A geological characterization is achieved by a detailed mud logging and cutting analysis. Gamma-ray logs and spectral gamma-ray help to determine the exact depth of mud logging data and to characterize lithology, clay contents and alteration minerals.
After the production test, in order to precisely characterize the geothermal fluid, a downhole sample should be collected. Before collecting a minimum of two samples in the well’s open-hole section (feasible at once), the well should have been producing at least two times its volume. The recommended chemical analysis of the downhole sample are: GLR (Gas Liquid Ratio), a complete gas analysis, including at least CO2 and H2S, a complete fluid analysis, including physico-chemical properties (T°, pH, redox potential, Total Dissolved Salt content, electrical conductivity and total alkalinity), cations, anions and traces.
A hydraulic characterization of the reservoir is realized by combining well testing data (see below) and additional logging results. For example, a Production Logging Tool (PLT), which includes a flow log, identifies the flow zones within the reservoir section. Temperature logs, realized at the thermal equilibrium and shortly after the well drilling, also allow detecting flow zones. A neutron porosity log and a resistivity log can complete the hydraulic characterization by locating high porosity zones.
A density log allows defining the vertical stress in the framework of the reservoir mechanical characterization. A full-wave sonic log enables the calculation of the rock’s mechanical parameters, like the Young modulus. Leak-off tests and Formation Integrity tests can define the fracture pressure of the formation and the Shmin value (minimum horizontal stress).
A reservoir structural characterization should be carried out by acoustic or electrical imaging, which will image the reservoir structures (fractures, faults, layering, etc.) and thus enable an accurate estimation of the distribution of fractures orientations (dip direction, dip, thickness) in a given well.