April 8, 2021 “Luncheon” Webinar- Robert Brune

Robert (Bob) Brune, Geophysical Consultant, Colorado.

Abstract:

Frac monitoring pilot/demonstration surveys have been acquired in the Marcellus shale (dry gas) using fundamentally new techniques.  Some aspects of these surveys include:

• Primarily utilize seismic shear body waves and Krauklis (fluid oscillation) interface waves, both of which are sensitive to frac’s, have good resolution, and have good signal strength.
• Use controlled compressional (P) and shear (S) seismic wave sources (Vibroseis on surface, including dipole configurations), such that measurements are possible at any time (“Life-of-Frac”), i.e., pre-pumping, during pumping, ‘soak’, flowback, and/or production.
• Compact, efficient, and economical field deployment, using surface multi-component geophones for measurements, with no well intervention needed.
• Simple differential data analysis workflows between baseline and monitor survey(s), with mappable results, suitable for real-time guidance of pumping.
• The field data acquisition and processing use existing equipment and software, but in novel ways.

Krauklis waves are a type of seismic wave that propagate along the frac face boundary between rock and fluid.  They exhibit several particular characteristics, as seen in field data from these surveys.  These include emergent onset and high Q resonance.  Frac size (tip-to-tip) relates to resonant frequency.  Field data imply small individual frac sizes, analogous to small frac planes seen in passive microseismic source mechanism inversions.  Krauklis waves have slow propagation velocity, with dispersion.  Krauklis waves are excited by P, and particularly S, body waves; Krauklis waves then re-radiate body waves back to the surface.  Krauklis waves have spatially evanescent decay away from the frac’s.  Krauklis waves depend strongly on fluid characteristics as well as frac geometry.  Weak Krauklis effects are also seen in some baseline (pre-pumping) data, interpreted as due to a priori natural fractures.

Seismic shear velocity changes from pre- to post-pumping are seen in the data.  Changes may be interpreted based on dominant azimuth Horizontal Transverse Isotropy (HTI) type models, or on random azimuth type models (e.g., Berryman).  There are four travel times times, or velocities, that are pertinent:  ‘fast’ and ‘slow’, for both the baseline survey and also for monitor survey(s) at any time in the Life-Of-Frac.

The data acquired to date show Krauklis wave changes at distances up to 2,000 and 3,000 ft from treatment wells, at times of hours to days after pumping.  Depletion / stress shadow type effects are seen, due to production on an adjacent pad.  Krauklis data (and corroborating passive surface microseismic data) show one treatment well with frac’ing dominantly on only one side.  Near another treatment well, up to 20 msec. time shifts are seen in data post-pumping vs pre-pumping, which are significantly above a detectability threshold of a few msec.  There is potential to quantify dominant vs multiple (random?) frac azimuths.

There are various additional topics that are pertinent for interpretation of this type of data.  These include Krauklis wave frac tip radiation patterns; high pressures at frac tips; Krauklis waveguide phase inversions; dipole shear sources; mutual admittance for heterodyne sources for very low frequencies (for larger frac’s); velocity changes for random azimuth frac’s; frac compliance analysis and frac density effects; possible significance of bulk viscosity for methane; reservoir depletion effects; stress shadowing; velocity hysteresis vs relative saturation; etc.

These frac monitoring techniques are presented here as a significant step forward, available today to guide improved frac completions.  These pilot surveys showed actionable results with significant economic impact:  well sequencing, pumping rates, volumes, proppant load, and additives can be impacted in near real time.  These data could have further significant impact on planning, such as for well, stage, and cluster spacing.  This technology has the potential to monitor the evolution of the effective SRV through a soak period, flowback, and initial production.  There are also future technology development paths for enhancements, refinements, and … likely surprises!

However, significant commercial issues are problematic:  this is new technology, it’s unfamiliar to staff, frac plans are not readily altered, …  At this time, this new technology is effectively an ‘orphan’.  No E&P Operator, nor any Service Company, is presently an advocate for this technology.

Data Montage:  Upper left inset is a trace with an emergent onset, high Q, high amplitude Krauklis wave.  Upper map is Krauklis wave post- vs pre-pumping; treatment well is long red line; 12.5 by 12.5 m subsurface bins within the surveyed area.  Lower left is a shear body wave time shift, post- vs pre-pumping, for near vertical propagation through the SRV, for a single Source, Receiver polarization, but with a composite of S-to-R azimuths, showing ~20 msec time shift.  Lower right is a surface receiver line for pre- vs post-pumping, showing emergent, high Q resonant, high amplitude, slow propagation velocity Krauklis wave in the post-pumping data.

Robert (Bob) Brune is a Geophysical Consultant with a broad technical background in Geophysics and Oil & Gas.  In recent years he has been focused on various fundamental technology issues, including rotational seismic, HSE issues, elastic wave, seismic acquisition technology, as well as fracture monitoring.  He has approached frac monitoring technology from several vantage points, based on his background and experience.

Rober has experience in the oil and gas industry includes work in Exploration (including as an Exploration Mgr. for Sohio); Production (Mgr. of Regional Field Development in Alaska for BP); and in Technology (Mgr. of Geophysical Technical Services and R&D for Sohio), as well as experience in the service industry including work at GSI and at TGS (President, Offshore; and Chief Geophysicist), as well as consulting work for various companies, particularly in S. America.  Robert also has experience working at the U.S.G.S. in Menlo Park, CA with seismic sources.

Robert has a B.S. in Geology (U. of Missouri at Rolla; National Merit Scholar; graduated at 19); an M.S. in Geophysics (Stanford University; NSF scholarship; inaugural group of SEP consortium); and an M.S. in Computer Systems (U. of Denver).  His Petroleum Engineering educational background is from Colo. School of Mines.