After nearly 10 years of WAG experience on Ula, production evidence shows that the process is working. In order to optimize the existing scheme and to justify investment in its further expansion, detailed measurement of residual saturations and system efficiency was necessary.
In 2006 the 7/12-A-9A well was drilled within the most mature WAG sector of the field specifically to make detailed core & wireline measurements of WAG effectiveness and efficiency, primarily by combining residual water and oil saturation measurements with analyses of hydrocarbon compositions. Existing SCAL data had demonstrated the potential for miscible WAG to reduce the residual oil saturation from Sorw's of ~35-50% to Sorm's of ~5%. Would direct field evidence demonstrate that this was possible within the heterogeneous & dynamic conditions of the reservoir?
One hundred meters of reservoir column was cored and subsequently logged as part of the large data acquisition project. Coring and plugging followed strict guidelines to reduce fluid losses and contamination. The core analysis program included routine porosity & permeability measurements, Dean-Stark water saturations along with direct water resistivity measurement of extracted the water and gas chromatography of the extracted oil. A comprehensive wireline log program was run to complement the goals of the core analysis including, gamma-ray, density-neutron, sonic resistivity, pressure measurements and fluid samples. In addition, both the NMR, pulsed neutron and carbon-oxygen tools were run to provide non-resistivity based fluid saturation estimates.
The result of the data acquisition in 7/12-A-9A has been the successful characterisation of the WAG sweep through the Ula reservoir at this location in the field. The data clearly describe a two-fingered sweep pattern. Within the high permeability fingers, irreducible oil saturations are approaching the SCAL determined Sorm values with oil stripped of its lighter-end components and currently containing high Sw's. Overlying these intervals are lower permeability intervals which also have very low Sorm's with compositions stripped of their lighter-end components but containing high Sg's. Below the high permeability fingers are large volumes of reservoir unaffected by the WAG process and containing high irreducible Sorw's.
The findings of the study have given support to the planned Phase 1 of the Ula extended WAG program which includes two horizontal WAG injector wells drilled on the flank of the field and set stratigraphically deep to sweep these reservoir intervals as yet unaffected by the current WAG process.
Simon began his career in Norway in 1997 and worked for three years as a petrophysicist for Statoil. In 2000 he joined BP Norway where he worked as a petrophysicist for the Ula and Gyda Fields. In 2003 he joined the Skarv Development Team as a reservoir geologist. He is currently working as a reservoir/production geologist on the Ula Field and has most recently been reponsible for interpretation and integration of data from the 7/12-A-9A.
Jonathan started work as a geologist with BP in 2000, working on a variety of projects in exploration/appraisal in Aberdeen. He is currently working as a geologist on the Ula subsurface team and was responsible for the data acquisition planning, operations and follow-up on the 7/12-A-9A well.