Kenneth D. Mahrer

Deep-Injection and Closely Monitored Induced Seismicity at Paradox Valley, Colorado

The U.S. Bureau of Reclamation’s Paradox Valley Unit (pvu) extracts aquifer brine from nine shallow wells along the Dolores River, Paradox Valley, in southwestern Colorado and, after treating, high pressure injects the brine 4.3–4.8 km below the surface. pvu injects at rates between ∼800 and ∼1300 L/min. Since 1991, pvu has emplaced over 4 × 106 m3 of fluid and induced more than 4000 surface-recorded seismic events. The events are recorded on the local 15-station Paradox Valley Seismic Network. The induced seismicity at Paradox separates into two distinct source zones: a principle zone (>95% of the events) asymmetrically surrounding the injection well to a maximum radial distance of ∼3 km, and a secondary, ellipsoidal zone, ∼2.5 km long and centered ∼8 km northwest of the injection well. The expansion of these zones has stabilized since mid-1999, about three years after the onset of continuous injection. Within the principal zone, hypocenters align in distinct linear patterns, showing at-depth stratigraphy and the local Wray Mesa fracture and fault system. The primary faults of the Wray Mesa system are aseismic, striking subparallel to the inferred maximum principal stress direction, with one or more faults, probably, acting as fluid conduits to the secondary seismic zone. Individual seismic events, in both zones, do not discernibly correlate with short-term injection parameters; however, a 0.5 km2 region immediately northwest of the injection well responds to long-term, large-scale changes in injection rate and the surpassing of a threshold injection pressure. Focal mechanisms of the induced events are consistent with simple double-couple, strike-slip moments and subhorizontal extension to the northeast. In addition, the fault planes are consistent with principal stress directions determined from borehole breakouts. More than 99.9% of the pvu seismicity is below human detection (∼ M 2.5). However, approximately 15 events have been felt locally, with the largest being a magnitude M 4.3. Because of the M 4.3 and two earlier-felt M ∼3.5 events and injection economics, pvu changed injection strategies three times since 1996. These changes reduced seismicity from ∼1100 events/year to as low as ∼60 events/year.

A review and perspective on far-field hydraulic fracture geometry studies

Abstract A review of the published literature and expert opinions shows that the traditional oil and gas industry paradigm for far-field hydraulic fracture geometry – simple, deterministic fracturing – is changing to include the potential for creating complex fracture geometries. For six decades, the established paradigm has purported the creation of two mirror-image fracture wings radiating from opposite sides of the wellbore with each wing growing into the far-field as a single (deterministic) fracture along a single plane. This review found that the roots and development of this paradigm are based in theory and laboratory studies that were pre-disposed to the bi-wing, deterministic geometry. These studies lacked critical factors (e.g., realistic wellbore-casing descriptions; realistic fluid rheologies; simulated pumping histories; including stoppages and slowdowns; formation heterogeneities, etc.), that can foster the creation of complex fracture geometries. In addition to a historical perspective, this review summaries studies from the last decade that were designed specifically to investigate fracture geometry. The data from these studies show multiple, far-field fracture geometries. These data include (1) recovered cores drilled laterally through the fracturing, (2) mining through the fracturing, (3) treatment-induced (i.e., passive) microseismicity, (4) controlled source seismic exploration, (5) borehole overcores and videos, (6) anomalous treatment pressure histories, and (7) surface tilts. The literature from laboratory simulations, theoretical models, and natural hydraulic fracture analogues support these findings, the creation of multiple fracture geometries in the far-field.

Radio frequency electromagnetic tunnel detection and delineation at the Otay Mesa site

Using three radio frequency electromagnetic field procedures: surface‐to‐surface, borehole‐to‐surface, and borehole‐to‐borehole, we detected and delineated a horizontal, 2 1/2 × 5-ft (3/4 × 1 1/2-m), and 45-ft (14-m) deep tunnel crossing the US‐Mexico border near Otay Mesa east of San Diego, CA. We used continuous monochromatic signals to delineate the tunnel by (1) locating and identifying signature variations in signal transmissivity from the tunnel void in the local rock (sandstone) and (2) inducing and mapping the (secondary) radiation from electrical cabling within and running the length of the tunnel. In the surface‐to‐surface study we evaluated both a single magnetic dipole and a dipole gradiometer receiver and both a magnetic dipole and a grounded long‐wire transmitter operating between 22 kHz to approximately 300 kHz to isolate the induced radiation. We found that both sources operating at approximately 100 kHz gave the best transmitter‐cable coupling, and the nulling effect of the gradiometer wa...

An empirical study of instability and improvement of absorbing boundary conditions for the elastic wave equation

One of the persistent problems with numerical solutions to the elastic wave equation is the finite size of the numerical grid. As with a physical body, the grid boundaries will reflect incident energy. If not eliminated or reduced substantially, these reflections will invade the grid interior and interfere with the desired solution. One method for eliminating reflections is creating a large and/or expanding grid. This method may be impractical since it can be quite costly in both computer time and memory. Another method is making the grid boundary “transparent” to outgoing energy. This method is ideally done by designing absorbing or nonreflecting boundaries which are mathematically equivalent to a one‐way, or outgoing, elastic wave equation only. In practice, an outgoing elastic wave equation is an approximation since the wave equation is not generally separable into outgoing and incoming parts. Two absorbing boundary condition approximations commonly used are those from Reynolds (Reynolds, 1978) and Cla...

Review of the radio frequency (RIM) method and its utilization in near-surface investigations

Electromagnetic subsurface investigation methods, with regard to frequency, span more than 12 orders of magnitude, from millihertz (e.g., complex resistivity) to gigahertz (e.g., ground penetrating radar). The range between kilohertz and low megahertz is the radio frequency band and this, unsurprisingly, is where the radio frequency investigation method (RIM) operates.

A field test of an electrodeless arc discharge, borehole seismic source

Waveforms generated by an impulsive, 1.2 kJ, seven‐conductor wireline electrodeless arc discharge borehole seismic source or sparker at Texaco’s Humble, TX field test site were recorded by three borehole sensor arrays: two free‐hanging hydrophone streamers in in‐line boreholes at 82 m and 170 m from the source well and a grouted, three‐component geophone string in a borehole 110 m from the source well. A repeatability test of the source, consisting of single firings of the source at a rate of 1 firing per 5 s, showed very clean, very strong, Ricker‐like wavelets. Despite a high‐degree of attenuation (exact value of Q is not known), the useful frequency passband of the wavelets was from 200 Hz to 1200 Hz for the data recorded by the 82-m offset hydrophones and 200 Hz to 500 Hz for the 170-m hydrophones. Using 62 single‐firing wavelets recorded in the 82-m offset well gave mean and median crosscorrelations greater than 0.96 with standard deviations less than 0.02. A stack test, consisting of 1, 2, 4, 8, 16,...

Hydraulic fracture height in cased wells

Abstract A new method determines the top and bottom of the hydraulic fracturing in a cased treatment well from the microseismicity induced within the fracturing. The method uses a wall-locking sonde to passively record three-component data over a range of depths within and outside the fracturing immediately following either the fracture treatment or subsequent fluid injection into the fractured formation. The processed data (i.e., the background motion after removing the obvious events) show an anomalous inversion as a function of depth that delineates the fracture height. Specifically, the ratio of the horizontal motion component, H, to the vertical component, Z, inverts subdividing the recording traverse into three regions: those above and below the affected zone defined by H Z and the affected zone with H Z >1 . The hydraulic fracture treatment creates the horizontally elongated, in situ stress-aligned affected zone that is comprised of a dilatant communicating network of new and preexisting fractures, joints, pores, and weaknesses, nucleates from the casing perforations. The zone has anomalously reduced elastic properties (i.e., seismic velocities) and acts as an embedded seismic and hydraulic waveguide with the borehole running through it. Pressure gradients, temperature gradients, and stress recovery within the low-velocity zone induce a pervasive microseismic cloud for several hours after the pressurization. The microsource cloud and the low-velocity zone create the data anomaly and its alignment with the extent of the affected zone. Computer simulations of the in situ setting and recording corroborate the H Z inversion and its interpretation.

An open letter to authors whose native language is not English

Author’s note: Since starting this department, I have been regularly asked to dedicate a column to authors who are struggling primarily because their native language is not English. This is my reply. My help and advice are “fixes” for common weaknesses that I have encountered in manuscripts submitted to Geophysics and other technical journals. I am not implying that all nonnative‐English‐speaking authors are inferior writers. This certainly is not the situation. Many have a superior command of English and are capable, accomplished technical and nontechnical writers.

Some Considerations When Abstracting

My dictionary (Urdang, 1973) defines the abstract as “something that concentrates in itself the essential qualities of anything more extensive or more general, or of several things; essence.” As it applies to writing, Houghton (1975) says, “An abstract can be defined as a summary of the in/ormation in a document.” Although these definitions are clear and direct, they sometimes get muddled or lost during the writing of a manuscript. As a result, the abstract falls short of its function and the manuscript is greatly weakened. The reviewing, editing, and possible publication of the manuscript can be substantially delayed because of its weak and insufficient abstract. To help expedite matters, I offer the following comments concerning abstracts. I offer these for your benefit, for the benefit of your readers, and for the benefit of ditors, viewers, and publication staffs everywhere.

Injecting Brine and Inducing Seismicity at the World's Deepest Injection Well, Paradox Valley, Southwest Colorado

Publisher Summary This chapter describes the injecting of brine and inducing of seismicity at the worlds deepest injection well of Paradox Valley, Southwest Colorado. Beneath a remote section of Southwestern Colorado, the Paradox Valley unit (PVU), a U. S. Bureau of Reclamation Project injects high-pressure waste brine injectate into a ∼4.3 km deep limestone formation in order to reduce salinity in the Colorado River. The study of the Paradox Valley–Dolores River brine seepage problem began in 1971, the project operated more than 90 shallow (12–21 m) brine extraction, monitoring, and test wells bordering the Dolores River. Paradox Valley seismic network (PVSN) identifies some characteristics of fluid flow by mapping earthquake locations. Using 346 earthquakes, seismic-source characteristics were calculated, which suggest mainly strike-slip motion with the minimum principal stress pointing northeast and subhorizontally. In addition to the geophysically interesting results, this project shows that by coordinating seismic data with injection operations, adjusted brine disposal to maintain economic viability while reducing the proclivity for felt earthquakes are obtained.