| Title | Development of downhole cooling charts to prevent drilling problems in geothermal wells through wellbore temperature simulation |
|---|---|
| Authors | S. Ishikawa, S. Naganawa |
| Year | 2022 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | downhole cooling, wellbore simulation, geothermal gradient, drilling depth, vertical depth |
| Abstract | In high temperature geothermal well drilling in Japan, it is sometimes reported that MWD tools get damaged because of its insufficient heat resistant temperatures of approximately 175°C to 200°C. To avoid such troubles of downhole tool failure, it is important to properly cool the inside of the wellbore by drilling fluid circulation. The effect of downhole cooling was analyzed and evaluated by downhole temperature simulation, and it was found that the well diameter and pump rate have a significant effect on the downhole temperature. The objective of this study was to create a chart for different well diameters that would allow us to determine the drilling pump rate required to reduce the downhole temperature to 175°C. The wells used in the simulations were set up based on drilling data from NEDO (the New Energy and Industrial Technology Development Organization) Geothermal Development Promotion reports. Numerical simulations were carried out using the modified wellbore temperature simulation program GEOTEMP2 originally developed at Sandia National Laboratories. A simple correlation chart between well conditions and pump rates was computed for mud and water as drilling fluids. The correlation chart has the vertical axis as the target depth and the horizontal axis as the geothermal temperature gradient. The ranges of the vertical and horizontal axes were determined from well data drilled in past NEDO geothermal development promotion studies, and the geothermal formation temperature was assumed to vary linearly with depth as assigned by the temperature gradient. As a result of the simulations, it was found that drilling fluid with mud provided better cooling than only using water for all well diameters. In addition, it was confirmed that the required pump rate increases as the diameter of the well decreases. |