Use of Potential Gradients in Massive Hydraulic Fracture Mapping and Characterization

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Abstract Mapping of massive hydraulic fractures allows for their efficient use to stimulate natural gas production in tight formations. A mapping technique has been used by Sandia Laboratories in conjunction with industry in the Green River Basin at Pinedale, Wyoming and in the Wattenburg field near Denver, Colorado. Comparison of field data to model calculations shows that the electrical potential gradients produced by the direct electrical excitation of the produced by the direct electrical excitation of the fracture well and fracture fluid can be used to map and characterize massive hydraulic fractures. The direction and any asymmetry of the fracture can be determined. Introduction In April 1973, a special Natural Gas Technology Task Force issued a preliminary report on three major gas deposits in the Rocky Mountain region which cannot be exploited with current extraction techniques. The regions considered were the Green River Basin in Wyoming, the Piceance Basin in Colorado, and the Uinta Basin Piceance Basin in Colorado, and the Uinta Basin in Utah. The low permeability of the gas bearing sands in these basins dictates that extremely large fractures are required to provide adequate productivity. The report indicates that the gas deposits could be stimulated by massive hydraulic fracturing (MHF). Massive hydraulic fracturing stimulation consists of multi-stage sand and fluid injections that would potentially create long fractures (appx. 5000 ft.) over a large gross pay interval. To assess the efficiency of the MHF process, characterization information is needed on the azimuthal direction, length, and height of the permeable portion of the hydraulic fractures. In addition, for production application of MHF, similar information will be needed to affect optimum well placement to achieve overall efficient drainage. Sandia Laboratories is currently developing geophysical diagnostic techniques to characterize and map massive hydraulic fractures. These techniques include the use of passive seismic signals created by the fracturing and surface electrical potentials created by injecting current into the casing of the fracture well. In this paper, the modeling and field results of recent tests for the surface potential technique will be given. The electrical MHF mapping technique consists of measuring potential gradients at the surface of the earth. The potential gradients are a result of using the fracture well casing along with the associated fracture filled with a conducting fracture fluid as one current electrode and a distant well casing as the other current electrode.