CASE STUDY OF MASS FLOW DECLINE IN PRODUCTION WELL NM11

Abstract

NM11 is an inactive production well at Ngatamariki geothermal field drilled 3km deep. It is a large diameter well with an 18-5/8” production casing. The well was first flowed in 2013, initially providing approximately 1000t/h. A few months after starting production, the mass flow from the well began to decline. Calcite scale was detected in the well 10 months after beginning production. Managing calcite scale in production wells and power plants is a major challenge in geothermal field operations. The well was ultimately rendered unproductive and understanding the mechanism will lead to appropriate management options for future wells.
Geological, reservoir engineering, and geochemical data are investigated to determine possible causes for the decline observed in the well’s productivity. The geology of the well and characteristics of two cores are inspected and compared to neighboring wells. PTS results before production and after 6 months of production show significant permeability reduction in all feedzones over the first 6 months of production. Wellbore models are used to match the PTS results and give modelled productivity index (PI), pressure, and enthalpy for each of the 9 feedzones. Geochemical data showed the well had calcite scale deposition occurring. Calcite scaling is induced by flashing, typically at inferred flash point depth. Scaling is a likely cause of productivity decline in shallow feedzones but
does not explain the productivity index in lower feedzones which have not experienced flashing. Thermal expansion and collapse of weak formation are possible contributors to the observed permeability decline.