| Title | Identifying formation scale in geothermal wells |
|---|---|
| Authors | K. McLean, D. Jasmin, N. Kabigting, B. Cacho, C. McMurdo, R. Romey, J. Setiawan |
| Year | 2024 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Geothermal wells, formation scaling, chemical cleaning, identifying scale type, vertical discharge, productivity/injectivity index, pressure transient analysis |
| Abstract | Operational geothermal wells tend to block up over time with mineral deposits, leading to declines in well capacity and ultimately generation. The mineral deposits are called “scale” – and can be calcite, silica, silicates, or a mix. The issue affects both production and reinjection wells. In production wells, scale forms where there is boiling. In reinjection wells, scale forms as minerals exceed the saturation limit of the reinjected fluid. Scaling can be within the wellbore (“wellbore scale”), or the reservoir beyond the wellbore (“formation scale”). Wellbore scale is easier to identify as it can be accessed directly within the wellbore using wireline tools, such as calipers or gauge rings. Formation scale is more difficult to identify as it sits outside of the accessible wellbore and cannot be detected by any of those direct methods. The presence of formation scale has typically been inferred indirectly, by ruling out other culprits. If a well is declining rapidly, and the decline cannot be explained by changes in reservoir conditions, and no wellbore scale is found, then formation scale is assumed to be the culprit. However, a higher level of certainty is desirable to decrease the risk to the operator of unsuccessful chemical cleans, as methods to remove formation scale (chemical cleaning) are expensive. There are several methods for identifying formation scale more directly and with more certainty. These include: vertical discharge methods, tracking of productivity/ injectivity index, and tracking of skin factor and permeability from pressure transient analysis. This paper reviews all methods of identifying formation scale, both indirect and direct, for production and reinjection wells, and provides examples. This is intended as a guide to decrease the risk of chemical cleaning jobs, and can also form the basis for well monitoring program design. |