Vladimir Grechka

We examine the feasibility of high-resolution microseismic imaging of unconventional reservoirs. Because downhole microseismic data have almost an order of magnitude higher frequencies than seismic reflection data, a comparable increase in image resolution might be expected, bringing potential resolution of microseismic images to a few meters. We demonstrate that such a resolution is, indeed, achievable and present two case studies illustrating the reservoir features that can be imaged. Our first example, constructed with the P-waves recorded in the Woodford Field, reveals internal fabric of the Woodford reservoir and formations surrounding it, suggesting the possibility of discriminating the stimulated and unstimulated zones of the Lower Woodford shale. Encouraged by those results, in our second example, this time from the Bakken Field, we use the shearwaves, more sensitive to uids than the P-waves, to find out whether hydraulic fractures themselves could be imaged. Our seismic volume contains peculiar geobodies, growing precisely from perforation holes spaced at a 40 ft interval in a treatment well landed in the Middle Bakken, the geobodies penetrating through the Lower Bakken shale reservoir and terminating at the top of the Three Forks dolomites. While our interpretation of the extracted geobodies as hydraulic fractures remains an interpretation, the remarkably high resolution of seismic images obtained in both case studies is unquestionable. Presentation Date: Wednesday, October 17, 2018 Start Time: 1:50:00 PM Location: 208A (Anaheim Convention Center) Presentation Type: Oral

The recent downturn in the oil industry hit geophysicists especially hard. As entire geophysics departments were shut down, U.S. service companies reported losing 38% of their workforce by March 2016¹. Layoffs across the U.S. oil industry as a whole have been much milder though, registering only a relatively modest decline of 12% from the peak employment in October 2014 to its lowest point in January 2017². As the industry began to recover in 2017, the hiring of geophysicists has been slow; for example, the website http://www.rigzone.com/oil/jobs/search/ accessed on 8 September 2017, listed four geophysics job openings out of a total of 3755 oil and gas jobs — a meager 0.1%. Such a statistic makes one wonder why the oil industry, having shed so many geophysics jobs, is in no hurry to replenish them.

We examine the feasibility of high-resolution microseismic imaging of unconventional reservoirs. Given frequencies in typical downhole microseismic data that are almost an order of magnitude higher than in seismic reflection data, a comparable increase in image resolution might be expected, bringing potential resolution of downhole microseismic images to a few meters. We demonstrate that such a resolution is indeed achievable and present two case studies illustrating the reservoir features that can be imaged. Our first example, constructed with P-waves recorded in the Woodford Field, reveals internal fabric of the Woodford reservoir and formations surrounding it, allowing us to discriminate the stimulated and unstimulated zones of the Lower Woodford Shale. Encouraged by those results, in our second example, this time from the Bakken Field, we use the slow shear waves, which are more sensitive to fluids than P-waves, to find out whether hydraulic fractures themselves could be imaged. Our seismic volume contains peculiar geobodies, growing precisely from perforation holes spaced at a 40 ft interval in a treatment well landed in the Middle Bakken. These geobodies penetrate through the Lower Bakken Shale reservoir and terminate at the top of the Three Forks dolomites. While our interpretation of the extracted geobodies as hydraulic fractures remains an interpretation, the remarkably high resolution of seismic images obtained in both case studies is unquestionable.

Microseismic data acquired in a single observation well parallel to the axis of rotational symmetry of surrounding rocks — typically, in a vertical well drilled through a horizontally layered isotropic or vertically transversely isotropic formation — cannot be uniquely inverted for six independent components comprising the full seismic moment tensor. To constrain the inversion for such a survey geometry and medium symmetry, one might assume certain physical properties of seismic sources, the properties relating otherwise independent moment components to each other, regularizing moment tensor inversion, and helping reduce its ambiguity. Our paper examines one possibility of this kind: the assumption of a tensile fracture, rupturing the focal region along a plane of its greatest weakness. Mathematical formulation of inversion of single-well microseismic records for the parameters of tensile fractures reveals that the true solution, always recoverable from properly acquired data, might be accompanied by two spurious solutions. The analysis of those solutions leads to a criterion that, although not perfect, makes it possible to select the correct solution for the majority of elastic models. After being tested on synthetic, our methodology is applied to a field data set recorded with multiple vertical downhole arrays, demonstrating that the results of dual-well moment tensor inversion can be replicated with single-well data.

A recently proposed paraxial ray-based technique for relative location of microseismicity is extended to accommodate several master events with respect to which other events, termed the slaves, are located. The multi-master extension addresses two issues inherent for the existing single-master algorithm: a gradual decrease of its accuracy with the distance from the master and less than satisfactory performance in the presence of strong velocity variations. Those deficiencies are handled by applying an improved paraxial traveltime formula, exact in homogeneous elliptically anisotropic media, and by distributing multiple masters in the subsurface to sample its heterogeneity. The contributions from different master events to the hypocenter of a given slave are automatically weighted to enhance the influence of adjacent masters, ensuring the precise slave location, and to suppress the influence of distant ones, tending to increase the slave-location errors. Tests of the multi-master relative event-location method on synthetic and field microseismic data demonstrate its precision and flexibility, as well as applicability to both surface and downhole microseismic geometries. Presentation Date: Monday, October 17, 2016 Start Time: 3:45:00 PM Location: 144/145 Presentation Type: ORAL

A recently proposed paraxial ray-based technique for relative location of microseismicity is extended to accommodate several master events with respect to which other events, termed the slaves, are located. The multi-master extension addresses two issues inherent for the existing single-master algorithm: a gradual decrease of its accuracy with the distance from the master and less than satisfactory performance in the presence of strong velocity heterogeneity. Those deficiencies are handled by applying an improved paraxial traveltime formula, exact in homogeneous elliptically anisotropic media, and by distributing masters in the subsurface to sample its heterogeneity. The contributions of different master events to the hypocenter of a given slave are automatically weighted to enhance the influence of adjacent masters, ensuring the precise slave location, and to suppress distant ones, tending to increase the slave-location errors. Tests of the multi-master relative event-location method on synthetic and field microseismic data demonstrate its precision and flexibility as well as applicability to both surface and downhole microseismic geometries.