The Brent field is embarking on the largest offshore oilfield depressurization project ever attempted. To allow cost-effective management of this process an extensive surveillance program is being executed; gathering formation fluid type as well as reservoir pressure data. Pressure data acquisition is well established, but for formation fluid type existing technology is pushed to its limits.
Cased-hole formation fluid interpretation has always been difficult in Brent because of the low formation water salinity combined with seawater injection. Now with oil rims moving to the crest into the original gascap and depressurization changing the properties of the gas and light oil, it has become a real challenge.
Initially Brent has used time-lapse capture sigma measurements1,2 to monitor the oil being swept by injection water. This was significantly improved upon, when the inelastic Carbon/Oxygen measurements3,4 were introduced, but the most reliable results were still obtained from simple open-hole logging data.
After the introduction of the monobore completions in Brent wells5 , surveillance does not have to be restricted to slim (1 11/16") surveillance tools anymore since tool restrictions are removed in the monobore design. As a result, the larger (3 5/8") neutron and density open-hole tools can be employed in casing.
Calibrated cased-hole Neutron/Density surveys together with the original open-hole data have proven to be very successful in identifying the current formation fluids. They have also proven to be more robust against reservoir pressure decline and borehole content than Carbon/Oxygen methods.