Cotton Valley Hydraulic Fracture Imaging Project
Authors: Walker Ray N. 
Published: December 2007

This paper describes an experiment to image hydraulic fractures in the Cotton Valley sands at approximately 9,500' depth. The purpose is to determine key geometrical attributes of those hydraulic fractures. A pilot study of one monitor well with a single geophone at normal well spacing and preliminary results from a more detailed study involving two monitor wells with arrays of geophones are presented.

In the pilot study, it is shown that microseismic events induced by hydraulic fracturing at a depth of 9,500' were detected at distances exceeding 1,300' from the origin. Analysis of results indicated that a more complex geophone array was necessary. An industry consortium of major operators, service companies, government agencies and national laboratories was formed to perform a more detailed study using 96, 3-component geophone sondes distributed among two monitor wells. These monitor wells offset the treatment well using nominal 80 acre spacing rules and will be used as producers. A unique microseismic recording system specifically designed and manufactured for these operating depths (~9,500') is presented. Imaging is accomplished by using various data analysis techniques including real-time event location.

Applications of the technology include optimization of hydraulic fracture designs and accurate mapping of fracture geometry. Fracture models will be developed to accurately depict fracture growth as defined by the finding of this project.

Introduction

Hydraulic fracturing is the primary completion technique for the Cotton Valley formation of East Texas. Operators in the East Texas area will make major investments in hydraulic fracturing during future years. Technology that accurately depicts geometrical attributes of hydraulic fractures will have a major impact on the future development of tight gas reservoirs.

The fracture imaging study in the Carthage Cotton Valley Field, Panola County, Texas is focused on the determination of certain key geometrical attributes of a hydraulic fracture - height, length and fracture azimuth. Information regarding the internal structure and real time development of the created fractures is gathered.

A consortium was formed with operators active in the Cotton Valley fields of East Texas. The project was driven primarily by the operations groups of each company and kept on track by cost control and process management.

The primary data collected in this study are seismograms associated with micro earthquakes induced by hydraulic fracturing. Microseismic data provide the only source of information with the spatial resolution suitable for the imaging of fractures away from the borehole. Additionally, velocity data sets are being developed through a crosswell, tomography study to aid in the imaging process.

Event location software is being utilized to analyze the above data and produce the required geometrical attributes of the fracture. The analysis consists of matching recorded arrival times with modeled times derived from a rate-traced velocity model calibrated locally with checkshot data. A detailed velocity model derived from the crosswell survey is utilized. Modeled travel times for both compressional and shear arrivals are matched in a least squares sense to recorded compressional and shear first arrivals. One of the benefits of having multiple data analyses is the ability to compare the reliability of the various analysis methods employed, and so determine the most efficient field procedure and event location technique combination for future operations in a normal production environment and at normal well spacings.

Three wells were utilized in the experiment: one injection well and two seismic monitor wells. P. 131^