Leigh House

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.

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.

Minimum traveltime calculation in 3-D graph theory

Traveltime calculation is a crucial part of seismic migration schemes, especially prestack migration. There are many different ways to compute traveltimes. These methods can be divided into three categories: (1) Ray tracing (Julian and Gubbins, 1977; Cervený et al., 1977). These treat the problem as a initial value problem by shooting rays from the source to the receivers. Or they can also treat the problem as a two‐point boundary value problem. An initial raypath is bent using perturbation theory until Fermat’s principle is satisfied. Nichols (1994) also computed traveltimes with the amplitude information attached to it in two dimensions. (2) Finite‐difference methods (Reshel and Kosloff, 1986; Vidale, 1988; van Trier and Symes, 1991). These solve the eikonal equation directly by using different numerical schemes such as the Runge‐Kutta method, wavefront expansion, or upwind finite difference. (3) Graph theory (Moser, 1991; Fisher and Lees, 1993; Meng et al. 1994). This method recasts the traveltime prob...

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.

3-D Elastic Numerical Modeling of a Complex Salt Structure

Summary Reliably processing, imaging, and interpreting seismic data from areas with complicated structures, such as sub-salt, requires a thorough understanding of elastic as well as acoustic wave propagation. Elastic numerical modeling is an essential tool to develop that understanding. While 2-D elastic modeling is in common use, 3-D elastic modeling has been too computationally intensive to be used routinely. Recent advances in computing hardware, including commodity-based hardware, have substantially reduced computing costs. These advances are making 3-D elastic numerical modeling more feasible. A series of example 3-D elastic calculations were performed using a complicated structure, the SEG/EAGE salt structure. The synthetic traces show that the effects of shear wave propagation can be important for imaging and interpretation of images, and also for AVO and other applications that rely on trace amplitudes. Additional calculations are needed to better identify and understand the complex wave propagation effects produced in complicated structures, such as the SEG/EAGE salt struc

Improved Relative Locations of Clustered Earthquakes Using Constrained Multiple Event Location

A new method for improving relative locations of clustered earthquakes is presented and applied to a suite of microearthquakes induced by hydraulic frac- turing. The method is based on the assumption that clustering of earthquake hypo- centers is obscured by the uncorrelated scatter of individual hypocenters. The method is implemented as an additional constraint in a Joint Hypocenter Determination (JHD) scheme. The method shifts event hypocenters toward the center of mass of the events within some volume surrounding the event location if the RMS misfit between pre- dicted and measured arrival times does not increase significantly. The method uses the same basic assumption of Jones and Stewart (1997), which is that there is greater clustering in actual earthquake locations than there is in locations determined using conventional techniques. Our method differs in that it is implemented as part of the JHD process so it operates on raw travel-time data rather than on derived hypocenters. The method produces hypocenters from a demonstration field dataset that are similar to those obtained by Phillips et al. (1997), from time-consuming precise manual repicking of relative arrival times of events. The clustering constraint can easily be incorporated as an additional constraint in earthquake location/velocity tomography codes and may lead to improved velocity structure determination and earthquake location pattern identification and interpretation.

Seismological Studies of a Fluid Injection in Sedimentary Rocks, East Texas

— Induced microseismicity data from a large volume fluid injection into sedimentary rock was analyzed to study the fracture system, fluid pathways, and state of stress in the lower Frio formation in east Texas. Seismicity data are from two arrays of 25 3-component geophone packages sited in two monitoring boreholes. From a total of 2,894 event triggers, a subset of 54 microearthquakes was chosen for their high quality seismograms and clear P and S arrivals. Arrival times were picked with a precision of 0.5 to 1.0 ms, and microearthquakes were located with hypocentral uncertainties estimated as less than 10–20 m. Hypocenters farthest from the injection well define a nearly horizontal tube of seismicity approximately aligned in the direction of the injection well. A simultaneous inversion of arrival times for transverse isotropic velocity structure and hypocenters yielded P-wave anisotropy of −14% and S-wave anisotropy of −2%. Thus, velocities along vertical ray paths are higher than those along horizontal paths, probably because of lithologic differences. Single-event focal mechanisms were determined for 47 events, and many of them are normal fault type. The minimum principal stress derived from the focal mechanisms is nearly horizontal and trends approximately north-south, consistent with the regional stress state. An imaging analysis of the seismograms shows the presence of strong seismic scatterers at positions that correlate with boundaries seen in the hypocenters; both features probably result from a similar set of heterogeneities. This study demonstrates the abundance of information that can be extracted from induced seismicity data and underscores the value of induced seismicity monitoring for studying the fluid and fracture systems created by fluid injections.

Next-generation Numerical Modeling: Incorporating Elasticity, Anisotropy And Attenuation

A new effort has been initiated between the Department of Energy (DOE) and the Society of Exploration Geophysicists (SEG) to investigate what features the next generation of numerical seismic models should contain that will best address current technical problems encountered during exploration in increasingly complex geologies. This collaborative work is focused on designing and building these new models, generating synthetic seismic data through simulated surveys of various geometries, and using these data to test and validate new and improved seismic imaging algorithms. The new models will be both 2- and 3-dimensional and will include complex velocity structures as well as anisotropy and attenuation. Considerable attention is being focused on multi-component acoustic and elastic effects because it is now widely recognized that converted phases could play a vital role in improving the quality of seismic images. An existing, validated 3-D elastic modeling code is being used to generate the synthetic data. Preliminary elastic modeling results using this code are presented here along with a description of the proposed new models that will be built and tested.

A national laboratory industry collaboration to use SEG/EAEG model data sets

A collaboration between universities, U.S. national laboratories, the oil and gas industry and was initiated in the spring of 1995 to exploit and enhance the value of the data sets produced by the SEG/EAEG 3-D modeling project (SEM). The project, part of the Department of Energy’s Advanced Computing Technology Initiative (ACTI), is titled “Testing Advanced Computation Tools for 3-D Seismic Analysis Using the SEG/EAEG 3-D Model Data Set” and is usually called ACTI/SEG. Project goals are to complement and extend the usefulness of the SEM data set. The two major efforts will involve testing/improving numerical modeling and developing new imaging/inversion methods. All of these efforts are geared toward the two models generated under the SEM effort.

2-D And 3-D Elastic Modeling With Shared Seismic Models

Several elastic models, both 2-D and 3-D, are being built for use in calculating synthetic elastic seismic data. The models will be made available to the research community, along with the synthetic data that are being calculated from them. These shared models have been proposed or contributed by participants in a collaborative industry, national laboratory, and university research project. The purpose of the modeling is to provide synthetic data to better understand elastic wave propagation and the effects of structural and stratigraphic complexities. The 2-D models are easier to design and change and synthetic calculations can be run relatively quickly in them. It will be possible to alter their layer properties and calculate time-lapse data sets from them. Field data will be available to accompany many of the 2-D models. 3-D models are more realistic, but more difficult to design and change. They also require considerably more computing resources to calculate synthetic data from them. A new 3-D model is being designed, and will be used for computing synthetic elastic data.