Surface-Facilities Design for First CO2 EOR Project in Saudi Arabia
A demonstration project of carbon capture, utilization, and storage (CCUS) through enhanced oil recovery (EOR) was conducted in Saudi Arabia. The main objectives of the project are quantifying the amount of carbon dioxide (CO2) sequestered in the reservoir; addressing the risks and uncertainties involved, including remaining oil saturation and the migration of CO2 within the reservoir; identifying operational concerns; and estimating the incremental oil recovery (beyond waterflooding). This paper covers the details of the overall facilities design.
CO2 is a great injectant for EOR—for environmental, technical, and economic reasons. It is an excellent solvent, especially for light crudes, and can considerably enhance overall oil recovery. CO2 swells the oil and reduces oil viscosity significantly. CO2 EOR has attracted a lot of attention over the years because one can sequester CO2 into a reservoir and increase recovery at the same time. This dual objective will continue to expand CO2 EOR application and deployment worldwide.
The surface facilities for a CCUS and CO2 EOR project must be designed on a case-by-case basis. That is because the design is based on the amount of CO2 available, its composition, the location of the facility and its proximity to the injection site, reservoir conditions, production forecasts and processing requirements, handling of produced fluids, and comprehensive environmental assessment. Surface facilities for CO2 EOR and CCUS projects generally are very expensive to build and inherently involve tradeoffs among different options. Economics plays an important role in whatever option is selected for a given set of conditions. The choice to conduct such a project will require a thorough evaluation of alternatives. Any such design will go through a number of steps, including
- A feasibility study to address the viability of the project from a technical and economic perspective
- A prefront end engineering and design (pre-FEED) study that looks at the design on a larger technical and engineering scale
- A full FEED study that addresses the nuts and bolts of the actual facilities
Important aspects to consider in the design phase of these facilities include a comprehensive material-selection process because wet CO2 is very corrosive, injection and production manifolds, compression requirements, separation issues, and metering of produced fluids. The latter is extremely important because the performance of the project ultimately hinges on accurately measuring the amount of CO2 sequestered and the amount of fluids produced.
CO2 Capture Plant
The CO2 for the project is sourced from an exhaust stream in an existing natural-gas-liquids recovery plant that receives its feed from nearby gas plants. The feed is contacted in a conventional diglycolamine column to remove the CO2. The CO2 from the stream was initially being routed to an oxidizer to remove the impurities before being vented to the atmosphere. The CO2 is relatively pure and saturated with water. The initial scope of the project was to reroute the stream from the oxidizer into a fit-for-purpose CO2 capture facility. The main purpose of the capture facility is to dehydrate and compress 40 MMscf/D of wet CO2 to the required transfer pressure (3,500 psi). This is accomplished with a water-cooled integrally geared compressor with an intermediate triethylene glycol (TEG) dehydration unit, an aftercooler and a dense-phase pump (DPP) with suction and discharge coolers.
CO2 Compression Unit. Following the capture and separation of CO2 from the gas mixture in the gas-treating unit, the CO2 is compressed, cooled, and transported through a pipeline. The compression unit was designed to handle the wet CO2 and includes a seven-stage integrally geared compressor, a TEG gas dehydration unit (GDU) between the stages of the compressor, an interstage water cooling system, and a DPP to increase the dry CO2 pressure.
Water Cooling System. A dedicated water cooling system for the CO2 compressor was installed. The water cooling system consists of one open-loop and one closed-loop system. Both circuits of the water cooling system are designed to remove heat from rotating equipment and other process equipment in the CO2 compression unit.
GDU. To prevent numerous operational issues that might occur from transporting CO2 through pipelines, such as hydrates formation and corrosion, CO2 must be dried and dehydrated. The GDU is downstream of the fifth stage of CO2 compressor.
DPP and CO2 Pipeline. The DPP is a multistage centrifugal pump downstream of the CO2 compressor. At the discharge of the seventh stage (at 1,600 psig and 160°F), the CO2 reaches the supercritical phase (dense phase). The DPP increases the pressure of the supercritical CO2 before it is discharged into the CO2 pipeline.
Injection- and Produced-Fluid Handling
The project includes a row of four injectors and four producers and two observation wells. All the wells have been equipped with downhole monitoring sensors to provide full surveillance of reservoir parameters during the project period.
Injection Manifold. The pilot is designed for the CO2 to be injected in a water-alternating-gas mode. CO2 will be injected into two of the injectors while water is injected into the other two. After 1 month, the injection will be alternated. Therefore, the injection manifold for the four injectors was designed and constructed such that switching the injection mode is safe and efficient.
Injector-Well Instrumentation and Metering. Each of the four injectors is equipped with a Coriolis flowmeter for mass flow measurements to measure two separate fluids with different volumetric properties (water and supercritical CO2).
Producer-Well Instrumentation and Metering. All four producers are equipped with a multiphase flowmeter (MPFM) to measure the amount of gas, oil, and water of each individual producer accurately. The MPFM consists of a temperature and pressure transmitter, a differential pressure transmitter, a gamma densitometer, an in-line capacitance sensor, and inductive sensors. The purpose of the capacitance sensor is to measure the fraction of oil, water, and gas flowing through the meter.
Produced-Fluid Handling. The produced fluids from the four CO2 EOR producers are transported through a newly installed trunk line to the dedicated and built-for-purpose high-pressure production trap (HPPT) at the gas/oil separation plant (GOSP).
HPPT. The produced fluids from the four CO2 EOR producers are separated inside the three-phase HPPT vessel to oil, water, and gas (Figure above). The vessel was constructed using a cladded special alloy to accommodate the high concentrations of CO2. The separated oil is combined with the existing GOSP oil stream to the low-pressure production trap (LPPT). The separated water is sent to a water/oil separator and saltwater disposal system. The produced gas from the new HPPT is compressed with a dedicated vertical compressor, combined with the high-pressure gas discharge, and piped to a nearby gas plant for further processing.
CO2 Compressor. A vertical, seal-less, variable-frequency-drive centrifugal CO2 compressor using active magnetic bearings was selected for produced-gas compression. The compressor was designed to handle the maximum amount of gas produced during the life cycle of the project.
Chemical Treatment. Before entering the HPPT, the EOR fluid is treated with demulsifiers, scale inhibitors, and corrosion inhibitors.
CO2 Gas Analyzer. Before the gas enters the EOR compressor, a sample is taken from the CO2 gas stream of the new HPPT and analyzed using a gas-composition analyzer. This analyzer is a key component to quantify the amount of CO2 being produced, which will assist in calculating the amount of CO2 sequestered.
Best Practices and Early Observations
The design and construction of the capture plant and produced-fluid handling facilities was a multidepartmental, multidisciplinary, and multiorganizational effort. It involved personnel from the research center, reservoir management, reservoir simulation, gas operations, facilities planning, project management, production and production engineering, loss prevention, and environmental protection, to name just a few. Comprehensive studies including hazard and operability, environmental assessment, facilities integration, effect on existing facilities, and long-term performance prediction were some that were conducted with due diligence.
The key to success was a close collaboration of all concerned personnel and departments through a multidisciplinary team. A special CO2 EOR team was established that oversaw all aspects of the project and glued the personnel and organizations together. The project has completed more than 10 million construction man-hours with zero incidents.
One of the challenges faced was sand accumulation at the injection manifold. The four injectors for the CO2 pilot were injecting water for 2 years before the start of CO2 injection. Before the injection of CO2, the injection manifold was almost completely buried under sand from drifting sand that had accumulated. The sand was removed and appropriate sand barriers installed around the injection manifold to minimize or prevent sand from accumulating again.
No leak issues have occurred along the 85-km pipeline that transports the CO2 from the capture plant to the injection site. No leaks have been observed along the trunk line that transports the produced fluids from all four EOR producers to the GOSP. Some leaks have been detected in the choke valves and flanges at the injection manifold and injectors. These leaks sometimes occurred after the switch over of the wells from water to CO2 and vice versa. The monthly switch cycle and more-frequent opening and isolating of valves increases the chances of water and CO2 leaks. This is also perpetuated by the differences in density, pressure, and properties between the CO2 and water. Live monitoring and remote-control accessibility to the injectors enable the engineers to identifying any leak immediately and act accordingly.
After 18 months of CO2 injection, the results from the project have been very positive. The CO2-capture and the produced-fluid-handling facilities have performed well.
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