Abstract
Central Luconia has been explored with hundreds of well since the 1950s. During that time, all offshore wells were drilled using hyperbolic positioning system which has lower accuracy compare to current satellite positioning system, which was only introduced in early 1990s. With this knowledge, the old exploration well's locations (which was drilled in 1970s) pose potential hazards in terms of seabed obstruction and potential well collision during the future development wells drilling. Without a reliable seismic to well tie, interpreter has difficulty in identifying the top of carbonate event for depth conversion, thus impacting the well delivery, static model building and subsurface reserves estimation.
Onsite verification was carried out using a multibeam echosounder (MBES), a Side Scan Sonar (SSS), and a Sub Bottom Profiler (SBP) in accordance with standard site survey procedures, but the existing wellhead location was unable to be detected because the wells had been abandoned and cut off at the seabed level. Magnetometer was deployed to further investigate the existing wellhead location; the sensor was towed approximately about three (3) times water depth from the stern of the vessel and altitude 10m from the seabed. To navigate the towed sensor, Ultra Short Baseline (USBL) transponder was attached close to the sensor to get real time underwater positioning. Five (5) survey lines were designed centered at the suspected existing wellhead location with the coverage of 60m radius. During data acquisition, the magnetic anomalies were recorded in the system via receiver and total magnetic data was used for further analysis to derive the as-found wellhead location. During the interpretation, the area of ambient magnetic field distortion was identified and marked as anomaly which represents "area of suspected wellhead". The magnitude and pattern of such distortion was used for interpretation and combined with the coordinates from the positioning system (surface and underwater) onboard the survey vessel.
The general total magnetic field reading is ranging between 40920nT and 41130nT with the magnetic anomaly/wellhead had magnetic value from 100nT to 115nT. The total magnetic field analytical signal value is ranging from 0 to 3.5. The target magnetic anomaly refers to the area with greatest analytical signal value where it is also the area with most drastic change of the total magnetic field. From the survey results, the as-found wellhead position varies from 48m - 53m compared to existing wellhead position. With the confirmation on the old wellhead location, this helps to derisk the well collisions study for future development well and also improves the seismic to well tie analysis to provide higher confidence in the Top Carbonate pick and a better inverted seismic match in the reservoir interval for properties distribution.
