A case study of an underexplored field is used to consider the issue of building the most probable option of a geological model that takes into account all possible uncertainties and associated geological risks. Since the moment of discovery, the field was categorized as unallocated fund and has been poorly covered by seismic studies, deep drilling, small testing scope; it was understudied by laboratory tests of core and wellhead samples. In 2013, the field was put up for auction with the reserves estimated and approved by the regional commission. After the field was acquired at the auction, an appraisal project was completed in 2015, which increased the estimated gas reserves by approximately 400 bcm in comparison with the reserves approved by the regional commission. The reason for the significant increase is explained by re-processing and re-interpretation of the seismic data, improved amplitude characteristics, taking into account near-surface effects, changes in the structural-tectonic basis of productive reservoirs and their correlation. Both reserves estimates have not been reviewed by the State Commission on Reserves, no 3D geological models have been built.
In view of the total amount of gathered information, the appraisal project looks more preferable than the original estimate, but a significant increase in reserves looks alarming and requires confirmation of the estimated reserves. The key parameters in the probabilistic estimation of reserves are: area, net sand, fluid contact level, flow properties, as well as formation fluids properties.
First, we verified the reliability of structural maps and discovered a number of deviations. Based on the results of the analysis, on the basis of re-processing and re-interpretation of 2D seismic profiles, we found a mathematical method for building an updated structural framework of reservoirs. The tectonic structure data allowed to update tectonic faults in the structural framework, and the fracture amplitudes for each reservoir were determined on the basis of seismic data.
The logging interpretation results were revised and updated on the basis of the author's understanding of the geological structure, the structure of the two reservoirs with updated fluid contacts was detailed, the tectonic faults are generally considered to be non-conductive because of the lack of information on their conductivity. An analysis of the key parameters variability allowed to identify areas of increased risk and made it possible to avoid overstating the resource base. The recommendations for field appraisal, E&A well locations and depths have been given. The resulting data formed the basis for a basic version of the 3D geological model with the most probable volume of reserves and consideration of the existing uncertainties.
References
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2. Fokin A., Managing exploration risks & uncertainties, Rogtec, 2011, no. 9, pp. 76–84.
3. Anokhina M.S., Virskiy D.A., Risks and probabilistic assessment of prospective structures resources in southeastern shelf of the Pechora Sea (In Russ.), Nauchno-tekhnicheskiy Vestnik OAO “NK “Rosneft'”, 2015, no. (39), pp. 28–32.
