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The thermal effects of three dimensional groundwater flow

University of British Columbia

The proper measurement and understanding of heat flow values is essential in studies of tectonics (regional heat flow), oil maturation, low and high temperature geothermal environments, and earthquake research. Numerical solutions of the equations of fluid flow and heat transport in porous media are used to quantify the effects of three-dimensional regional groundwater flow on the thermal regime. The Galerkin Finite Element method is used to solve the coupled equations governing fluid and heat transfer. Simplex tetrahedral elements are used to subdivide the region. The resulting finite element equations are non-linear with temperature and a "leap frog" iteration technique is employed to solve the coupled equations. A series of computer simulations are carried out to investigate how typical three dimensional flow systems can influence heat flow measurements. Generic modeling is carried out on hypothetical basins with a 40 km separation between the regional topographic highs and lows. Emphasis is placed on understanding the conditions under which groundwater flows severely perturb the thermal field. The results of the sensitivity study indicate that the transition from a conduction-dominated to an advective-dominated thermal regime is sharp, and depends on: the length, depth, and width of the basin, water table configuration, permeability, existence of extensive or discontinuous aquifers, and hydraulic anisotropy. In addition, the advective threshold is a function of location within the basin. Simulations show that surface heat flow reflects the spatial distribution of recharging and discharging fluid associated with the three dimensional groundwater flow system. For the water table configurations studied, there is a high sensitivity of surface heat flow to large scale variations of water table topography. A significant difference is noted between two and three dimensional simulations of the same water table configuration. The results underscore the need to recognize the effects of large scale relief in the direction orthogonal to any section line. The lateral variation in surface heat flow is considered to be a poor indicator of active groundwater flow systems in an advectively-disturbed three dimensional basin. Suggestions are made for a two step strategy to be employed for locating boreholes for heat flow measurements. A number of closely spaced boreholes in several directions may be needed to correctly interpret heat flow measurements. Near-correct basal heat flux values can be measured in areas of the basin where there is a transition from regional recharge to discharge. At these locations heat transfer is found to be laterally advective and vertically conductive.

Text

2010-04-22

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http://hdl.handle.net/2429/24058

Geological Sciences

University of British Columbia

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