The Application of One VSD Driving Multi-ESPs Technology (OVDMT) in Offshore Heavy Oil Field
Authors: Li Chengjian , Liu Zongzhao , Yang Zhai , Wang Yinzheng 
Published: December 2007

SZ36–1 oil field is the largest developed offshore heavy oil field in China, located in Liaodong bay of Bohai, where the winter temperature is very low and the water depth is about 30 m. Since 1993, 4 blocks of this field have been directly put into production with electric submersible pump (ESP), watercut varies from 0% to 40%, ESP has played an important role in keeping an annual output of 1.3~2.1×106 tons. However, the combined actions of such factors as heavy oil, power supply system failure, low air temperature and cold sea water pose difficulties in restarting of ESP after shutdown. What's worse, 1~2 sets of the ESP motors were damaged each time when restarting. Aiming at the problem of difficult-to-start and limited platform space, we have developed the OVDMT. Now, with the aid of OVDMT, ESP can be softly started with VSD (variable speed driver). The device is capable of controlling multi-ESPs, thus reducing the areas of space-taking ESP surface equipment. So far, 3 sets of this system have been installed in SZ36–1 oil field, controlling 40 ESP wells, the average runlife of ESP in this field has been increased from about 360 days to 460 days.

Introduction

With the rapid development of oil production and ESP manufacture industry, ESP has become an important artificial-lift method. Of course, it also holds true in Bohai Oil Company (BOC). Up to May, 2000, the number of ESP wells makes up 58.5% of total production wells in BOC. The produced liquid and oil rate by ESP accounts for 68.1% and 65.0% in total production rate of BOC separately.

In accordance with original design project (ODP), the large-scale offshore heavy oil fields that belong to BOC, such as Project II of SZ36–1 oilfield, QHD32–6 oilfield and PL19–3 oilfield will be put into and maintain production with ESPs, which means that ESP will play a decisive role in oil production in BOC oilfields.

So far, SZ36–1 oilfield is the largest integrated offshore heavy oilfield in China with a reserve of about 2.7×108t. Stock oil density is 900-955kg/m3. Dead oil viscosity ranges from 500 to 5000mPa.s at 50°C. Reservoir underlays -1300~-1400m, pool temperature is amid 60~65°C. This field lies in Liaodong bay in Bohai where mean seawater depth is 30m or so and the lowest air temperature is -20°C in winter. According to the ODP, this field will be divided into 10 areas and put into production step by step. As pilot test zones, Area A and B separately began to produce oil in 1993 and 1995. Area J went into all-round production in 1997. Now this field has the capability of producing 2.16×106t oil annually from 52 ESP wells. The conventional switchboard control mode (Mode I, shown in Fig. 1) dominated absolute majority of ESP surface equipment. This mode could not soft-start ESP motor and damaged motor was the expected result when restarting. It is fit for light oil and high-water-cut oilwell. There were only 4 wells using VSD control mode II (Fig. 2, hereafter called Mode II) which could provide soft-start function and match with the production demands of heavy oil wells, but it was space-taking and money consuming.

After Platforms AI and AII came into production in 1993 and 1994 separately in pilot test Area A, it was found that the restart of mode I controlled ESPs was very difficult when main power went off or ESP wells shutdown for some reason or other, especially in winter. To resume ESP running, a serial of dangerous steps had to be taken, such as increasing operating voltage, reversing phase sequence, adjusting overload current setting, extending overload delay time, diesel soaking wellbore, seawater displacing dead oil, etc. Unfortunately, 1 or 2 ESP motors were damaged each time restarting. After carefully analysis, we found out that the main reason was that heavy and viscous oil creates great resistance for crude oil to flow in pumps and tubing. And the ever-worse pump performance and the low air and water temperature deteriorated the problem.