W. Scott Phillips

Hydraulic stimulation of natural fractures as revealed by induced microearthquakes, Carthage Cotton Valley gas field, east Texas

We produced a high‐resolution microseismic image of a hydraulic fracture stimulation in the Carthage Cotton Valley gas field of east Texas. We improved the precision of microseismic event locations four‐fold over initial locations by manually repicking the traveltimes in a spatial sequence, allowing us to visually correlate waveforms of adjacent sources. The new locations show vertical containment within individual, targeted sands, suggesting little or no hydraulic communication between the discrete perforation intervals simultaneously treated within an 80‐m section. Treatment (i.e., fracture‐zone) lengths inferred from event locations are about 200 m greater at the shallow perforation intervals than at the deeper intervals. The highest quality locations indicate fracture‐zone widths as narrow as 6 m. Similarity of adjacent‐source waveforms, along with systematic changes of phase amplitude ratios and polarities, indicate fairly uniform source mechanisms (fracture plane orientation and sense of slip) over ...

Induced Microearthquake Patterns in Hydrocarbon and Geothermal Reservoirs: Six Case Studies

Abstract — The injection or production of fluids can induce microseismic events in hydrocarbon and geothermal reservoirs. By deploying sensors downhole, data sets have been collected that consist of a few hundred to well over 10,000 induced events. We find that most induced events cluster into well-defined geometrical patterns. In many cases, we must apply high-precision, relative location techniques to observe these patterns. At three sedimentary sites, thin horizontal strands of activity are commonly found within the location patterns. We believe this reflects fracture containment between stratigraphic layers of differing mechanical properties or states of stress. At a massive carbonate and two crystalline sites, combinations of linear and planar features indicate networks of intersecting fractures and allow us to infer positions of aseismic fractures through their influence on the location patterns. In addition, the fine-scale seismicity patterns often evolve systematically with time. At sedimentary sites, migration of seismicity toward the injection point has been observed and may result from slip-induced stress along fractures that initially have little resolved shear. In such cases, triggering events may be critical to generate high levels of seismic activity. At one crystalline site, the early occurrence of linear features that traverse planes of activity indicate permeable zones and possible flow paths within fractures. We hope the continued development of microseismic techniques and refinement of conceptual models will further increase our understanding of fluid behavior and lead to improved resource management in fractured reservoirs.

Detailed joint structure in a geothermal reservoir from studies of induced microearthquake clusters

Microearthquake clusters form distinct, planar patterns within five study regions of a geothermal reservoir undergoing hydraulic fracturing at Fenton Hill, New Mexico. The patterns define individual, slipping joint surfaces of dimension 40–120 m, containing 80–150 events each. Sharp, straight edges truncate the clusters; we interpret these as marking intersections with aseismic joints. Each edge orientation is consistent with an intersection between the active joint and a plane oriented parallel to one of the other clusters we identify. Therefore it appears that cluster shapes constrain the geometry of seismic and aseismic joints; both could be important components of the fluid-flow network. The distribution of inferred slip plane orientations is consistent with but fails to provide sufficient constraint to differentiate conclusively between two, very different, stress field estimates, one measured using pressurization and wellbore breakouts, the other using focal mechanisms of the largest microearthquakes. An impermeable joint model, requiring pore pressure in excess of the normal stress on a joint before slip can occur, was inconsistent with many of the inferred slip plane orientations. The high-quality locations were possible because events from the same cluster generated nearly similar waveforms, permitting the precise determination of relative arrival times. Standard deviations of arrival-time residuals fall between 0.1 and 1.1 ms for these clusters. Major axes and aspect ratios of the 90% confidence ellipsoids range from 6 to 28 m and 1.5 to 8, respectively. Small events dominate the seismic energy release and thoroughly populate the identified, active joints, allowing the hypocenters to reflect details of the joint structure. To further investigate the reservoir structure, we applied a source-array, slant-stack technique to waveforms from the well-located clusters, yielding directions that scattered energy left each cluster. By studying paths of scattered waves we expected to pinpoint impedance contrasts that might have indicated concentrations of fluid-filled joints. However, results show that scattered energy in the S wave coda left the source region in the same direction as the direct S wave. Direct waves may have excited borehole tube waves that became trapped in the vicinity of the geophone tool, overwhelming any energy scattered from the reservoir.

Traveltime tomography: A comparison of popular methods

Noisy or inconsistent traveltime data yielded tomographic images that contain geologically unrealistic fluctuations. In addition to diverting attention away from structural patterns, these high‐wavenumber fluctuations can generate shadow zones and caustics that destabilize iterative solution schemes requiring ray tracing. We evaluated the performance of a number of popular methods that have been designed to reduce this effect, using synthetic crosswell data containing Gaussian noise. Quantitative comparisons between tomography methods were based on the misfit with the true model, solution stability under different sets of noise of the same level, and resolution‐covariance relationships. Other important factors included versatility and simplicity. Versatility is the ability to treat data with a wide range of noise levels as well as data generated by different structures. Simplicity is characterized by the number of adjustable inputs such as smoother shape, starting model, and damping or regularization para...

Precise Microearthquake Locations and Fluid Flow in the Geothermal Reservoir at Soultz-sous-Forêts, France

I relocated two microearthquake clusters induced by hydraulic stimulation of a hot-dry-rock geothermal reservoir in the Rhine Graben near Soultz-sous-Forets, France. The two clusters were chosen from a collection of clusters identified within a 16,000-event data set obtained in 1993. I determined P - and S -wave arrival times, manually, choosing distinctive peaks if waveforms were similar at a given station, or first breaks if waveforms were emergent or nodal, judging weights accordingly. Requiring a minimum of five arrival times yielded populous clusters of 226 and 355 events. In both cases, relocation revealed two distinct planar patches of activity, 100 to 200 m across, intersecting and truncating one another along a common edge. Linear segments traverse the planar patches and may mark intersections with seismically inactive fractures. The earliest activity occurred along these segments. These segments may therefore mark permeable zones that could be important components of the fluid-flow network. The interpretation of linear segments as flow paths is supported by path dimension constraints derived from fluid-flow modeling studies and by the expected confining effects of hydrothermal sealing found in cored fractures. Orientations of the planar features are verified through consistency with orientations found in core and logging studies. In addition, focal mechanisms obtained by constraining slip planes parallel to active planes indicate normal to right-lateral strike slip, consistent with the measured stress field. Focal mechanisms indicate mass deficits at inside corners, implying that the concentrations of activity at fracture intersections may result from slip on one fracture causing extension and a resulting permeability increase in the other. The improved location precision results from a ten-fold improvement in the precision of arrival-time estimation for phases that were similar, as estimated from the standard deviation of residuals, grouped by station and phase, before and after repicking. Only a two-fold improvement was noted for poorly correlated or nodal phases. For this data set, the manual phase picking yielded more interpretable detail in the location patterns, compared to results of an automatic, cross-correlation technique. The manual results could be used to guide improvements to automatic techniques to obtain precise locations of the entire data set.

Amplitude corrections for regional seismic discriminants

Abstract — A fundamental problem associated with event identification lies in deriving corrections that remove path and earthquake source effects on regional phase amplitudes used to construct discriminants. Our goal is to derive a set of physically based corrections that are independent of magnitude and distance, and amenable to multivariate discrimination by extending the technique described in Taylor and Hartse (1998). For a given station and source region, a number of well-recorded earthquakes is used to estimate source and path corrections. The source model assumes a simple Brune (1970) earthquake source that has been extended to handle non-constant stress drop. The discrimination power in using corrected amplitudes lies in the assumption that the earthquake model will provide a poor fit to the signals from an explosion. The propagation model consists of a frequency-independent geometrical spreading and frequency-dependent power law Q. A grid search is performed simultaneously at each station for all recorded regional phases over stress-drop, geometrical spreading, and frequency-dependent Q to find a suite of good-fitting models that remove the dependence on mb and distance. Seismic moments can either be set to pre-determined values or estimated through inversion and are tied to mb through two additional coefficients. We also solve for frequency-dependent site/phase excitation terms. Once a set of corrections is derived, effects of source scaling and distance as a function of frequency are applied to amplitudes from new events prior to forming discrimination ratios. Thus, all the corrections are tied to just mb (or M0) and distance and can be applied very rapidly in an operational setting. Moreover, phase amplitude residuals as a function of frequency can be spatially interpolated (e.g., using kriging) and used to construct a correction surface for each phase and frequency. The spatial corrections from the correction surfaces can then be applied to the corrected amplitudes based only on the event location. The correction parameters and correction surfaces can be developed offline and entered into an online database for pipeline processing providing multivariate-normal corrected amplitudes for event identification. Examples are shown using events from western China recorded at the station MAKZ.

Development of the active doublet method for measuring small velocity and attenuation changes in solids

The measurement of small changes in elastic wave velocity and attenuation is important to a broad range of problems, such as earthquake prediction and early detection of rock failure in mines. Previous authors proposed a method for estimating small temporal velocity changes in the earth’s crust by analyzing progressive relative phase delays between the scattered waves of two signals generated by nearly identical earthquake sources, called doublets, recorded at different times at the same receivers. Several improvements have been made to the original method and are presented here. The reliability of measured velocity changes has been increased by using active, repeatable sources instead of natural earthquakes. The robustness of the analysis technique has been improved by eliminating unnecessary intermediate phase regression steps and thus reducing the sensitivity to spurious data. Finally, the phase‐delay algorithm has been extended to allow measurement of small attenuation changes from relative amplitude ...

1 Hz Lg Q tomography in central Asia

We have applied tomographic techniques to a data set of over 1700, regional distance, Lg amplitudes from 12 stations for paths inside a 30° by 40° region of Central Asia. Our purpose is to create high resolution path correction maps for use in regional distance source discrimination and magnitude determination, as well as to study Lg propagation characteristics in one of the most heterogeneous portions of the earth. For 1.0 Hz data, tomography reduced variance 33%, relative to a best fit, uniform Q model. LgQ varied with geologic region, with low values in Tibet, the Tien Shan and the Pamir range (Q=200 to 400), intermediate values in basins such as the Tarim (Q=500) and high values for platforms and older crust (Q>800). The relatively efficient propagation across the Tarim differs from the poor propagation that is generally observed across sedimentary basins.

A method to allow temporal variation of velocity in travel-time tomography using microearthquakes induced during hydraulic fracturing

Abstract Hydraulic injections produce fluid-filled fractures that reduce the seismic velocity of the rock compared to intact rock. The travel times of microearthquakes induced by the injections may be used to discern changes in the rock velocities, as well as locating the microearthquakes. Determining the volumes of rock where the velocities have changed provides indirect evidence for the location of the injected fluid, and the character of the changes produced in the fractured rock. Available data are generally insufficient to resolve both the spatial and temporal changes within the rock. To extract information about temporal changes, and to obtain an improved image of the velocity structure, we chose a parameterization scheme in which the velocities of each block are allowed to change from the background velocity only after a threshold number of microearthquakes have occurred in the block. Regularizing by constraining the velocity of all the altered blocks to be similar helps stabilize the inversion. The regularization can be relaxed somewhat to allow the velocity of an altered block to be different from other altered blocks if the travel-time data are compelling. The parameterization scheme is justified since observations show that the volume of the seismically stimulated rock increases linearly with the volume of the injected fluid. We applied the method to data collected in a region of Precambrian crystalline rock that was injected with 21,600 m 3 of water. We use travel times from a total of 3886 microearthquakes that were induced by the injection. The mean RMS travel-time residual decreases about 7%. The velocity structure contains a low-velocity zone located near the injection region. Other distinct low-velocity zones are identified. The pattern of microearthquake locations found using our method appears to contain more structure than the pattern found in locations determined using a homogeneous velocity structure. Two clear low-velocity regions found near the point where water was injected into the rock are separated by a region whose velocity did not change. The region of unaltered velocity had a large number of microearthquakes.

Path correction using interpolated amplitude residuals: An example from central China

To isolate effects of the laterally varying lithosphere on regional seismograms, we geographically smoothed source- and distance-corrected amplitudes from 853 events recorded at station LZH in central China. This procedure was applied to P n , P g , S n and L g phases and various coda windows for eight bands between 0.5 and 8 Hz. We assume smoothing reduces effects such as source radiation, leaving an estimate of the path effect that can be used to correct data. A cross-validation procedure showed that path corrections performed best with low-frequency L g data, reducing variance up to 39% for 1 Hz L g , and up to 57% for 0.75 Hz P/L g ratios, compared to correction for distance alone. These corrections reduce scatter in magnitude estimates and discriminant ratios without needing to know detailed geological structure and will be important for effective test-ban monitoring.