Scott Losing, Engineering Program Manager at Emerson Automation Solutions, and Andrew Prusha, Global Upstream Oil and Gas Manager for Emerson’s Flow Controls Products, recently published an article in the Issue 2, 2021 of Oilfield Technology. The article describes how electric control valve drives are helping oilfield operators address methane emission regulations and maximize production. The article is titled Switching On To Electric Control Valves’ Potential and is summarized below.
Wellheads are often located in very remote sites, lacking both compressed air and power, so their pneumatic controls are frequently powered by natural gas. While these controls work well, they can be prone to maintenance problems if the gas quality is poor. More troublesome, they continuously vent methane. Limiting this venting has been targeted by numerous countries to curb methane emissions (Figure 1).
Figure 1: Recently introduced by the US Environmental Protection Agency (EPA), CFR 40 Part 60 Subpart OOOO seeks to dramatically reduce methane emissions.
New standards are forcing wellhead operators to either replace natural gas-driven pneumatic devices with low bleed alternatives, or to instead install air compressors for powering the devices. In either case, significant CAPEX may be required.
Fortunately, there is another alternative, made possible by advances in electronic drive technology and the availability of limited power at well sites from solar systems, batteries, and small natural gas generators.
Electric Valve Drives
New powerful, low-cost electric drives have recently been introduced that can be easily retrofitted to many existing control and on/off valves. Scott and Andrew describe their capabilities:
These drives only require low voltage (12V and/or 24V) and low current, while providing fast and reliable control along with diagnostics. This gives users the ability to satisfy the new methane reducing environmental regulations while providing remote control and monitoring of their well sites. More importantly, the new drives and instrumentation enable profitable, advanced control strategies that were not easily implemented before these upgrades.
Here are just a few of the many new control options enabled by this technology.
Oil Separator Level Controls
An oil separator is a simple process device that uses gravity to separate well fluids into oil, water, and gas (Figure 2). Water collects upstream of the weir, oil flows over the weir, and gas leaves the top. In remote well sites, simple on/off control is used to maintain an oil/water interface about halfway up the weir, along with a sufficient level of oil on the back of the weir to keep gas from escaping.
Figure 2: An oil separator uses gravity to separate incoming well fluids into gas, oil and water.
Unfortunately this coarse method of control tends to create pulses of oil and gas which are difficult to measure. Flow is often understated, reducing profits. However, electronic control valves provide an alternative, and much better, solution. These valves can be programmed to operate in a linear fashion, so oil and gas flow become continuous. This results in tighter level control and better flow measurement. The authors describe the result:
Electronic control valves were recently employed on a number of oil separators in North Dakota, US. The control improvements reduced gas loss down the oil line by 80% and improved oil flow measurement accuracy by 5%, effectively increasing well production.
Gas Lift Enhancements
Another area of opportunity is improved control of gas lift. Gas lift injects pressurized natural gas down the well annular space to reduce the well fluid density and help push the liquids up the well (Figure 3). Gas lift can increase production, but it results in a loss of natural gas. Therefore the gas flow must be metered carefully, providing just enough to improve oil production.
Figure 3: Gas lift uses natural gas injected at the base of the well to help push fluids to the surface. Electronic controls and valves enable more advanced control schemes to maximize production, while minimizing wasted energy and natural gas usage.
Pneumatic controls are typically set once during start up and left unchanged. However, electronic controls and control valves can automatically adjust the flow to optimize production as conditions change, increasing net profit.
Plunger Lift Control Schemes
Plunger lift is used on ageing gas wells when falling well pressure becomes insufficient to push entrained liquids to the surface. These liquids can pool at the bottom and eventually stop all gas flow. Plunger lifts usually employ some type of plunger to help push the liquids up the well (Figure 4).
Figure 4: Plunger lift uses a plunger and trapped wellhead pressure to push liquids to the surface.
The controls block the gas flow at the top, let the plunger fall to the base of the well, and then release the gas when the well has pressured up. The surge of gas drives the plunger and any liquids to surface, allowing the gas to flow until the liquids build up again. Another plunger cycle is then started.
Plunger lift control requires logic to detect falling gas flow and automate the plunger cycle. There is also significant advantage to controlling the rate of gas flow surge so that it drives the plunger correctly but does not overwhelm downstream flow measurement. Electronic drive control valves make all this possible, generating significant cost improvements, as Scott and Andrew describe:
Electronic transmitters and control valves enable plunger lift operations and are able to control and measure the resulting gas surge flow. This improved gas flow measurement saves about US$85,000 a year on an average well.
Rod Lift Flumping Controls
Rod lift is another enhanced recovery method, using a downhole pump to push liquids to the surface (Figure 5). The gas pressure in the well must be carefully controlled or the gas flow can vapor lock the pump, reducing efficiency and production. This condition is known as “flumping”.
Figure 5: A rod lift well can experience reduced liquid production due to ‘flumping’ when high gas flow from the well creates vapor lock conditions on the pump.
Electronic controls and valves can be used to maintain enough backpressure on the well to maximize gas flow, while maintaining liquid production. The elimination of flumping can generate approximately US$2000/day in increased production on a 250 bpd well.
When considering a wellhead instrument upgrade, it is worth the time to investigate the latest technology. Implementing electronic transmitters and electric control valves can significantly increase production and improve your bottom line.
Figures all courtesy of Emerson
About the Authors
Scott Losing has 20 years of design and development experience with Emerson flow computers and control valve actuation. He completed his BS in Computer Engineering Iowa State University and is the Engineering Program Manager at Emerson Automation Solutions for their Fisher easy-Drive Electric Actuators.
Andrew Prusha is Global Upstream Oil and Gas Manager for Emerson’s Flow Controls products in Marshalltown, IA. He has seven years of experience representing Fisher valve & instrument products from a variety of roles.
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