Robert M. Potter

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.

Interwell tracer analyses of a hydraulically fractured granitic geothermal reservoir

Field experiments using fluorescent dye and radioactive tracers (Br{sup 82} and I{sup 131}) have been employed to characterize a hot, low-matrix permeability, hydraulically-fractured granitic reservoir at depths of 2440 to 2960 m (8000 to 9700 ft). Tracer profiles and residence time distributions have been used to delineate changes in the fracture system, particularly in diagnosing pathological flow patterns and in identifying new injection and production zones. The effectiveness of one- and two-dimensional theoretical dispersion models utilizing single and multiple porous, fractured zones with velocity and formation dependent effects are discussed with respect to actual field data.

Preliminary Assessment of a Geothermal Energy Reservoir Formed by Hydraulic Fracturing

Two, 3-km-deep boreholes have been drilled into hot (approximately 200/sup 0/C) graphite in northern New Mexico in order to extract geothermal energy from hot dry rock. Both boreholes were hydraulically fractured to establish a flow connection. Presently this connection has a large flow impedance which may be improved with further stimulation. Fracture-to-borehole intersection locations and in situ thermal conductivity were determined from flowing temperature logs. In situ measurements of permeability show an extremely strong dependence upon pore pressure--the permeability increased by a factor of 80 as the pressure was increased 83 bars (1200 psi). An estimate of the minimum horizontal earth stress was derived from fracture extension pressures and found to be one-half the overburden stress.

Results of Experiment 2035, Preliminary Analysis

The purpose of this experiment was to run temperature, spinner and tracer surveys in EE-2, while injecting water, so as to determine the extent, if any, of damage to the casing cement, and also to determine locations and flow fractions of the fracture between the bottom of the casing and top of the sand.

Experiment 2033. Proposed Injection Test into the Upper Fracture Zone in EE-3

This is a detailed examination of microseismic event locations obtained from experiments 2018, 2020, 2023 and 2025. It suggests that the major difference in pressure behavior between systems that open at lower pressure and those that open at higher pressure can be ascribed to a lithologic boundary. The boundary appears to be defined by a discontinuity in joint orientation rather than an abrupt change in stress field. Examination of rock types determined from cuttings in EE-2 and EE-3 have defined two major intrusive events that contact roughly coincides with the inferred fracture orientation boundary.

Fracture Evaluation Procedures

Following fracturing it will be necessary to evaluate the success of each treatment. The primary method will be to measure impedance.