Project expansion plans and recent increases in operating temperature led Denbury Resources Inc. (DRI) to re-evaluate the CO2 compression facilities at the Little Creek Tertiary Recovery Project.
DRI operates a tertiary CO2 flood project in the Little Creek field between Lincoln and Pike counties north of McComb, Miss. There are 32 producing wells and 28 injection wells in the project. CO2 is transferred to the field for injection from a supply facility via a 20-in., 90-mile (144-km) long, high-pressure pipeline. Pressure of the purchased CO2 at the end of the pipeline is at about 1,100 psi, and must be boosted to 3,000 psi for injection. Average temperature of the product at the end of the pipeline is 81?F (27?C). Ordinarily the boost in pressure required the purchase of additional compressors.

Two 700 hp compressors, each rated for 32 MMcf/d, were originally being used to inject the CO2. Increases in the temperature of the CO2 from the pipeline had reduced the compressor performance to 27 MMcf/d. This made them less effective (i.e., less product was being injected). In addition, the compressors were:

• Expensive to purchase and/or rent;
• Very complicated to install (requiring extensive site preparation);
• Costly to operate (with large power requirements); and
• Maintenance intensive (resulting in loss of CO2 injection and reduced oil production during costly downtime).
Denbury's objective was to increase the volume of CO2 injected into this field, while reducing downtime and operating costs.

The solution

Woos Group ESP's (WGESP) engineering group worked with DRI engineers to design a totally new approach. One of the compressors was replaced with specially designed SPS technology based on two horizontal, surface-mounted electric submersible pumps (ESPs). Due to the high-pressure, multiphase, non-lubricating environment, a detailed analysis and testing of all pump components was required during the system design and engineering stage. Aerospace technology was employed to develop reliable rotating-shaft seals. This resulted in a tandem cartridge, self-lubricating, mechanical shaft seal and supporting seal flush on the suction side of the SPS intake. WGESP patented the bolt in (component) packaging of this seal assembly that makes for a more aesthetic installation and ease of change out. Additionally, the appropriate stage count of the pumps and selected stage designation was engineered to move the required volume of CO2 at the reduced densities, which were hampering the existing compressors.

Minor design modifications to the plumbing system at the DRI field facility were required to ensure dense phase liquid was being delivered to the pump assemblies.

The SPS technology was installed, replacing one of the two compressors, and operated on a 6-month field test. The units were then dismantled and analyzed to ensure integrity of all components prior to re-installation. Significant improvements were realized.

How it compares

The purchase price of a compressor designed to move 32 MMcf/d to 40 MMcf/d (depending on CO2 density) is US $1.2 million. The SPS skid units are designed to move 25 MMcf/d to 30 MMcf/d, also depending on CO2 density. The price of both complete skid units was approximately $285,000. One of the units is equipped with a variable frequency controller to regulate overall volume, and the other with a "soft start" to run at a continuous volume. The purchase price of the surface pumping system was less than a 3-month rental of a comparably rated compressor package.

Routine maintenance on each of the compressors (not including unforeseen extensive repairs) for the last three years averaged $22,000 annually. Maintenance on the pair of surface pump skid units has been less than $750 annually ($375 each).

A compressor requires a 9-month delivery - from order to delivery - and requires a full month to install. The pumps were delivered in less than 8 weeks, and take about 7 to 10 days to install. Additionally, the site preparation and pad area was one-tenth the size of the compressor pad, and the noise level at the facility was dramatically reduced.

The new system installed at the Little Creek unit has transported 26.7 MMcf/day of product at a 450 hp load. Compared to the 700 hp compressor volume of 27 MMcf. This equates to 36% less horsepower required to run the pumps and translates into considerable savings in monthly power consumption costs.
This project required the movement of a large volume of CO2 at high intake and discharge pressures. The surface pumping system provided a smaller, quieter, more efficient solution that was easier to install, required less maintenance and was more economical than the original compressor system.
The new system allowed DRI to more economically expand the tertiary flood in the Little Creek unit, which is currently injecting 90 MMcf/d of purchased CO2 product (handled with one existing compressor and the two new SPS units), and capturing and re-circulating 130 MMcf/d (220 MMcf/d total volume) through 28 active injection sites.

The pump system has continued to function as designed with no problems and no downtime, and has required little or no maintenance. The efficiencies gained in the injection system design have resulted in increased and more continuous production output.

DRI is considering replacing the remaining compressor with additional SPS units.