James T. Rutledge

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.

Induced microearthquake patterns and oil-producing fracture systems in the Austin chalk

Abstract Microearthquakes collected during hydraulic stimulation allowed us to study fracture zones in Austin chalk oil reservoirs at two sites in the Giddings field, Texas. We deployed three-component, downhole geophone tools in production wells at depths of 2100 m and greater, one near Cooks Point, and two on the Matcek lease near Caldwell. At Cooks Point, we collected 482 microseismic events during a 4000 m 3 (25,000 bbl) hydraulic stimulation in an offset well. We collected 770 events during a similar operation on the Matcek lease. Many seismograms contained reflected phases that constrained location depths to the production zone at the base of the Austin chalk. By restricting all microearthquake locations to production depths, we located 20% of the Cooks Point events and over 60% of the Matcek events. At both sites we observed only the fracture wing closest to the observation stations. Locations formed elongated patterns extending up to 1 km from the stimulation well and trending N60°E, parallel to the known, regional fracture trend. The Cooks Point seismic zone measured over 100 m in width, while long stretches of the Matcek seismic zone narrowed to 30 m or less. We believe that the width of the seismic zone reflected the density of conductive fractures and thus, the volume of the reservoir accessed by the stimulation. Indeed, production rates in the first year following stimulation were much higher at Cooks Point, where we observed the wider of the two seismic zones.

Microseismic event location for monitoring CO2 injection using double-difference tomography

Increases in pore pressure and volume caused by CO2 injection will perturb the ambient stress field and possibly trigger brittle failure on small fractures or faults, resulting in microearthquakes. Monitoring this induced seismicity can help understand the migration of the CO2 front, and the stress and strain changes in the reservoir (Miyazawa et al., 2008). As part of research efforts of the Southwest Regional Partnership on Carbon Sequestration supported by the U.S. Department of Energy and managed by the National Energy Technology Laboratory, a large number of microseismic events have been recorded since early 2008 using a geophone string cemented into a well for monitoring CO2 injection at the Aneth oil field in Utah. We use double-difference tomography to obtain high-precision locations of microseismic events and improve the spatial resolution of the velocity structure simultaneously. The double-difference seismic tomography method was developed for improving earthquake source locations and tomograph...

Current status of seismic and borehole measurements for HDR/HWR development

Seismic and borehole measurements provide significant information about HDR/HWR reservoirs that is useful for reservoir development, reservoir characterization, and performance evaluation. Both techniques have been widely used during all HDR/HWR development projects. Seismic measurements have advanced from making passive surface measurements during hydraulic fracturing to making passive observations from multiple boreholes during all phases of HDR/HWR development, as well as active seismic measurements to probe regions of the reservoir deemed to be of interest. Seismic data provide information about reservoir extent, locations and orientations of significant fractures, and areas of thermal drawdown. Recent advances include the ability to examine structures within the seismically active zone using statistics-based techniques and methods such as seismic tomography. Seismic method is the only means to obtain direct information about reservoir characteristics away from boreholes. Borehole measurements provide high-resolution information about reservoir characteristics in the vicinity of the borehole. The ability to make borehole measurements has grown during the course of HDR/HWR development as high temperature tools have been developed. Temperature logging, televiewer logs, and electrical property measurements have been made and shown to provide useful information about locations of fractures intersecting wellbores, and regions where water leaves and enters injection and production wellbores, respectively.

Using Microseismicity to Map Cotton Valley Hydraulic Fractures

Spatial variations in event location and source parameters (magnitude, energy and stress release) for several thousand microseismic events recorded during six hydraulic fractures in May and July, 1997, within two deep gas wells in the Cotton Valley formation, are used to identify patterns in fracture growth and the dimensions of the fractures. The distributions of event locations suggest that all six hydraulic fracture were oriented at N78°E, in line with the major principal stress orientation in the field. Spatially, the microseismicity generally exhibited asymmetric behaviour with preferential growth to the east of the treatment wells (symmetry varying from 1:1 to 4.5:1). Only in two instances could the fracture be considered as symmetric, however, temporal growth patterns, as identified in the source parameter distribution were highly asymmetric. The fractures were constrained in depth to the perforation zones and varied in overall length from 1300 ft. to 2200 ft.. Observed terminations in microseismic activity were considered to be related to crosscutting structures that altered or impeded fracture wing growth. Differences in apparent stress drop distributions suggested that these parameters can be used to outline differences in fracture behaviour and to define regions of the fracture where fracture growth does not necessarily contribute to the development of a propped zone. The results further suggest spatial variations in microseismicity, correlated with the treatment histories, can provide more information on the dynamics and heterogeneities associated with fracture growth.

East Texas Hydraulic Fracture Imaging Project: Measuring Hydraulic Fracture Growth of Conventional Sandfracs and Waterfracs

This paper presents detailed analyses of hydraulic fracture microseismicity and engineering data created during the joint operator Cotton Valley Hydraulic Fracture Imaging Project in East Texas. The project was a joint operator consortium with the goal of evaluating hydraulic fracture growth of conventional sandfracs and waterfracs with very low sand concentrations. A variety of fracture diagnostic tools were used on ten fracture stages in three wells including microseismic and downhole tiltmeter fracture mapping, fracture modeling, stress tests, radioactive tracers, pressure transient well tests, and production logging. We also introduce a methodology that uses full triaxial waveform analysis of the microseismic signals to obtain seismic source parameters, which characterize failure modes during hydraulic fracturing. This information could potentially be used for a detailed description of fracture geometry, growth and complexities and may give some indications about created versus propped fracture lengths. The paper compares the microseismic created lengths and propped lengths with those from frac models.

Borehole testing of a micromachined silicon accelerometer

Side-by-side field tests of borehole seismic packages containing either micromachined analog servo accelerometers or conventional piezoelectric accelerometers currently used in borehole seismic applications, show important differences in their relative performance. Laboratory testing of the micromachine shows dramatically improved distortion and low frequency response compared to conventional geophones (Gannon et al., this volume).

Reservoir microseismicity at the Ekofisk oil field

A triaxial, downhole geophone was deployed within the Ekofisk oil reservoir for monitoring ambient microseismicity as a test to determine if microearthquake signals generated from discrete shear failure of the reservoir rock could be detected. The results of the test were positive. During 104 hours of monitoring, 572 discrete events were recorded which have been identified as shear-failure microearthquakes. Reservoir microseismicity was detected at large distances (1000 m) from the monitor borehole and at rates (> 5 events per hour) which may allow practical characterization of the reservoir rock and overburden deformation induced by reservoir pressure changes.

Using the coda‐wave interferometry method and time‐lapse VSP data to estimate velocity changes from geological carbon sequestration in a brine aquifer

Injection and movement/saturation of CO2 in a geological formation can cause changes in seismic velocities and attenuation, resulting in changes in seismic-wave scattering and propagation. Accurately estimating seismic-velocity changes from time-lapse seismograms can provide valuable information about where CO2 moves. We investigate the capability of the coda-wave interferometry method for monitoring geological carbon sequestration using field time-lapse VSP data. The coda-wave interferometry method can estimate relative temporal changes in seismic velocities. Pre-injection and post-injection field VSP data sets were acquired for monitoring of injected CO2 in a brine aquifer. We estimate the temporal velocity changes at the centers of a moving time window using the coda-wave interferometry method, and then obtain the mean velocity change by averaging the temporal velocity changes over time. The time-lapse VSP data along three azimuthal directions with different offset ranges from the observation well are used in this study. Generally, the estimated mean velocity changes from field VSP data from different shots show similar changing pattern with receiver depth. Mid offset shots (shot 5, 6, and 8) give largest estimation, while closest offset shots (shot 1, 2, and 4) give mid estimated changes and shot 9 with offset 1.5km give smallest estimated velocity changes. The estimated velocity changes are as large as 0.5%. This demonstrates that the coda-wave interferometry method can be used for monitoring of CO2 injection using time-lapse VSP data. The different changes along different azimuths may be caused by the local geology heterogeneity.