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Monitoring Well Completion Evaluation with Borehole Geophysical Density Logging |
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| Authors: | EN Yearsley RE Crowder LA Irons |
| Institution: | Elliot N. Yearsley is a geological engineer and log analyst for Colog Inc. (Borehole Geophysical Services, 1019 Eighth St., Golden, CO 80401). He received his B.Sc. in geological engineering from the Colorado School of Mines (CSM) in 1981, later returning to CSM to earn a M.Sc. in engineering mechanics in 1989. Between degrees, Yearsley was employed as a geological engineer for Tenneco Oil Co., where he pioneered the use of measurement while drilling (MWD) formation evaluation. He is currently working on new borehole geophysical techniques applied to geotechnical and hydrogeological investigations, including monitoring well completion evaluation and full waveform acoustic analysis applied to permeability determinations.;Robert E. Crowder is president of Colog Inc. (Borehole Geophysical Services, 1019 Eighth St., Golden, CO 80401), and has 12 years of diversified experience in borehole geophysical applications, including environmental, geotechnical, and mineral investigations. Prior to forming Colog he was operations manager for Colorado Well Logging for eight years. He has taught numerous courses for client groups in borehole geophysical logging and log interpretation, both domestic and international. He received his degree in engineering geophysics from Colorado School of Mines in 1978.;Larry A. Irons is a senior associate geophysicist at Ebasco Environmental Services (143 Union Blvd., Ste. 1010, Lakewood, CO 80228). He graduated from the University of Nebraska in 1977 with a B.Sc. in geology. During 1977–79 he conducted research for the Antarctic Research Program of the University of Nebraska, earning an M.Sc. in geology with a mathematics minor. During 1979–86 Irons worked as an exploration geophysicist for several major oil companies. He is currently conducting remedial investigations of hazardous waste sites using borehole, magnetic, gravity, electromagnetic, and seismic geophysical methods. |
| Abstract: | Grout continuity and the location of the bentonite seal and sand pack in PVC-cased monitoring wells can be evaluated with cased-hole geophysical density logs. This method relies upon density contrasts among various completion conditions and annular materials. Notably, the lack of annular material behind pipe (i.e., void space) creates a low-density zone that is readily detected by borehole density measurements. Acoustic cement bond logging has typically been applied to the evaluation of cement in the annular space of completed oil and gas production wells, and in some cases to ground water monitoring wells. These logs, however, can only be obtained in the fluid-filled portion of the borehole, and their interpretation is severely hindered by the presence of the micro-annulus between casing and cement. The influence of the micro-annulus on cement bond logs can be mitigated in steel-cased wells by pressurizing the wellbore during acquisition of the log, but this procedure is not feasible in PVC-cased monitoring wells. The micro-annulus does not affect cased-hole density logs or their interpretation. Empirical measurements made in the laboratory with density probes provide information on their depths of investigation and response to specific completion conditions. These empirical data, and general knowledge of the density of annular completion materials (sand, bentonite, cement), are used to support interpretations of cased-hole density logs acquired in the field. Three field examples demonstrate the applicability of geophysical density logs to the evaluation of PVC-cased monitoring well completions. |
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