A laboratory and numerical investigation of steady-state, two-regime, radial flow to a well from rough, horizontal, deformable fractures

Atkinson, Lee Chaflin (1986) A laboratory and numerical investigation of steady-state, two-regime, radial flow to a well from rough, horizontal, deformable fractures. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (16MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

A numerical and laboratory study was undertaken to obtain a better understanding of the production-drawdown response of a well completed into an aquifer whose primary flow conduits are horizontal fractures. -- In the first phase of the study, a finite-element model was developed to evaluate the effects of fracture aperture and relative roughness on the nature of flow within the fractures and the compounding effect of the fractures closing in response to drawdown-induced increases in effective stress. The flow code of the model simulates steady-state, two-regime (i.e., both laminar and turbulent) radial flow in a series of horizontal, rough fractures of fixed aperture using empirically derived flow laws from the literature. The deformation code models the rock as an elastic medium and follows a user-defined constitutive curve for the stress-deformation behavior of the fractures in order to simulate their closure under the fluid pressure distribution determined by the flow routine. A coupled solution, using fluid pressure as the linking parameter, is obtained by iterating between the two routines. -- For the laboratory phase of the investigation, a concrete cylinder 1.5 m in diameter by 3.0 m high was fabricated with a single, cast-in-place horizontal fracture. More than 60 tests involving constant discharge, variable discharge, and constant injection rate were conducted on the physical model. In addition to flow rate, the laboratory arrangement permitted the varying of boundary fluid pressures, the load applied to the top of the cylinder, and the wellbore size. -- The numerical model closely simulated the results of tests when the fracture was open (i.e., with no contact area) using the true geometric characteristics of the fracture. When the fracture surfaces were in contact, however, the closest simulations were obtained using an "effective aperture" in the range of 0.3-0.8 of the best estimate of the true geometric aperture. -- The results of this investigation demonstrate that non-linear flow in fractures within a few tenths of a meter of the wellbore can comprise a significant portion of the total drawdown in a well. The results also indicate that the coefficient of the well loss term in the classical step-drawdown equation is not a constant (as usually assumed) but increases as the discharge rate increases. One of the more practical findings of this study is that reaming some wellbores in fractured rock aquifers could minimize well losses and lead to improved efficiency in their performance. -- Key words: aquifer evaluation; aquifer testing; dewatering; drawdown; fracture permeability; groundwater flow; groundwater movement; mine drainage; turbulent flow; well hydraulics; well testing; wells.

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