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Tracer injection for identifying reservoir and transport parameters in geothermal sources has become an important procedure for designing an optimum reinjection program. Field tests have been analyzed using convection-diffusion equation to predict fracture size, width and probable thermal breakthrough times. The present study was conducted in a laboratory fractured reservoir model whose physical properties were known. The tracer, KI, breakthrough profiles were analyzed for different injection-production depth schemes. For different patterns different preferential flow paths existed which were then affected by flow through auxiliary paths. A correlation between heat and mass transport was used to define an apparent heat transfer coefficient for the flow. 3 tabs., 5 figs., 7 refs.
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The model proposed has been developed to study the flow of tracers through naturally fractured geothermal reservoirs. The reservoir is treated as being composed of two regions: a mobile region where diffusion and convection take place and a stagnant or immobile region where only diffusion and adsorption are allowed. Solutions to the basic equations in the Laplace space were derived for tracer injection and were numerically inverted using the Stehfest algorithm. Even though numerical dispersion is present in these solutions, starting at moderate dimensionless time values, a definite trend was found as to infer the behavior of the system under different flow conditions. For practical purposes, it was found that he size of the matrix blocks does not seem to affect the tracer concentration reponse and the solution became equivalent to that previously presented by Tang et al. Under these conditions, the behavior of the system can be described by two dimensionless parameters: the Peclet number for the fractures, P{sub e{sup 1}}, and a parameter [alpha] ([alpha] = [xi]{sqrt}P{sub e2}), where [xi] is [xi] = [phi]{sub e} D{sub e}/v(w-[delta]) and P{sub e2} is the Peclet number for the matrix. Tracer response for spike injection was also derived in this work. A limiting analytical solution was found for the case of [alpha] approaching zero and a given P{sub e{sup 1}}, which corresponds to the case of a homogeneous system. It is shown that this limiting solution is valid for [alpha]
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A parallel fractures model, having equal width and spacing, has been developed to study the flow of tracers through naturally fractured geothermal reservoirs. The model is capable of handling either a single fracture or a system of two or more parallel fractures, interacting with associated porous bodies. The reservoir is treated as being composed of two regions a mobile region where diffusion and convection are allowed and a stagnant or immobile region where only diffusion and adsorption are allowed. Both regions are interconnected by means of a very thin fluid film contained within the immobile region which controls the fluid and mass transfer between both regions. The mobile region represents the system of fractures, where tracer is free to flow reaching high velocities, whereas non-homogeneities of the reservoir rock, such as microfractures and dead-end fractures are represented by means of an equivalent porous body where fluid remains immobile. The boundary-value problem for the system is stated and its solution into Laplace's space is presented. Numerical inversion of this solution was performed by means of the Stehfest algorithm. Preliminary results showing results obtained from the proposed model are included. Further work is underway to apply the model for interpretation of actual tracer flow field data. 4 figs., 1 tab., 11 refs.
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Tracer tests performed at the geothermal reservoir at Wairakei, New Zealand have been analyzed, using a mathematical and physical model in which tracer flows through individual fractures with diffusion into the surrounding porous matrix. Model calculations matched well with the observed tracer return profiles. From the model, first tracer arrival times and the number of individual fractures (the principal conduits of fluid flow in the reservoir) joining the injector-producer wells can be determined. if the porosity, adsorption distribution coefficient, bulk density and effective diffusion coefficient are nown, fracture widths may be estimated. Hydrodynamic dispersion down the length of the fracture is a physical component not taken into account in this model. Future studies may be warranted in order to determine the necessity of including this factor. In addition to the tracer profile matching by the matrix diffusion model, comparisons with a simpler fracture flow model by Fossum and Horne (1982) were made. The inclusion of the matrix diffusion effects was seen to significantly improve the fit to the observed data.
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Fluid flow and tracer transport in a fractured Hot Dry Rock (HDR) geothermal reservoir are modeled using fracture network modeling techniques. The steady state pressure and flow fields are solved for a two-dimensional, interconnected network of fractures with no-flow outer boundaries and constant-pressure source and sink points to simulate wellbore-fracture intersections. The tracer response is simulated by particle tracking, which follows the progress of a representative sample of individual tracer molecules traveling through the network. Solute retardation due to matrix diffusion and sorption is handled easily with these particle tracking methods. Matrix diffusion is shown to have an important effect in many fractured geothermal reservoirs, including those in crystalline formations of relatively low matrix porosity. Pressure drop and tracer behavior are matched for a fractured HDR reservoir tested at Fenton Hill, NM.
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Two tracer tests on doublet systems in a fractured geothermal system were carried out in Klamath Falls, Oregon. The purpose of the tests were to obtain data which would lead to information about the reservoir and to test the applicability of current tracer flow models. The results show rapid breakthrough times and indicate fracture flow with vigorous mixing of injector fluid before production of same. This leads to the idea that thermal breakthrough is not directly related to tracer breakthrough in the Klamath Union doublet system. There has been no long-term enthalpy loss from exploiting the resource for 40 years. In order to reduce the data, models were developed to analyze the results. Along with a porous media flow model two mathematical models developed to analyze fractured geothermal systems are used to help decipher the various tracer return curves. The flow of tracers in doublet systems was investigated. A mathematical description is used for tracer flow through fractures as a function of time and various nonlinear parameters which can be found using a curve fitting technique. This allows the reservoir to be qualitatively defined. These models fit the data well, but point to the fact that future improvement needs to be considered for a clearer and more quantitative understanding of fractured geothermal systems. 22 refs., 32 figs., 11 tabs.
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Tracer tests have been an important technique for determining the flow and reservoir characteristics in various rock matrix systems. While the interwell tracer tests are aimed at the characterization of the regions between the wells, single-well injection-backflow tracer tests may be useful tools of preliminary evaluation, before implementing long term interwell tracer tests. This work is concerned with the quantitative evaluation of the tracer return profiles obtained from single well injection-backflow tracer tests. First, two mathematical models of tracer transport through fractures, have been reviewed. These two models are based on two different principles: Taylor Dispersion along the fracture and simultaneous diffusion in and out of the adjacent matrix. Then the governing equations for the transport during the injection-backflow tests have been solved. Finally the results were applied to field data obtained from Raft River and East Mesa geothermal fields. In order to determine the values of the parameters of the models that define the transport mechanisms through fractures a non-linear optimization technique was employed. 26 refs., 10 figs.
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A three dimensional fracture system synthesis and flow simulation has been developed to correlate drawdown characteristics measured in a geothermal well and to provide the basis for an analysis of tracer tests. A new dual permeability approach was developed which incorporates simulations at two levels to better represent a discrete fracture system within computer limitations. The first incorporates a discrete simulation of the largest fractures in the system plus distributed or representative element stimulation of the smaller fractures. The second determines the representative element properties by discrete simulation of the smaller fractures. The fracture system was synthesized from acoustic televiewer data on the orientation and separation of three distinct fracture sets, together with additional data from the literature. Lognormal and exponential distributions of fracture spacing and radius were studied with the exponential distribution providing more reasonable results. Hydraulic apertures were estimated as a function of distance from the model boundary to a constant head boundary. Mean values of 6.7, 101 and 46 .mu.m were chosen as the most representative values for the three fracture sets. Recommendations are given for the additional fracture characterization needed to reduce the uncertainties in the model. 20 refs., 6 figs.
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Discrete fracture and continuum models are being developed to simulate Hot Dry Rock (HDR) geothermal reservoirs. The discrete fracture model is a two-dimensional steady state simulator of fluid flow and tracer transport in a fracture network which is generated from assumed statistical properties of the fractures. The model's strength lies in its ability to compute the steady state pressure drop and tracer response in a realistic network of interconnected fractures. The continuum approach models fracture behavior by treating permeability and porosity as functions of temperature and effective stress. With this model it is practical to model transient behavior as well as the coupled processes of fluid flow, heat transfer, and stress effects in a three-dimensional system. The model capabilities being developed will also have applications in conventional geothermal systems undergoing reinjection and in fractured geothermal reservoirs in general.
Author: Bruce A. Robinson
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Category : Hot dry rock systems
Languages : en
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