After a well is completed, hydrocarbons do not reach the wellhead ready for transport to market. Efficient and effective production over the life of a project requires the right design, installation and equipment. To ensure a safe, quality output, wellsites should incorporate separation, filtration, dehydration, heating and cooling, and stabilization equipment. Pumping systems also play a crucial part in maintaining a smooth well pad operation up to the gathering system or the pipeline.
Well pad production facilities have five main purposes:
- Control flow of well fluids
- Separate well fluids (gas, oil, water, solids)
- Process and measure oil and gas
- Store and transport product for sale
- Process water, solids and low-pressure vapors for recovery or disposal
As with any well pad production facility, a key component to sizing the equipment and pump system is a clear understanding of the expectations and flow circulation requirements of the wellhead fluid up to the pipeline. Understanding these variables will help end users properly size the equipment and ensure a quality product.
As these fluids and gases pass through each stage of the process, pumps play an integral role in the treatment of the gas, oil and water. Examples of production and midstream equipment processes that require pumps are at the glycol dehydration unit and at the lease automatic custody transfer (LACT) unit.
Glycol dehydration is the process of removing water vapor from the natural gas stream (see Figure 2, page 20).
Three main reasons to dehydrate natural gas are to prevent hydrate formation (which reduces the occurrences of line plugging), deter the formation of free water in pipelines (which reduces line capacity), and to help reduce the possibility of pipeline corrosion. A dehydration unit—commonly referred to as a "dehy" unit—is used to bring natural gas in contact with a liquid desiccant, triethylene glycol (TEG), which absorbs water vapor from the natural gas stream. The dehy also includes a regenerator that heats the TEG to drive off the absorbed water.
Moving TEG through the dehydration process requires a special glycol pump. As with any production equipment, the type of pump used is based on several factors relating to user preference, real estate, regulations and economics. Two basic pump types commonly used to pump glycol are energy exchange pumps (which require no electricity) and electric-motor-driven pumps. Although both pumps perform the same function, each one has its place in the market.
Energy exchange pumps are typically used for regenerators that will operate at remote locations without electrical service. These pumps efficiently circulate rich (with absorbed water) and lean (water-free) glycol through the absorber tower and the regenerator to facilitate the removal of water from the natural gas stream. These pumps serve double-duty and use the pressurized rich glycol and a small amount of pressurized gas returning from the absorber tower as a source of power. This energy is transferred in the pump to move the low-pressure lean glycol from the regenerator back into the high-pressure absorber tower. Multiple pumps are commonly used in parallel, with one pump reserved as a standby or backup. Pump capacity ranges from 8 to 450 gallons per hour (gph) at pressures ranging from 100 to 2,000 pounds per square inch (psi).
A small but measurable increase in emissions occurs when using an energy exchange glycol pump. When electric power is available, users often prefer to use electric-motor-driven glycol pumps. Although these pumps require a power source, the benefits can outweigh the costs depending on factors such as power availability and permitting requirements. While not as common as energy exchange pumps, electric-drive pumps work well in dehy systems. When used in these applications, electric-drive pumps are never exposed to rich, high-pressure glycol returning from the absorber. Instead, the electric-drive pump moves the low-pressure, lean glycol from the regenerator to the high-pressure absorber tower. Electric-drive gear pumps deliver pressure up to 1,500 psi with a flow rate ranging from 24 to 7,000 gph.
Once well fluids are separated into a final product, liquid hydrocarbon products—crude oil and condensate—are stored in tanks to be trucked to market or sent to the pipeline for sale. At this point, the oil should meet an American Petroleum Institute (API) standard and flow through a final sampling and measuring process to account for quality and quantity. The value for this crude typically depends on volume, basic sediment and water (BS&W) content and American Petroleum Institute (API) gravity. LACT units are designed to facilitate this process through high-accuracy measurement probes and flow meters. Pumps are typically necessary to transfer the "on-spec" oil to the LACT unit and to deliver it at a set pressure required by the pipeline.
Custody transfer (CT) is a critical stage of the process where payment is accounted for based on quality and quantity. On-spec oil enters LACT units from the storage tank (see Figure 1). These units commonly use either American Naational Standards Institute (ANSI) or API 610 centrifugal pumps; however, unloading sites may require positive displacement pumps. Centrifugal pumps are often associated with high net positive suction head (NPSH) available and high flow rates at low pressures.
If pumping conditions are reversed, positive displacement pumps are used when NPSH available and flow rates are low at high pressures. LACT units need to meet basic requirements to correctly size a centrifugal pump:
- Discharge pressure
- Specific gravity
- Flow rate
Flow rates vary per customer specifications and range from 80 to 300 gallons per minute (gpm). Booster pumps, a supplement to LACT units, can require up to 875 gpm at differential heads of up to 195 feet. Pump manufacturers have extensive experience in sizing pumps for hydrocarbon condensates. For example, for a specific gravity of 0.73 with a differential head of 195 feet at 875 gpm, OH2 pumps are a good fit for this liquid type.
Knowing the fluid conditions and basic pumping requirements of condensate liquid being transferred to LACT units will promote a successful transfer of ownership of a high-quality, reliable product.
Reduced downtime and reliability are imperative to the success of processing oil and gas to meet specification for sale. Glycol dehydration and LACT units play a pivotal role in accomplishing this task.