Mobil Oil Canada's Wimborne Producing Team identified an economic opportunity to increase production and reduce operating expenses in the Wimborne field by converting several gas lift wells to electric submersible pumps (ESP). The economic success of the project demands that a one year ESP run life be achievable in the Wimborne field's high hydrogen sulfide (H2S) environment. This paper reports on work performed to increase the life of ESP's in wells with high concentrations of H2S (20% +). The field experience had shown corrosion could be severe enough to destroy any copper alloys and some stainless steel alloys despite their location in the unit. Research work and failure analysis led to the development of unique ESP equipment designed to maximize run life in the Wimborne field. The work represents a joint effort among a major oil company, a ESP manufacturer and an academic research institution to develop methods to eliminate the sources of failures.
The Wimborne field is approximately 60 miles north of Calgary, Alberta, Canada (Figure 1) and is operated by Mobil Oil Canada. The field has of 45 wells that produce from a sour dolomite known as the Leduc formation. The average hydrogen sulfide (H2S) percentage in the gas stream is 20%. The reservoir is pressure maintained by a strong underlying aquifer and has remained on primary production since it came on line in 1958. Table 1 gives a summary of the well characteristics.
Presently, the primary form of artificial lift is gas lift. Gas lift was first introduced to Wimborne in 1982. Then the justification for gas lift was based on safety concerns, corrosion considerations, and the availability of gas compression at the plant. However, in creased water cuts due to a maturing field accompanied with further field development have put a strain on the existing gas handling facilities.
With the average gas lift well now requiring 1.0 million standard cubic feet per day (scf/day) of injection gas. It was essential that alternative forms of lift be evaluated in an attempt to reduce operating expenses and optimize the facilities. Investment in additional gas compression to meet demand would have required capital in millions of dollars with costly long term process expenses. Other disadvantages of the gas lift system were associated with missed opportunities to increase production from increased drawdown. Depth, high water cuts, and high production volumes set limits on the gas lift systems potential to increase draw-down.
In the mid 1980's several ESP's were installed. The first ESP installed, operated approximately six months, but, no succeeding installation would repeat the six month run life. The average combined run life of the additional installations was less than a month. Due to the short run life of the ESP's, all ESP wells were converted back to gas lift. The failures were recorded as electrical problems associated H2S attack of the electrical connectors, main cable, motor flat leads, and the motor.
The support of an in-house ESP specialist, advances in ESP technology, and support from a major manufacturer based in Tulsa, Oklahoma produced the confidence to attempt ESP applications in Wimborne once again. The project was limited to a few wells that were selected as experiments. If a suitable run life could not be achieved, the project would be deemed un-economical. If a suitable run life was achieved, then more wells would be converted to ESP's.