an integrated stress map of northern South America, which shows that the stress field in the region varies systematically in orientation and relative magnitude. This change in the stress field appears to reflect both relative plate motions and the negative buoyancy of the subducted Caribbean slab.; In the third project I evaluated the geomechanics and wellbore completion methods of Coalbed Methane (CBM) wells in the Powder River basin. It has been identified that water-enhancement procedures after under-reaming the coal result in hydrofrac'ing of the coal (and possibly the adjacent strata), which can cause excess CBM water production and inefficient depressurization of coals. Water-enhancement tests data from ∼550 wells were used to obtain the magnitude of the least principal stress, which indicates the orientation of the hydrofracs. Vertical fractures (VF), and horizontal fractures (HF) occur in the basin. Water production from wells with HF is minimal and excessive water production is always associated to wells with VF. In wells with high water production, gas production is significantly delayed compared to wells with VF and low water production, which are excellent gas producers. Since wells with HF are usually poor gas producers, it is suggested that such wells are hydrofrac'ed (and propped) to enhance gas production. Additionally, in areas of vertical fracture growth the injection during the water-enhancement tests should be restricted, so propagation of induced fractures into the overlying formations is prevented.