TRACER TEST ANALYSIS FOR CHARACTERIZATION OF LAUGALAND GEOTHERMAL FIELD
This paper presents the tracer data analysis obtained from the single-phase, conservative tracer experiments conducted during a two-year reinjection project at Laugaland geothermal field in N. Iceland. Three tracer tests were conducted during the two-year period (1997-1999). The tracers injected were sodium fluorescein in Test 1 and Test 3 and potassium iodide in Test 2. We concentrated on Test 1 only, the reasons for which are explained in the paper. Axelsson et al. (2000; 2001) have conducted a comprehensive analysis of the Laugaland reinjection project tracer test data, including both qualitative and quantitative aspects using multi-flow-channel models. In this study, we focus only on the quantitative analysis of tracer test data using analytical interpretation methods . It has been proved extensively that the first temporal moment analysis leads to an estimation of the tracer swept pore volume in a closed boundary with a balanced injection/production rate scenario. But in this paper, we prove that the first temporal moment does give the swept pore volume, even for open boundaries with an unbalanced rate scenario such as existed at the Laugaland geothermal field. The produced tracer concentration history was used to estimate the interwell reservoir relative flow capacity versus storage capacity (F-C plot). The F-C plot is a simple and powerful semi-quantitative tool to evaluate the conceptual model’s relative flow and storage geometries. The relative flow and storage capacities calculated from the flow channel geometry obtained from multi-flow-channel modeling (Axelsson et al. 2001) for one production well LN-12 agreed well with the continuous F-C plot obtained from the tracer return data. The second temporal moment of tracer production history was used to derive an equivalent dispersion coefficient. The reservoir interwell heterogeneity was characterized using the Lorenz coefficient derived from the F-C plot. The Lorenz coefficient for well pair LJ-08/LN-12 was 0.509, indicating that it is moderately heterogenous. Reservoir information was obtained for interwell LJ-08/LN-12 and, to some extent, LJ-08/TN-04. The tracer data for other wells could not be analyzed, since the streamlines were not constant, and deconvolution techniques need to be developed to do meaningful interpretation. Recommendations for possible future Laugaland reservoir modeling are offered in order to obtain a better description of the reservoir.
Analysis of field tracers for reservoir description
Abstract A multiple-well, multiple-tracer project has been successfully simulated using a comprehensive three-dimensional, chemical flood, compositional simulator developed at the University of Texas. Several modelling capabilities were taken advantage of in the course of this study including many factors, such as transverse dispersion, variable well rates, areal permeability variation, and mobile oil, which cannot be handled by streamline models commonly used to model field-wide tracer flow. This study illustrates the potential of accurate reservoir simulation to be used as a tool to incorporate various field data for reservoir description purposes. Both areal and vertical variations of permeability and oil saturation were determined as well as both longitudinal and transverse dispersivities. This was a highly successful use of compositional simulation for this purpose which, although it relied most heavily on interwell tracer data, also made use of core and log data and opens the door for more completely integrated efforts along these lines. All of these aspects, taken together, clearly demonstrate the advantages and practicality of the use of a compositional chemical simulator, based upon accurate, higher-order finite differences and extremely fast vector processing, to do field scale interwell tracer analysis quantitatively.
Application of Higher-Order Methods in Compositional Simulation
A third-order differencing scheme is described and compared with several alternative methods commonly used to discretize the convection term of the component conservation equations of compositional simulators. This method can be used at higher cell Peclet numbers than those used with other common convective differencing schemes, resulting in a substantial saving in computation time for the same accuracy Comparisons are made with analytical solutions and with 2D and 3D simulation results for single-point upstream weighting, two-point upstream weighting, and Chaudhari's method.