| Abstract |
The productive reservoir in the Mak-Ban Geothermal Field has been found to include both shallow and deep reservoirs that are separated by a low permeability formation called the Andesite Lava Marker (ALM) (also known as LimBrigo barrier), which occurs at ~4,500 ft (~1,400 m) below sea level (bsl). The majority of the production wells are completed in and produce from both reservoirs and the separation of the two reservoirs has been manifested by the existence of intra-zonal flows under shut-in conditions. Historically, the intra-zonal flows have been seen as upflows and the overall production has been very stable. However, in recent years, it has been noted that there have been significant negative impacts to production which are believed to be caused by a relative increase in cooler fluid inflow from the shallow reservoir. This has resulted in both flow instability and downflows in wells under flowing conditions that have caused step-drops in individual well production capacity. To evaluate the impact of the shallow fluid entries in more detail, recent downhole Pressure-Temperature-Spinner (PTS) and Pressure-Temperature (PT) surveys have been analyzed and wells that have developed downflow in their wellbores were identified. The downflows are easily identified because they are nearly isothermal and register negative spinner velocity. The mass flow rate and enthalpy of the downflowing fluid were estimated and used to assess its impact on the deep reservoir. Downflow temperatures range from 207°C to 250°C. Although this is not much cooler than the typical reservoir temperatures of 280°C to 320°C, the hydraulic head and higher density of the downflow have the capability to suppress production from the deep entry zones and this was determined as the main cause of the reduction of productivity in affected wells. The individual well downflows were calculated to be in the range of 10 to 200 kph (1.2 to 25.2 kg/s), and this was considered too small to significantly cool down the deep reservoir or to have a significant impact on neighboring wells, considering that the fluid is also injected at different depths in the deep reservoir. Numerical modeling studies show that there is little or no impact on neighboring wells provided the feed zones are more than 400 ft (122 m) apart. It is also possible that the mass flow rate of the downflowing fluid will increase as mass withdrawal from the deep reservoir continues and a separate numerical modeling study is being undertaken to address this issue. To better understand the dynamics of the downflow, both geochemical and production/injection data were evaluated. Current analysis showed that there is not a single model to explain why the downflows have started in individual wells, therefore, each well needs to be reviewed on a case-to-case basis. To mitigate the downflows, alternatives are being investigated to determine what can be done to isolate the cooler fluid inflow zones and stop the downflows. Zonal isolation, where the slotted production liner is cut and pulled and replaced by a cemented blank casing, is the preferable method but this is not always possible due to well completion issues. It will be necessary to also develop alternative technologies, such as injection of resins or similar products to plug off the inflow zones located behind the liners. Although this may not result in a full plugging of the zone, it may reduce the inflow to the point where the well can continue to flow from the deep reservoir. |