ALBERT

Description: We have developed stimulation tests of a model discrete fracture network (DFN) in the Marcellus Shale reservoir, Morgantown, West Virginia. The microseismic response observed from the modeled stage is characteristic of that observed in several stages along the length of the horizontal well, so that the workflow developed in this paper can be easily extended to other stages in this and other Marcellus Shale wells. The model DFN is designed using log data, including fracture image logs from vertical pilot and horizontal wells. Data from these wells provide geomechanical properties, fracture trend and intensity, and stress orientation. Microseismic cluster trends provide additional constraints on geomechanical model development. Results from stimulation tests are used to modify the reservoir DFN and geomechanical model. Modifications ensure consistency with borehole observations. Fractures observed along the length of the horizontal well consist predominantly of one set, whereas two sets are observed in the vertical pilot well. These two sets are required in the model DFN to reproduce the stimulation trend inferred from microseismic data. Northeast asymmetry in the microseismicity associated with hydraulic fracture treatment is interpreted to result from a horizontal drop of [Formula: see text] toward a previously drilled well. The asymmetry is interpreted to result from stress reduction associated with treatment of an earlier parallel well, the presence of a cross-strike structure parallel to the well, or a combination of the two. Limited downward growth, inferred from the microseismic response, required an increase of the minimum stress in model strata underlying the Marcellus.

Description: Microseismic monitoring by downhole geophones, surface seismic, fiber-optic distributed acoustic sensing (DAS), and distributed temperature sensing (DTS) observations were made during the hydraulic fracture stimulation of the MIP-3H well in the Marcellus Shale in northern West Virginia. DAS and DTS data measure the fiber strain and temperature, respectively, along a fiber-optic cable cemented behind the casing of the well. The presence of long-period long-duration (LPLD) events is evaluated in the borehole geophones, DAS data, and surface seismic data of one of the MIP-3H stimulated stages. LPLD events are generally overlooked during the conventional processing of microseismic data, but they represent significant nonbrittle deformation produced during hydraulic fracture stimulation. In a single stage that was examined, 160 preexisting fractures and two faults of suboptimal orientation are noted in the image logs. We identified two low-frequency ([Formula: see text]) events of large temporal duration (tens of seconds) by comparing the surface seismic data, borehole geophone data, and DAS amplitude spectra of one of the MIP-3H stages. Spectrograms of DAS traces in time and depth reveal that the first low-frequency event might be an injection noise that has footprints on all DAS channels above the stimulated stage. However, the surface seismic array indicates an LPLD event concurrent with the first low-frequency event on DAS. The second LPLD event on DAS data and surface seismic data is related to a local deformation and does not have footprints on all DAS channels. The interpreted events have duration less than 100 s with frequencies concentrated below 10 Hz, and are accompanied by microseismic events.

Description: The Tensleep oil reservoir at Teapot Dome, Wyoming, USA, is a naturally fractured tight sandstone reservoir that has been considered for carbon dioxide enhanced oil recovery ([Formula: see text]-EOR) and sequestration. Interpretation of open fractures identified in wireline image logs from the field suggests that the reservoir fracture network is dominated by early formed structural hinge-oblique fractures with interconnectivity enhanced by hinge-parallel and hinge-perpendicular fracture sets. Previous studies show that 3D seismic scale discontinuity attributes are dominated by more recent hinge-parallel and strike slip trends. The most negative curvature attribute that we used highlights concave features attributed to subtle traveltime delay through fracture zones and small faults or flexures associated with the fracture swarms. The poststack discontinuity extraction workflow incorporated seismic spectral blueing (SSB) to enhance the resolution of the seismic data. The SSB process is followed by computation of the short-wavelength most negative curvature. Subsequently, the minimum similarity attribute is applied to accentuate regions with minimum similarity of curvature. An edge-illumination process is then applied to the minimum similarity of the most negative curvature output. Discontinuities extracted through edge illumination locate regions of minimal similarity in curvature along fracture zones or small fault boundaries. This workflow enhances hinge-oblique discontinuities without azimuthal filtering and provides a fracture intensity attribute, which is used as an input to distribute the fracture intensity through the model discrete fracture network. Qualitative correlation of production data to extracted discontinuities suggests that wells located on hinge-oblique discontinuities are more productive than other wells in the field.

Description: The data we analyzed are from a Marcellus Shale gas field in Greene County, southwestern Pennsylvania. We first investigated the relationship between microseismic event trends and discontinuities extracted from 3D seismic data and their relationship to [Formula: see text]. This analysis was followed by an examination of the relationship of cumulative gas production to radiated energy, stimulated reservoir volume (SRV), and energy density (ED). We have determined that microseismic event trends observed in multiwell hydraulic fracture treatments were similar to the trends of interpreted small faults and fracture zones extracted from 3D seismic coverage of the area. Hydraulic fracture treatments conducted in six laterals produced clusters of microseismic events with an average trend of N51°E and, to a more limited extent, N56°W. The N51°E microseismic event trend coincided closely with the average N52°E trend of interpreted minor faults and fracture zones extracted from the 3D seismic data. That relationship suggested that microseismic events form through reactivation of old faults and fracture zones in response to an easterly trending [Formula: see text]. We also found that variations in gas production correlated with variations in radiated microseismic energy ([Formula: see text] of 0.985), SRV ([Formula: see text] of 0.974), and ED ([Formula: see text] of 0.989). SRV is a measure of the volume of space occupied by induced microseismicity, whereas energy release per unit volume (ED) can be directly related to rupture area created through hydraulic fracture stimulation. We suggest that ED serves as a better estimator of production potential in unconventional shale reservoirs.