Interyac: a New Graphics Interface for Tetrad Users

Reservoir simulation studies are continuously demanded for the Mexican geothermal fields, in places such as, Cerro Prieto, Los Azufres, Los Humeros, etc. Over the last ten years, TETRAD code has been intensively used in Mexico as the main tool for numerical simulation. In order to overcome problems and limitations with the use of GridView, SimView and Fplot the graphic DOS based software for the pre and postprocess analysis in reservoir simulation works with TETRAD , a new interface has been developed. CFE, the Mexican government utility and IIE, the Mexican electrical research institute, have joined efforts to develop a Windows-based program, INTERYAC, with expanded and improved analysis capability for reservoir simulation that traditionally required the use of DOS programs such as, GridView, SimView and Fplot. INTERYAC was designed for pre and post-graphic processing of TETRAD input and output data files. In addition, INTERYAC is able to handle directly GRD and CTR data files from SURFER. In its pre-process module, the simulation grid blocks can be easily generated and modified. Users can record modification notes while using INTERYAC. Additional features include automatic grid block numeration, reading data from ASCII files, plotting of any X-Y plane or 3D surfaces and customized IS file generation. The INTERYAC post-process module can read and graph GV and PLT files from TETRAD. Reservoir data can be processed into a PLT file to adjust and match actual data. INTERYAC provides an "animation" feature to analyze time-dependent parameters including the movement of fluid between simulation blocks. INTERYAC has been tested and validated with Cerro Prieto geothermal field simulation results. A 2944block model was utilized to match the productionreinjection history of the field. WHAT IS INTERYAC? INTERYAC v1.0 is a computer system designed for graphic preand post-processing of input and output data to the TETRAD geothermal reservoir simulator. This system was developed to work in Windows 95, 98 and NT environments. During the pre-process, INTERYAC easily generates and edits the input files (IS files) to TETRAD from the definition of the reservoir numerical model. This involves the discretization or subdivision of the reservoir into a certain number of grid block elements, in its horizontal dimensions (X and Y) as well as the vertical (Z), assigning initial conditions to parameters. Graphic representations of the model by layer (XY plane), by cross section (XZ and YZ planes) and 3D can be shown and plotted by INTERYAC. Then it is possible to visualize and easily edit the numerical values of any parameters at any element of the grid, as well as generating contour maps of the data. One of the most important features of INTERYAC, is the capability to generate values for missing blocks in a simulation grid from the interpolation of values for the existing blocks. In addition, these values can be obtained from configuration files generated by SURFER (GRD format) or Contours files (CTR format). Additional features for data processing are: ! Automatic numbering of grid blocks for any plane or layer. ! Automatic generation of a "well definition" section in the format required by the DAT file of the TETRAD simulator. ! Generation of records from comparing *.IS files. ! Plotting graphs of all planes and the 3D model of the reservoir. In post-processing stage, INTERYAC is a very versatile tool that allows the user to visualize and graphically analyze the data obtained from its simulation. Basically two types of analysis can be performed: ! Using Cartesian graphs to compare the measured data against the results of the simulator (PLT files and data measured at the field). ! Using graphic configurations (color and contour maps, volumetric flows, aquifers) to analyze the behavior of the simulated parameters through simulation time (GV files). In order to show more specifically detailed INTERYAC’s main features, the case of Cerro Prieto Geothermal Field has been selected, since several numerical simulation studies have been carried out in the past Electroconsult ( 1986) , GeothermEx ( 1990 and 1998), Ribó(1997). A brief description of the conceptual model for the Cerro Prieto Geothermal Field is shown below. CERRO PRIETO SIMULATION As it is well known, through more than 28 years of commercial exploitation, the Cerro Prieto geothermal system has been submitted to continous extraction of a water-steam endogenous mixture. Production has increased parallel to the increase in the electrical generation installed capacity: from an initial production of 676 t/h mixture required by mid-1973 to generate 32.5 MW, to 12 000 t/h reached in 1993 for a maximum generation of 635 MW. To date there are 286 existing wells in Cerro Prieto, with depths that range between 1500 m and 3400 m for the production and injection wells, and up to 4390 m for the exploratory wells. An average of 140 wells is usually connected to the steam gathering system in order to supply steam to the power plants. Boundaries and description of the reservoir. The analysis of the samples obtained during drilling establishes that the Cerro Prieto wells traverse through a litological column formed by five main units which, according to depth, are: Unconsolidated sediment unit, mudstone unit, brown shale unit, gray shale unit, and granitic basement. The production zones forming the geothermal reservoir are contained in the gray shale unit. The sandstones are the principal means by which the fluids move through the system. The gradual appearance of the epidote is registered at depths with temperatures above 220oC, observing that 300o-325o isotherms are directly related to the clear presence of the cementing silica and the mineral epidote. In general, the top of this unit deepens in a SW to NE direction, Figure 1. -6000 -5000 -4000 -3000 -2000 -1000 0