With the advent of the Quad O emissions regulations that are being mandated by the United States Environmental Protection Agency, along with a host of other methane emissions regulations, the regulatory landscape for operators is getting more complicated and costly to navigate. The focus of these regulations is to eliminate methane emissions associated with oil and gas production, with a combination of regulations that require a deep breath and lots of time to navigate.
Methane emissions can come from various sources on well sites. Most, if not all, valves used in production trees will leak some methane from the bonnet seals, albeit at a low rate. Well head stuffing boxes used with pump jacks are a regular source of methane emissions. Seals deteriorate over time, particularly seals used in dynamic systems like rotating shafts or reciprocating rods. The methane molecule is much smaller than an oil molecule, so seals may still function as intended, to keep oil in, but they often allow methane to vent.
This means there is a background level of venting, particularly with the proliferation of oil wells with large volumes of co-produced gas that are now the primary wells that are drilled in the U.S. These emissions form part of day-to-day operations and can be differentiated from “accidental” emissions caused by equipment failure or outside action. Example of these would be holes in flowlines caused by erosion, or an impact to a flanged connection causing it to vent.
Most emissions on well sites come from known point sources. Stuffing boxes and valve bonnets are typical sources on wells, safety valves on tanks, and pressure relief valves on other equipment typically vent methane to the atmosphere. These last sources are designed to vent as a safety precaution and operated irregularly, typically due to something happening somewhere else and in response to pressure anomalies. The operation of these safety valves goes unnoticed due to the low frequency of operation, but when they do operate, they can vent a significant amount of methane. A secondary issue with these valves is if they fail to reseal after an event. They will continue to vent methane until the issue is recognized, which could be months later.
Intermittent venting is a challenge to monitor. Periodic monitoring systems where a survey is performed once a quarter have a very low probability of catching intermittent venting, leading to the recognition that continuous monitoring is more appropriate. Continuous monitoring technology is available in many different forms. An excellent review was published by the International Association of Oil and Gas Producers on the various different technologies used for monitoring emissions, and these systems do function as designed. The challenge is that many of these systems are costly and commonly provide a partial solution. Examples are systems that do fence line monitoring. These systems rely on sensors that are distributed around the site, away from the point sources. The idea is that when an emission event occurs, the fence line sensors will pick it up and using a sophisticated model they can triangulate back to pinpoint the source. In reality, the resolution of the system is very poor, typically greater than a 16 feet radius (5m), which can narrow down the area to search but doesn’t pinpoint it. With an intermittent event, sending someone to investigate with a camera is futile as the intermittent nature of the event means that unless the investigator is looking directly at the source at the time of an incident, the probability of finding it is low.
Black Sand Technology has taken a different approach to the problem. As most fugitive emission come from known sources, it makes sense to place a sensor at the known source points to record emissions in real time. The sources are classified as NEC Class 1 Division 1 or 2 / ATEX Zone 0 or 1 in terms of hazardous gas environment. This means that the sensor needs to be compliant with relevant standards for hazardous locations, either explosion proof or intrinsically safe. The sensor package chosen for the new system is intrinsically safe with a small form factor. Black Sand Technology installs the base sensor into a simple stainless-steel housing with a ¾” NPT thread to allow the sensor to be mounted in a bracket or a housing close to the emission source.
The sensor was designed from the outset to be wired to a controller. The thought of a thousand sensors in a field using batteries for wireless transmission was the primary driver for a wired system, as operators need to have low maintenance costs and battery replacement would be a never-ending expensive task in terms of inventory and manpower.
A benefit of developing the system as a wired package meant that the data could be read by existing SCADA systems. This allows a monitored event to be correlated DIRECTLY to a monitored data source such as a pressure sensor. The gas sensor detects a safety valve operating in direct response to a pressure sensor recording an event, displayed in the operator’s own system. Sometime later the fence line monitor will detect the emission source, but as it is already known and classified the fence-line alert can be safely discounted.
The Multipoint Gas Monitoring system that Black Sand Technology has developed allows up to six sensors to be connected to a controller. The controller aggregates the sensor data and publishes it to Modbus to allow easy SCADA integration. The system is easy to install, with the primary control box connected to SCADA power and TCP/IP. The sensors can be mounted in free air or a custom designed Stevenson screen to allow the gas to be detected even in windy conditions.
A variation of the application is to integrate the sensor into existing pollution control devices on pump jack well heads. These come in many shapes and styles, from the Pollution Control Corp design which can be simple modified by drilling a ¾” hole in the top of the housing, to the Wellhead Systems Inc Environmental Control Shutdown Switch, where the sensor can be installed via a simple T between the ECS and the integrated vent.
The ultimate goal of installing a point monitoring system is to enable clear monitoring in the same dataspace of emission events. With emissions data in the same platform as all the other monitoring parameters enables reliable detection, monitoring, and mapping of vent sources in daily operations. This opens the door to condition-based monitoring opportunities, monitoring seal performance related to the amount of methane escaping from the seal, or monitoring safety valve performance in terms of seal and closure. As the landscape evolves, more applications for point monitoring that can integrate with legacy systems will become apparent, increasing the ability of operators to responsibly manage their site emissions in a practical and cost-efficient manner.
