Abstract

Research during the past two years into the triaxial borehole seismic method of fracture azimuth estimation has led to the development of new data processing and analysis procedures. Applications of these techniques to data acquired in a variety of treatment zones and under a variety of treatment conditions have successfully yielded precise estimates of seismic polarization planes which exist in the microseismic data. Methods have also planes which exist in the microseismic data. Methods have also been developed, or are being developed, to unambiguously relate the polarization planes to real fractures in the earth. In addition, polarization planes to real fractures in the earth. In addition, an advanced system of instrumentation has been developed and tested to simultaneously acquire bottom-hole pressure, micro-seismicity and temperature data in the treatment well.

Introduction

It has been shown by Smith that in order to optimize recovery from tight gas sand reservoirs it is important to have accurate data about the azimuths of the deeply penetrating hydraulic fractures created to stimulate production from individual wells. Similarly, Lacy has stressed the value of fracture azimuth information in the selection of optimum well sites for waterflooding and enhanced oil recovery operations. Research during the last decade led to the identification of a number of techniques which had the potential for providing reliable estimates of hydraulic fracture azimuth. These included both predictive, as well as deterministic methods. The predictive methods, such as an elastic strain recovery, differential strain curve analysis and wellbore breakout or ellipticity analyses yield estimates of the orientation of the principal stresses which, in turn, can be related to the fracture azimuth, given the assumption that the hydraulic fracture will lie in the plane normal to the minimum principal stress axis. The deterministic methods, such as the analysis of treatment induced earth tilt and microseismicity provide estimates of the azimuth of the fracture created by the stimulation process.

In early 1983, the Gas Research Institute initiated a comprehensive program to reduce the cost of gas produced from tight sand reservoirs. A key objective of this program is to significantly improve the cost effectiveness of hydraulic fracturing treatments as they are used to stimulate production from tight sand formations. In order to reach this objective, the need for reliable fracture diagnostic information was recognized. After an assessment of the existing technological base, the triaxial borehole seismic method was one of the technologies identified as having the potential for providing the desired fracture diagnostic information and was targeted for further research and development. A brief description of this method, as it has been practiced in the past, as well as a summary of the advances in the state of the art past, as well as a summary of the advances in the state of the art resulting from recent research, are contained in the following paragraphs. paragraphs. PREVIOUS EXPERIENCE PREVIOUS EXPERIENCE Research at Los Alamos National Laboratories in the mid-1970's associated with the Fenton Hill "Hot Dry Rock" experiment demonstrated that the creation of a hydraulic fracture stimulates weak microseismic activity. As depicted in figure 1, the sources of this activity have been found to occur randomly within a relatively narrow seam of rock which encloses the fracture. Thus, estimation of the spatial distribution of the microseismic sources can provide an approximate map of the hydraulic fracture.

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