CARRER: Estimation of Fracture Aperture Distributions From Wellbore Data
Carl Renshaw Principal Investigator
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9505773 Renshaw Fractures are recognized as important component of many subsurface flow systems. Consequently, an understanding of the hydrogeologic characteristics of fractures directly affects our ability to model such important societal problems as waste isolation, ore deposit genesis, natural resource recovery, and aquifer remediation. Over the past decade, numerous investigators have used theoretical, numerical and experimental methods to attempt to understand the flow and transport characteristics of individual fractures. While the experimental and conceptual models of the fracture geometry used by each of these investigators differs, the basic conclusion reached by each is the same; that flow and transport through an individual fracture is primarily controlled by the fracture apertures within the fracture plane and , in particular, the mean and variance of the fracture aperture distribution. Unfortunately, little data are available on the distribution of fracture apertures in a single fracture. Further, it is unclear to what extent fracture aperture data obtained from surficial exposures or laboratory samples, which have been subject to unloading and /or stress concentrations due to blasting, represent the in situ distribution of fracture apertures. Thus the primary objective of the proposed work is to develop and experimentally test an indirect.in situ distribution of fracture apertures. Thus the primary objective of the proposed work is to develop and experimentally test an indirect, in situ, technique for estimating the distribution of fracture apertures based on three different types of wellbore data: direct aperture measurement, hydraulic head measurements and tracer arrival time times. For flow and transport through a very simple fracture in an impermeable matrix, the proposed estimation procedure is demonstrated to predict the distribution of fracture apertures with a reasonable degree of accuracy. In order to adapt this procedure to real fractures, the possible diff usion and sorption of the tracer into and onto the fracture walls and rock matrix needs to be considered. Testing the estimation procedure on a laboratory scale sample with a measureable aperture distribution is also proposed. Such a test of the technique is a necessary prerequisite for the application of the estimation procedure to field data. The proposed research complements the education objectives as both seek to further integrate the tools and techniques of engineering hydrolgeology with more traditional geological analyses. For example, one of the education goals is to redesign the introductory hydrogeology curriculum to more thoroughly integrate the engineering and geological aspects of the field. This includes the development of exercises and interactive computer-based laboratories which require the students to interpret and analyze geologic data before they begin to model the hydrogeology of the system. Additionally, the introductory course is being redesigned to include many of the more recent engineering analyses not found in traditional courses. These include discussions on such topics as slope and fault stability, geostatistics, linear estimation and uncertainty, and optimization and management. The goal is not only to demonstrate the many different aspects of modern hydrogeology, but also to give the students sufficient background that they might apply these tools to problems in other geoscience fields beyond hydrogeology.