Geomechanical aspects of induced microseismicity during CO2 injection in Illinois Basin

Carbon sequestration activities are increasing in a global effort to mitigate the effects of greenhouse gas emissions on the climate. Injection of wastewater and oil-field fluids is known to induce seismic activity. This makes it important to understand how that risk relates to CO2 injection. Injection of supercritical CO2 into the Cambrian Mt. Simon sandstone in Illinois Basin induced microseismicity that was observed below the reservoir, primarily in the Precambrian crystalline basement. Geomechanical and flow properties of rock samples from the involved formations were measured in the laboratory and compared with geophysical log data and petrographic analysis. The controlling factors for induced microseismicity in the basement seem to be the hydraulic connection between the reservoir and basement rock and reactivation of pre-existing faults or fractures in the basement. Additionally, the presence of a laterally continuous low-permeability layer between reservoir and basement may have prevented downward migration of pore pressure and reactivation of critically stressed planes of weakness in the basement. Results of the geomechanical characterization of this intermediate layer indicate that it may act as an effective barrier for fluid penetration into the basement and that induced microseismicity is likely to be controlled by the pre-existing system of faults. This is because the intact material is not expected to fail under the reservoir stress conditions.

Quantitative measurement on coal components through the interpretation model of geophysical log: A case study from the Qaidam Basin, NW China

The Iqe coalfield is one of the most significant coal production bases in the Qaidam Basin. Over the last few decades, core explorations have targeted the Dameigou Formation for No. 7 coal seam (M7). Although many M7 coal samples have been analyzed for coal components in the laboratory, the systematic understanding of the components and changes of coal in the whole Iqe coalfield is still inadequate. In this study, we focus on building log interpretation models to accurately calculate the content of coal components of M7, including ash yield (Aad), volatile matter (Vdaf), fixed carbon (FCad), and moisture (Mad). Multiple regression analysis and statistical method, combined with the rock volume model, were used to establish log interpretation models of coal components. A total of 28 coal samples from ZK1, ZK2, ZK11-5, ZK23-4, and ZK36-9 wells in the Iqe coalfield were involved in the modeling, as well as well-logs parameters, such as radioactivity (GR), compensation density (DEN), acoustic (AC), and resistivity (RLLD). According to sensitivity analysis, the fitted Aad and Vdaf contents of M7 increase with the increasing of DEN and GR values, whereas the FCad content shows the opposite way. Furthermore, the positive relationship between Aad and Vdaf ( R2 = 0.59) and the negative relationship between Aad and FCad ( R2 = 0.92) as well as Vdaf and FCad ( R2 = 0.69) indicate that Aad is a key factor in coal and should be prior determined. Finally, based on the multiple regression analysis and rock volume model, we proposed log interpretation models for M7 coal components in the Iqe coalfield, these models have been examined successfully by the case studies from the same coalfield and will provide new insights into the application of geophysical log parameters for coal quality evaluation.