The Thermal Manifestation Monitoring Program at Muara Labuh Case Studies

Abstract

The thermal manifestations at the Muara Labuh field are monitored regularly to see their behaviors in response to the field production. The monitoring results are then used to identify and mitigate any potential hazards associated with the thermal area changes. This paper presents the monitoring results on the main thermal features Idung Mancung (IM) fumaroles complex, located in the vicinity of the steam cap production zone in the Northeast (NE) Sector of the field. Due to its proximity to the main production area, this thermal complex is considered to be susceptible to any reservoir changes. The thermal manifestations characteristics monitored are areal extent, numbers of the thermal funnels, the activities, physical and chemical characteristics, and geophysical signatures. Baseline data were taken from previous remote sensing studies, geology maps, and thermal manifestation geochemistry surveys prior to production periods. Remote sensing studies using true color composite (TCC) and NDVI suggest that the IM fumarole area covers the wider area prior to stage-1 project development in 2017. After production, periodic aerial photo analysis suggests no changes in IM fumarole thermal area growth. The fact is supported by the periodic Thermal Infra-Red Image (TIR), which shows no sign of thermal anomaly still contained inside the IM fumarole baseline area. IM fumarole field checks show that lack of thermal area changes and no sign that the manifestations have become more aggressive. Temperature and flow rate observation on several manifestation funnels indicates that rainfall rate variation affects the manifestation activities and temperature. As for the fumarole gas chemistry, the IM main fumarole shows a decreasing trend in NCG and CO2 contents from 2016 to 2021 and increasing in H2S, which indicates the return of degassing fluid. After one year of field operation, the microgravity and leveling survey in the IM fumarole area show slight changes from -0.1 to -0.14 mGal. The changes correspond with the area subsidence rate of -10 to -15 mm/yr. These changes are still considered low compared to other operating geothermal fields and did not directly affect the thermal manifestation activity.