An integrated geomechanical, microseismic, and laboratory study of the Bakken formation [electronic resource]
Keywords: Geophysics.

This thesis integrates geomechanics, microseismic, and laboratory studies to investigate the role of preexisting fractures and faults in development of unconventional reservoirs. This study provides a new method for delineation of reservoir structure using microseismic data with support from geomechanics and laboratory measurement. The interpretation from this study can be applied to address some of the practical questions in highly fractured reservoirs, including, but not limited to, well design and hydraulic fracture design. The implication provides insights for future strategic development of fractured and faulted unconventional reservoirs. The first part of this thesis focuses on the production of shale oil from the Bakken Formation and consists of three approaches (Chapters 2-5). A geomechanical model shows the current stress state to be characterized by a NF/SS regime, with SHmax orientation ~N45°E. The microseismic events were recorded in six vertical observation wells during hydraulic fracturing of parallel wells X and Z, and are independently processed by two contractors. Both sets of event locations show three characteristics: First, rather than occurring in proximity to the stages being pressurized, many of the events occur along the length of well Y, a parallel well located between X and Z that had been in production for ~2.5 years at the time X and Z were stimulated. Second, relatively few fracturing stages are associated with an elongated cloud of events trending in the direction of SHmax as is commonly observed during hydraulic fracturing. Instead, the microseismic events in a number of stages appear to trend ~N75°E, about 30° from the direction of SHmax. Earthquake focal plane mechanisms confirm slip on faults with this orientation. Finally, the microseismic events are clustered at two distinct depths, one near the depth of the well being pressurized in the Middle Bakken Formation and the other ~800 ft above in the Mission Canyon Formation. Approximately 60% of the microseismic events from stage 2 exhibit similar waveforms that are occurring in adjacent multiplet clusters. Two multiplet clusters are relocated using the double-difference technique, and the relocated hypocenters are more clustered, delineating reservoir structures that are consistent with the focal plane mechanisms. We argue that all three of these patterns result from the hydraulic stimulation being dominated by flow channeling along preexisting faults. Combined analysis of hypocenter locations, focal plane mechanisms, fault slip, and 3D seismic data indicate that steeply-dipping N75°E striking faults with a combination of normal and strike-slip movement were being stimulated during hydraulic fracturing. A simple geomechanical analysis was carried out to illustrate how this occurred in the context of the current stress field, pore pressure and depletion in the vicinity of well Y during the 2.5 years of production prior to stimulation of wells X and Z. Laboratory measurements of 6 pairs of core samples from the reservoir suggest that the time dependent deformation of these rocks can be characterized by a power-law constitutive law. The constitutive parameters determined from 3-hour creep measurements follow a range and trend similar to those of samples from other shale gas reservoirs. By applying the viscous relaxation model, the differential horizontal stresses are estimated from geophysical logs. With the NF/SS faulting regime in the Bakken and an assumption of a constant faulting regime in sedimentary lithology, a continuous principal stress profile is estimated. The least horizontal stress magnitude suggests that the Lodgepole and Three Forks Formations adjacent to the Bakken Formation are not acting as frac barriers during hydraulic fracture. Thus, the asymmetric distribution of the microseismicity suggests the out-of-zone microseismic events are associated with preexisting fractures and faults rather than purely hydraulic fracturing growth. Moreover, the pore pressure perturbation required for slip is consistent with the occurrence of microseismicity, where events occur at depths that require less elevated pore pressure. Brittleness determined from elastic properties is considered, but it cannot explain the microseismicity. This is because brittleness is not an intrinsic rock property and cannot be characterized in a consistent way. Therefore, applying brittleness for locating the hydraulic fracturing sweet spot needs to be done cautiously. The second part of this thesis focuses on the feasibility of injecting CO2 to enhance coalbed methane production, as well as the capacity for long term CO2 storage in coalbeds of the Power River Basin, Wyoming. Laboratory measurements are performed to study the adsorption/desorption, mechanical, and transport properties of coal with gas saturation of He, N2, CH4 and CO2, at either increasing pore pressure or increasing effective stress. Results suggest that coal from the PRB has strong adsorption capacity for CO2, and this strong adsorption is stable unless the pore pressure drops below 2 MPa. Also, CO2-induced swelling will cause permeability loss, but the loss is less than one order of magnitude. Laboratory results indicate that the coal seam in the study area might be a good candidate for an ECBM and CO2 sequestration project. However, its feasibility still depends on future numerical modeling predictions, and the results reported in this study can be applied for further modeling work.