An Application of Gravity Method to Estimate a Storage Capacity of Ngrayong Formation for Carbon Capture and Storage (CCS) Pilot Project of Gundih Field, East Java, Indonesia

Abstract

Indonesia has committed in Global Initiative as non-Annex I country to reduce 26% of GHG emission reduction further up to 41% reduction with incoming international support. Initiated by the scheme of SATREPS (Science and Technology Research Partnership for Sustainable Development) program co-funded by Japan International Agency, Indonesia works on the pilot project implementation of Carbon Capture and Storage (CCS). CCS is one of technology to be used to reduced CO2 gas emission in the atmosphere. The CO2 is captured from the source or atmosphere, then purified and liquefied. The liquefied CO2 then injected into underground reservoir. The underground reservoir could be a porous geologic formation such as an old hydrocarbon reservoir. At present, as guiding by regional and geoscience characterization at the further stage called as engineering characterization well integrity as an early stage of injection phase is done. Based on the evaluation of Regional Characterization study for CCS Pilot Project in Indonesia, Gundih Field was selected as the most suitable target. Gundih Field is a gas field located in the Northeast Java Basin, East Java, Indonesia. Accordingly, Geoscience Characterization has been done including structural and stratigraphic model of the region. Stratigraphically, the oldest formation at Gundih Field area consist of Basement, underlie uncomfortably by Tawun Formation, Ngrayong Formation, Wonocolo Formation, Ledok Formation, Mundu Formation and Lidah Formation. The Ngrayong Formation was decided as a targeted bed to be injected by CO2 liquid. Therefore, the study consists of injectivity, containment and capacity of the Ngrayong Formation reservoir. This formation mainly contains of sandstone intercalated by limestone beds. The age of this formation is Middle Miocene. There is one abandoned well available in the field namely Japon-1 well. The JPN-1 well will be utilized as an injected well. As the pilot project, the total CO2 planed will be stored in the formation around 20.000 tons within 2 years. Therefore, it could be calculated that approximately 52.300 m3 pore space is needed in the reservoir formation. Some studies related in to Geoscience Characterization for CCS implementation has been done, included geological and geophysical studies to evaluate the capacity of Ngrayong reservoir. In order to estimate the storage capacity of Ngrayong Formation, the gravity method was applied. The gravity acquisition was done in September 2014 using gravimeter Scintrex CG-5. The measurement was done in the surrounding of Japon-1 well for about 25 km2 area. In order to get better accuracy, the point of measurement was done with the interval of 250 m. Based on the measurement data the Complete Bouguer Anomaly Map was constructed on the assumption that the rocks had an average Bouguer density of 2.5 gr/cc. The result of Complete Bouguer Anomaly Map contain noise, then moving average filtering method based on spectral analysis calculation was applied. This Complete Bouguer Anomaly Map was used to make an interpretation of subsurface condition of Ngrayong Formation. By using this map, further process was done as separation between regional and residual anomalies. The separation anomalies were done as similar previous method using moving average filtering method based on spectral analysis. Using residual anomaly map, the 2.5D density model was calculated using forward modelling method. Some supporting data was used such as data of JPN-1 Well which the depth of Ngrayong Formation is 816 m, seismic data as the time structure map of nearly top of Ngrayong Formation and seismic section across the region, geology and stratigraphy of Rembang Zone. In order to get approximate volume of the formation, then the “pseudo-3D” model was developed. Based on this model the estimated capacity of the porous space of the reservoir could be calculated. According to well data of Japon-1, the porosity of Ngrayong Formation was 28%. Based on well stratigraphic data, the volume of sandstone in this formation is calculated 5%. By assumption that only 50% could be injected by CO2 fluid, the injection capacity of Ngrayong Formation using Japon-1 well could be calculated. The result of calculation, the porous space volume of Ngrayong Formation is approximately 17.168 x 10 m3 and pore space volume 30,044 x 106 m3. The result calculation of pore space volume of Ngrayong Formation is more than enough to accommodate injected CO2.