The Use of Inflow Performance Relationships to Identify Reservoir Response during Production Tests in a Geothermal Well

The behavior of four inflow performance relationships are analyzed and discussed. Two of them are developed for petroleum systems and two others are developed for geothermal fluid containing high and low salinity respectively. A methodology to determine the maximum flow rate (Wmax) that a well can produce for a specific stage of its productive life, using these inflow relationships is presented. Also it is showed the way to identify the intervals of the dimensionless pressure values (pD), for which the maximum flow rate can be considered as stable and as a reservoir response according to its bottom flowing pressure (pwf). Application of this methodology to four cases of production test in wells was carried out, founding that the intervals of pD values where the stabilized flow is identified is a function of the magnitude of the well flow capacity. INTRODUCTION Reservoir engineering for well characterization uses among other tools, the inflow curves also known as IPR curves (Inflow Performance Relationships). The inflow curve of a well is equivalent to the output curve but it is measured at bottom hole conditions. Both curves are individuals for each well and vary with the productive life of the well. The output curves are obtained from the measurements at surface conditions of the flow and pressure. The original application of inflow curves was done in the petroleum industry (Muskat, 1937; Evinger and Muskat, 1937; Horner, 1951; Gilbert, 1954; Weller, 1966; Vogel, 1968; Wiggins, 1994). Also several authors (Grant et al., 1982; Kjaran and Elliasson, 1982; Garg and Riney, 1984; Chu, 1988; James, 1989; Gunn and Freeston, 1991a) began to utilize the technique of output curves in geothermal reservoirs. Also different geothermal inflow performance relationships were developed assuming the fluid as: a) pure water (Iglesias and Moya, 1990), b) a mixture H2O-CO2 (Moya, 1994; Moya et al., 1998), c) a ternary mixture H2O-CO2-NaCl with low salinity (Montoya, 2003), and d) a ternary mixture H2O-CO2NaCl with high salinity (Meza, 2005). INFLOW RELATIONSHIPS There are many applications of inflow relationships and one of them is the determination of the maximum flow (Wmax) that a well can produce. The value of the maximum flow is useful in the design of exploitation, and to fix the reference value from which it can identify the decline in the well production. The inflow curves are related by their respective dimensionless relations, which utilize the variables of flow and pressure obtained in a well test production. The dimensionless expressions of these variables are: