Successful Reservoir Monitoring with 4D MicroGravity at Ras Laffan, State of Qatar

Abstract

Abstract Recent advances in microgravity technology have improved precision, making the microgravity method a cost effective tool for reservoir monitoring in a time-lapse (4D) mode. Surface recorded gravity can now be reliably measured to five microgal accuracy, allowing small changes in fluid type in subsurface reservoirs to be monitored. RasGas Company Limited (RasGas) has successfully been using this 4D microgravity method to monitor waste gas injection near the LNG plants at Ras Laffan Industrial City (RLIC). This method has proved to be a very cost effective and intrinsically safe way to monitor the movement of gas injected into an aquifer at approximately 5000 ft depth in an industrial environment. Gas injection at RLIC started in 2005. A requirement of the injection permit was to monitor the movement of the injected gas. RasGas evaluated available monitoring technologies and chose 4D microgravity over monitoring wells and 4D seismic as the best method to achieve the monitoring objectives. Monitoring wells would have been costly and raised the risk of leakage behind the wellbore. Placing the wells in a congested industrial area would have been difficult because positioning these wells would require prior knowledge of where the dynamic gas front was likely to be encountered with time. 4D seismic would have been very expensive, intrinsically unsafe for this industrial environment, and adversely affected by LNG plant noise. The microgravity technology employed by RasGas is less sensitive to the LNG plant noise than 4D seismic, provides good coverage of the area, can be positioned dynamically, and has proven to be a robust technology even in this very active construction area. RasGas has now successfully completed five microgravity surveys. The first two microgravity surveys, conducted in 2005, were baseline surveys to measure the in situ microgravity field at RLIC before the onset of the gas injection. The repeatability of the microgravity surveys in terms of background noise was also evaluated. A time variable reservoir model of the injection gas was used to predict a time variable change in the microgravity field at RLIC. This modeling indicated that after two years of injection, a 20 microgal anomaly would develop in the microgravity field near and around the injection wells and would continue to grow in strength and size for as long as the injection continued. Therefore, the method was approved for yearly monitoring. The ultimate precision of each survey varies on a year to year basis, but the average precision has been less that six microgals. The variability in the precision of the repeat measurements can be attributed to small errors in the elevation measurements, which are specified to be less than one cm, and bulk elevation changes due to the construction of the new plants in the RLIC area. To minimize the difference due to elevation changes, each station is corrected to an elevation datum established by three remote GPS base-stations. Other natural changes to the in situ microgravity field attributed to natural causes are either minimized or corrected. These natural changes include tides (both Lunar and Solar) and seasonal variations in the water table. For example, the seasonal variation in the water table was minimized by carrying out the surveys at about the same time of the year, in the fall, after the summer drought. Follow-up monitoring surveys were completed in November of 2007, 2008, and 2009. The 2007 survey result subtracted from the baseline survey showed a semi-circular microgravity anomaly centered on the injection wells with the expected magnitude and size predicted by the modeling. However, two unforeseen issues became apparent from this survey. The first was that approximately 15% of the gravity stations were unable to be re-acquired and had to be moved due to the active construction in the RLIC area. The second issue was that other unpredicted microgravity anomalies due to near-surface construction activity were discovered, which partially masked the reservoir monitoring anomaly objective.