Evolution of Reservoir Tracer Pathways During Multiple Phases of Exploitation

Abstract

Recurring tracer studies from two recently commissioned binary plants are investigated to quantify the evolution in fracture and matrix pathways throughout a highly permeable reservoir. Ormat implemented tracer tests within the first year of operation using several unique naphthalene sulfonate tracers and again during the third year following the start-up of a second phase of operation, effectively doubling the production in the wellfield. Tracer concentration profiles from repeated production-injection well doublets are fitted by two response curves representing both a low impedance fracture-dominated pathway and higher-impedance matrix-dominated pathway. Tracer curves are inverted for velocity, dispersion and swept area to compare changes in fluid pathways over time. Results from both wellfields suggest a net increase in fluid velocity and corresponding decrease in area swept for both fracture and matrix-dominated flowpaths. We also observe a small shift in percentage of tracer returns (injectate) towards fractures, corresponding with an increase in dispersivity, suggesting the utilization of more tortuous pathways through damage zones. Conversely, we see a decrease in dispersivity within the matrix reinforcing the idea of fluid focusing into lower impedance pathways. Overall, the wellfields in this study exhibit very high permeability and therefore do not mirror fracture evolution witnessed during pressure stimulation of very low permeability systems such as increased aperture or surface area. The results of this study highlight the importance of repeated tracer analysis when used to calibrate numerical models and long-term reservoir predictions.