ULA FIELD-LIFE AFTER THE FLOOD: CORE LOG EXPERIENCE FROM BEHIND A MATURING WAG FRONT

摘要

Following 10 years of Water Alternating Gas (WAG)experience on Ula, arrested production decline andincreasing gas-oil ratios indicate that the process isworking. This evidence supported special coreanalysis results which demonstrated that miscibleWAG could reduce residual oil saturations from ～35-50% following water flood (Sorw) down to ～5%after WAG (Sorm). However, for planning of anlarge scale expanded WAG scheme, the habitat of theremaining oil the injected gas pattern needed to bebetter understood.In 2006 well A9A was drilled within the most matureA3A-A15 WAG panel of the field specifically tomake detailed core & wireline measurements toestimate WAG efficiency. One hundred metres ofcore was recovered from the well. Coring & pluggingfollowed strict guidelines to reduce fluid losses &contamination. The core analysis program includedexhaustive routine porosity & permeability & Dean-Stark water saturation measurements along tolueneextraction of remaining oil. In addition, oilcomposition from gas chromatography analysis anddirect water resistivity measurements of waterextracted from the core were performed. Acomprehensive wireline log program was also run tocomplement the core analysis including, gamma-ray,temperature, density-neutron, sonic, resistivity,pressure measurements and fluid samples.Data acquisition in A9A has successfullycharacterised the WAG sweep within the A3A-A15WAG panel. The data clearly describe a two layeredsweep pattern. Within high permeability corridors,remaining oil saturations are approaching thelaboratory determined Sorm values and are currentlywater-flooded. Remaining oil in these intervals isstripped of its lighter-end components. Overlyingthese intervals are lower permeability layers whichalso have very low remaining oil saturations withcompositions stripped of their lighter-endcomponents and are currently gas bearing. Betweenthese WAG fingers are large volumes of reservoirunaffected by the WAG process and with relativelyhigh Sorw’s.The findings of the study support the planned growthof the Ula extended WAG program based upondown-flank horizontal WAG injector wells placedstratigraphically deep to sweep the lower reservoirintervals as yet unaffected by the current WAGprocess.