Subject : Reservoir pressure decline in Cogollo wells
Between Jan 1994 and March 1st, 2001 Cogollo wells have produced a cumulative oil volume of 17.1 MM STB from 20 well (six ofwhich are old wells from PDVSA). Four wells (UD494, UD536, UD564 and UD579) account for approximately 74% off take the total cumulative production from the Cogollo Reservoir. An increasing contributionto the Cogollo producing rates and developed reserves comes from old Rio Negro wells converted to Cogollo producers (re-completions/reactivations at Maraca level). Cogollo wells currently produceapproximately 16,200 bopd, of which about 9,500 from Maraca re-completions/reactivations.
Pressure decline in Cogollo wells:
A reservoir pressure survey campaign was carried out during the Feb 2001MPP shutdown allowing data gathering in wells that would otherwise not be shut-in for any extended period in view of production deferment.
Estimates of developed Ultimate Recoveries (UR) of currentCogollo wells were calculated for natural flow conditions and an abandonment reservoir pressure of 5000 psi (Cogollo FDP, 1999). The currently developed UR is estimated at 38.3 MMSTB under thesepremises, updated with the latest pressure survey results.
Shown on the next page are pressure decline profiles obtained from build up tests of eight Cogollo wells which together account for approximately12,500 bopd. Four of these wells (UD494, UD536, UD564, UD579) have several data points allowing extrapolation of the pressure decline profile with a good level of confidence. The other four wells(UD111, UD171, UD209, UD141) have only two data points each, which makes pressure extrapolation less reliable. However, the pressure decline extrapolation for these wells was done in line with trendsobserved in the four wells with more extensive pressure data coverage.
The attached plots show that under natural flow conditions UD536 will likely be the first of these eighth wells to reach the...
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