Magnetotelluric Reliability for Exploration Drilling Stage: Cases Study in Muara Laboh and Rantau Dedap Geothermal Project, Sumatera, Indonesia

Abstract

The most intriguing of exploring ‘green’ geothermal field is how reliable is collected data imaging subsurfaces real condition. Magnetotelluric (MT) capability to map distribution of low resistivity cly cap and to define geothermal reservoir geometry makes it as one of mandatory methods applied for assessing the resource potential and defining the exploration drilling strategy in Muara Laboh (ML) and Rantau Dedap (RD) geothermal fields. However, proving the reliability can be tough. Geothermal resource delineation using MT method is about interpreting 1D, 2D and 3D MT inversion profiles generated from the data collected during the survey. The 10 ohm-m contour is used to map the base of conductive layer (BOC) which represents the base of clay cap. The BOC and chloride spring elevation are then used to constraint a construction of liquid boiling point to depth (BPD) curve. This approach is used to predict the top of reservoir (TOR) prior to drilling the first well, and help to create well design. In term of determining reservoir size, particular characteristics of conductive layers are being used to locate the boundary of reservoir. Later, MT data reliability is evaluated with drilling data. At RD, 3D MT profile and predicted BPD are consistent with the drilling data. The TOR predited using MT is consisted with PT survey. However, in ML case, the 3D MT profile showgood correlation with clay cap distribution and reservoir geometry, but quite ambiguous in term of determining real TOR. Despite its limitation due to dimensional effect, static shifts and acquisition noise in the lower frequency, the utilization of 1D and 2D MT are considerably required as quality assurance check.