Analysis of Natural Fractures Observed by Borehole Video Camera in a Horizontal Well

Abstract

Abstract A video camera survey was used to examine 2217 feet of horizontal wellbore. The air-drilled hole was clean and generally free of debris. More than 200 curvelinear features were noted in the wellbore which were interpreted to be natural fractures. A method of orienting the features observed was developed and tested by building a replica of a section of a wellbore and examining features with known inclination and orientation. Special video enhancement and playback equipment was used to examine the video tape and orient features in the wellbore. Rose diagrams of oriented features show good correlation with rose diagrams of surface joints, seismic trends in the area and fractures oriented from 74 feet of oriented core taken in the well. The video camera survey also revealed that 2-phase flow conditions existed in the wellbore. LNG flowed through fractures into the wellbore staining the walls, sublimated, and was produced as a gas. This method of analysis provides information useful in designing completions and stimulations in low-pressure reservoirs. Introduction The BDM Corporation was contracted by the U.S. Department of Energy (DOE) in October, 1985, to select a site and drill, core, log, test, stimulate, and place in production a directional well in the Devonian Shales. The purpose of the well was to test the increased gas recovery efficiency of a well drilled horizontally through the shale at nearly right angles to the orientation of the natural fractures existing in the shale formation; that is, to test the effectiveness of natural fractures producing into a horizontal wellbore as compared to improved recovery efficiency by inducing additional natural fractures or extending existing natural fractures in contact with the horizontal wellbore. BDM geologists selected a site in Wayne County West Virginia, which was located near deep-seated basement faults. It was projected that the area near these faults would be extensively fractured and characterized by good production. The location selected was within 3000 feet of a well that had produced more than 1 bcf of gas over the past 35 years indicating good fracture development. The well was drilled along a target trajectory oriented S37deg.E which was nearly normal to the projected principal stress orientation of N52deg.E. Drilling operations started in October, 1986, and were completed on December 18, 1986. The well was drilled more than 2000 feet horizontally through the shale with air as the circulating medium, world's record operation for air drilling. Video camera surveys of vertical wellbores have been conducted in the Appalachian area since 1979. A video camera was used to inspect the wellbore to determine if there were conditions in the hole that might cause problems in installing the casing in the wellbore. In addition, it was hoped that the camera might detect natural fractures that crossed the wellbore and that the frequency and spacing of fractures could be determined from the video tape. The authors also hoped that a method could be found to determine the top and/or bottom of the wellbore on the video image,, because this would allow orientation of the fractures since the orientation of the wellbore was known. All of these objectives were achieved. It is believed that the survey performed on the RET #1 well in Wayne County, West Virginia, on December 14, 1986, was the first ever conducted in a horizontal wellbore. P. 9