The Banzala Field (Block 0, Angola) has produced oil with horizontal wells and ESP's as the artificial lift method for more than seven years. These wells were drilled with large sinusoidal undulations intentionally to cut the pay section a number of times. Banzala "A" horizontal wells underperformed expected production, possibly due to downhole slugging. In addition, declining reservoir pressures, rising GOR, and rising water-cut have also adversely affected production.
The objective of this simulation study of wellbore transient flow is to understand past production performance and to find ways to mitigate adverse well behavior. Simulation showed that low ESP efficiency could be related to down-hole slugging. GOR was the most significant factor for slugging and increasing water cut made slugging worse. The sinusoidal wellbore trajectory was studied to optimize ESP operating conditions. It was found that reducing sinusoidal amplitude by half and flattening the heel-end entrance angle from 79 deg. to 86 deg. would suppress slugging significantly.
Banzala Field. The Banzala "A" Jacket includes thirteen wells producing from the Mesa and Lago reservoirs. These wells were drilled with large sinusoidal undulations intentionally to cut the pay section multiple times.
BZ-A10 is a sinusoidal well in the Lago reservoir, see Fig. 1 excerpted from the reservoir simulation model. Since BZ-A10 is located close to the water injector BZ-A14, it had a higher water cut than other production wells. It also has a high-amplitude sinusoidal shape. BZ-A10 had an abrupt production drop accompanying increasing water production. Therefore, BZ-A10 was selected for a more-detailed study.
No PLT logs have been run yet to measure downhole flow distribution and characteristics; however, it is possible that poor well deliverability was caused by water accumulating in wellbore low spots and gas in higher points. This would not only have choked the flow but also caused worse slugging and unstable production. The use of ESP's could further complicate matters. Low ESP efficiency may relate to the gas-handling ability of the specific equipment used and the actual GOR.
Wellbore transient flow simulation was carried out to understand what was going on during the historical production period and to find mitigating solutions. Dynamic modeling was required to capture possible slugging. Horizontal wells 1, especially sinusoidal ones 6, may generate severe slugging which is not predicted by conventional NODAL analysis. If the wellbore geometry, flow velocity, GOR and WC fall into certain operating ranges, severe slugging may occur during which liquid accumulates in the low spots in one period and flushes into the ESP in another period.
Banzala A-10 Production Performance. Fig. 2 gives liquid flow rate QL, GOR, and WC history. Liquid rate is based on well test data. Note that the allocated production rate was about 20% lower than the well test rate. Analysis indicated that the well-test rate was more reliable and thus it was used.
Fig. 3 shows ESP downhole sensor measurement data (SCADA) for the BZ-A10 well. The entire production history from Feb. 2000 to Feb. 2006 can be divided into three different stages, as shown in Fig. 2 and Fig. 3.