Abstract

This paper presents detailed analyses of hydraulic fracture microseismicity and engineering data created during the joint operator Cotton Valley Hydraulic Fracture Imaging Project in East Texas. The project was a joint operator consortium with the goal of evaluating hydraulic fracture growth of conventional "sandfracs" and waterfracs with very low sand concentrations. A variety of fracture diagnostic tools were used on ten fracture stages in three wells including microseismic and downhole tiltmeter fracture mapping, fracture modeling, stress tests, radioactive tracers, pressure transient well tests, and production logging. We also introduce a methodology that uses full triaxial waveform analysis of the microseismic signals to obtain seismic source parameters, which characterize failure modes during hydraulic fracturing. This information could potentially be used for a detailed description of fracture geometry, growth and complexities and may give some indications about created versus propped fracture lengths. The paper compares the microseismic created lengths and propped lengths with those from frac models.

Introduction

The Cotton Valley formation in East Texas is generally completed with multi-stage hydraulic fracturing treatments. Since fracturing costs comprise a large portion of the overall completion costs, any diagnostic technology that measures fracture geometry and azimuth will have a major impact on fracture optimization and field development. In recent years, two fracture diagnostic technologies have emerged, which measure far-field fracture geometry.Microseismic fracture mapping1–10 provides measurements of fracture geometry and azimuth by creating maps of microseisms (micro-earthquakes) induced by hydraulic fracturing.Downhole tiltmeter fracture mapping also measures the fracture geometry by observing the deformation-induced tilt from the hydraulic fracture11–15.

A total of six fractures in two wells were imaged using microseismic fracture mapping from offset wells. One stage in a third well was mapped with downhole tiltmeters. One well was fractured using conventional crosslinked fluids and large amounts of proppant, the other two wells were fractured with low proppant concentration "waterfracs". Waterfracs using ultra-low sand concentrations (0.5 ppg) have become the primary fracturing technique in the East Texas Cotton Valley. Mayerhofer et al.16,17 and Walker et al.18 documented the successful application of waterfracs in the Cotton Valley. They concluded that wells with waterfracs have performed just as well as conventional sandfracs at substantially reduced completion costs.

A consortium of various East Texas operators was formed in 1997 to conduct a large-scale fracture-imaging experiment in the East Texas Carthage field. Several papers have discussed the project setup and basic imaging results.1–10 In short, the setup included two monitoring wells equipped with 48-level cemented-in, 3-component geophone assemblies. The geophones were strapped outside 2–7/8" casing and cemented inside an open hole in the monitor well CGU 21–9 and inside 7–5/8" casing in the monitor well CGU 22–9. Due to problems setting the casing with strapped-on geophones in the open hole, the CGU 21–9 was only completed to a depth of 9,100 ft instead of the planned depth of 9,800 ft. As a result some of the geophones in the lower portion of the array were damaged and not functional.

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