Several hundred microearthquakes with local magnitudes ranging between −4 and −2 were recorded during hydraulic stimulation experiments at the hot dry rock demonstration site, Fenton Hill, New Mexico. These events are probably caused by shear failure induced by high pore fluid pressures. Since the event locations seem to cluster in a narrow band near the hydraulic fracture, we were able to use micro‐seismic techniques to locate the hydraulic fracture and monitor its growth. We calculate the minimum pore pressure increase necessary to induce rock failure using a simple model of slippage on preexisting fractures. By comparing a pore pressure distribution, calculated using a one‐dimensional diffusion model with the distribution of event foci, we demonstrate that high pore pressures are a probable cause for these microearthquakes. Stress drops for these microearthquakes, calculated from spectral parameters, increased by a factor of 3 during the course of the experiment. The stress drop increase correlates with a migration of seismic activity away from the injection well bore, which suggests relatively low in situ shear stresses near the well bore caused by strain release in this region during previous pressurizations.

In this paper we report laboratory acoustic velocity and electrical resistivity measurements on Berea Sandstone and Austin Chalk samples saturated with a stoichiometric mixture of tetrahydrofuran (THF) and water. THF and water is an excellent experimental analogue to natural gas hydrates because THF solutions form hydrates similar to natural gas hydrates readily at atmospheric pressures. Hydrate formation in both the chalk and sandstone samples increased the acoustic P wave velocities by more than 80% when hydrates formed in the pore spaces; however, the velocities soon plateaued and further lowering the temperature did not appreciably increase the velocity. In contrast, the electrical resistivity increased nearly 2 orders of magnitude upon hydrate formation and continued to increase slowly as the temperature was decreased. In all cases resistivities were nearly frequency independent to 30 kHz, and the loss tangents were high, always greater than 5. The dielectric loss showed a linear decrease with frequency suggesting that ionic conduction through a brine phase dominates at all frequencies, even when the pores are nearly filled with hydrates. We find that resistivities were strongly a function of the dissolved salt content of the pore water. Pore water salinity also influenced the sonic velocity, but this effect is much smaller and only important near the hydrate formation temperature.

This paper was prepared for presentation at the 1999 SPE Western Regional Meeting held in Anchorage, Alaska, 26–28 May 1999.

An attempt was made to synthesize the results of active seismic experiments carried out by the Los Alamos National Laboratory's Hot Dry Rock Project staff for determining the geometrical and physical properties of the fracture system produced by hydraulic fracturing in a hot, low‐permeability rock. Interpretation of data from several reflection, transmission, and attenuation experiments using seismic probes in the frequency range from a few hundred to more than 104 Hz led us to postulate the existence of a highly complex fracture system consisting of major discrete vertical cracks intersected by several inclined joints which are surrounded by a large volume of rock containing small‐scale cracks. We found an interesting coincidence between the mean square fluctuation of P arrival time arid Q−1 of coda waves. Both are increased by nearly the same amount (35∼40%) when fluid pressure in the reservoir is increased. Another coincidence is the scale length of the inhomogeneity of roughly 3 m obtained from the frequency dependence of attenuation and also from the spatial variation of P arrival time. These results suggest that the seismic attenuation in the fractured region is caused primarily by scattering.

Summary This paper reports the first results of stress-oriented and aligned perforating of deviated wells at the Kuparuk River field, Alaska. Preferred perforation alignment and Preferred perforation alignment and spacing are calculated for each well so the fractures from individual perforations link to produce a single perforations link to produce a single "zipper" fracture plane along the deviated wellbore. Results of the first application of this technique are presented from the 26-well development presented from the 26-well development of Drillsite 2K. The results from use of three different oriented casinggun systems and pertinent data from Drillsite 2K fracture stimulation treatments are discussed. Comparisons to drillsites where nonaligned perforating strategies were used show a perforating strategies were used show a significant reduction in perforation friction, enabling the placement of larger, more productive fracture treatments. Application of this technique to deviated and vertical wells and its use at Kuparuk on developments after Drillsite 2K are discussed. Introduction Perforation design for a well that will be Perforation design for a well that will be hydraulically fractured is usually controlled by the requirements to place the stimulation treatment. Key parameters are the number, size, orientation, and phasing of perforations. Typically, the objective is either perforations. Typically, the objective is either to minimize or, in the case of limited entry treatments, to control the amount of perforation friction during the stimulation perforation friction during the stimulation treatmeat. No uniform criteria exist within the industry for defining perforation phasing or shot density. Different operators use different techniques. However, the pumping of a fluid stage to break down the well and to calculate the perforation friction loss is routine to verify that sufficient communication exists between the wellbore and the formation to place the fracture treatment. Often, a ballout treatment is pumped before the main stimulation to force additional perforations to breakdown. Although it is perforations to breakdown. Although it is generally acknowledged that the optimal placemeat of perforations in a vertical well is placemeat of perforations in a vertical well is 180 phasing in the fracture plane, which is perpendicular to the far-field minimum stress, there are, to the best of our knowledge, no reported efforts of routinely practicing such a technique. Laboratory practicing such a technique. Laboratory investigations into fracture initiation from deviated wells showed the importance of perforation placement on the length of perforation placement on the length of wellbore intersecting the fracture. During the past 7 years, more than 600 new development wells have been fracture -stimulated in the Kuparuk River field. The large number of treatments has provided the opportunity for significant advances in the technical and operational aspects of hydraulically fracturing deviated wells that are not aligned colinear to a direction of principal stress. The success of this stimulation principal stress. The success of this stimulation program was documented in Refs. 4 and 5. program was documented in Refs. 4 and 5. Perforation strategy during the initial development consisted primarily of perforating the net pay intervals in the Kuparuk A sand. Depending on the drillsite, this would result in the perforating of two or three separate zones. Before the wellbore tubulars and completion equipment were run, casing guns (4 1/2-in.) were shot with a typical shot density of 4 shots/ft and a phasing of either 90 or 120. We often used largehole shots every fifth hole. Most initial fracture treatments pumped in wells where this strategy was used had relatively high perforation friction drops ranging from 500 to perforation friction drops ranging from 500 to 1,500 psi. Post-treatment temperature and tracer logging often showed fluid entry into a few discreet points along the perforated interval, with the lowest zone of the A sand often showing no evidence of fracture stimulation. The poor communication at the wellbore is thought to have caused many treatment screenouts in the field.