Production Technology

VOC control system reduces field gas dehydration unit emissions

To satisfy regulations governing new dehydration units, a volatile organic compound (VOC) control system has been employed cost-effectively in a mature Wyoming field, generating additional revenue through improved condensate recovery.

Tom Atwood, TOCA Services LLC, Denver

A small, mature oil and gas field operated by Saga Petroleum LLC in Park County, Wyoming, draws field gas from 17 production wells that are served by two central compressor stations. Prior to 2003, 14 wellsite dehydration units were employed within the field. However, excessive operational and maintenance costs dictated installation of a centralized dehydration unit at each compressor station and then shutdown of the field units.

Wyoming Department of Environmental Quality (WDEQ) regulations required that the new dehydration units must comply with best available control technology (BACT). They must also achieve a minimum, 95% reduction in volatile organic compound (VOC) and hazardous air pollutant (HAP) emissions from the dehydrator reboiler vent stack. Vendor quotations for standard, off-the-shelf control units indicated an installed cost of more than $25,000 per unit.

OPTIMIZED SOLUTION

Based in Midland, Texas, Saga’s business strategy emphasizes and encourages solutions that are simple, durable, effective and flexible (i.e. easily transportable). As such, a novel VOC emissions control system was designed, fabricated, installed and tested during 2004, Fig. 1. The design was based upon a field gas production rate of 3 MMcfd to 5 MMcfd, and a GLYCALC estimate of up to 90 t/year of uncontrolled VOC emissions from each reboiler vent stack.

Fig. 1. To the right of the centralized dehydration unit are the VOC control system’s condenser module and combuster unit (tripod base and stack).

Based upon additional input from production company management, and operations and maintenance personnel, it was also agreed that the new control systems would be simple to operate, easy to install or relocate, and able to withstand cold weather conditions and comply with WDEQ standards.

TOTAL ORGANIC CONTROL APPARATUS

Overhead vapor from the reboiler vent stack is directed into an air-cooled condenser module that consists of a distribution header, an array of vertically mounted, stainless steel, finned tubes, and an accumulator vessel. A float-type level switch is used to activate operation of a gas-driven diaphragm pump that automatically transfers recovered condensate and water to the existing storage tank.

Temperature, pressure and level instruments provide the operator with full system knowledge, although no ongoing adjustment is required during normal operations. The condenser module is housed within a winterized shelter equipped with controllable, louvered side vents and a full-sized access door, Fig. 2. Non-condensable vapor from the condenser module is directed through a flame arrestor to the tripod-mounted combustor unit.

Fig. 2. Interior of the condenser module structure.

The combustor is equipped with a continuous gas pilot, recorder, sight port and adjustable air damper. A side-mounted hatch provides easy access to, and removal of, the burner for inspection and maintenance.

SYSTEM PERFORMANCE

The first unit was installed during June 2004 and demonstrated excellent heat transfer capability, even during the hot summer months when daytime ambient air temperatures often exceeded 100°F. Typical recovery was measured at about 12 gal of condensate and 15 gal of water per day. Non-condensable vapor (which included drive gas from the Kimray glycol recirculation pump) averaged approximately 3 cfm and was reliably destroyed in the combustor module.

Air Pollution Testing, Inc., of Arvada, Colorado, was hired to conduct a compliance test per WDEQ requirements. The control system’s destruction and removal efficiency (DRE) was demonstrated at 99.99%.

SPECIAL FEATURES

The condenser module is fabricated on a 2-by-5-ft steel skid that can be transported via a 3/4-ton pickup truck, and then offloaded and installed by a two-man crew in a half-day. The combustor unit is fabricated in two sections (tripod base & stack) that also can be transported via pickup and erected in less than an hour. This flexibility allows for efficient relocation of equipment, as circumstances change throughout the life of the field.

ECONOMICS

The entire VOC control system (condenser & combustor) was purchased and installed for under $18,000. Although this system was mandated by WDEQ requirements to operate the dehydrator, the recovery of formerly vented condensate produces additional sales revenue.

At today’s crude prices of more than $50/bbl, the revenue generated by condensate recovery is averaging more than $6,000/year. The original prediction ($4,000/year) was based on a measured recovery of about 10 barrels of condensate per month during the summer and fall of 2004. During the winter months of November 2004 through March of 2005, this recovery rate nearly doubled, due to the lower ambient air temperature. Thus, the original revenue prediction is significantly understated. In addition, the O&M savings associated with the shutdown of the 14 field dehydrators is probably closer to $22,000/year.

The additional produced water from this process is trucked off-site for treatment and disposal. This produced water averages about 2 bpd, so a service truck comes in about once a month to pump off the water. Future plans for this facility include permitting for an on-site treatment and disposal unit.

Another dimension to this story is that the technology can also be used for stand-alone condensate recovery (as opposed to being solely a pollution control system for permit compliance purposes). There are thousands of wellhead dehydrators in use throughout the Western states, Oklahoma, and Texas. If the US upstream industry installed condensate recovery modules on a significant percentage of these units, operators could potentially harvest 10,000 to 15,000 barrels of condensate per month, rather than lose them to the atmosphere. This represents a non-trivial resource and revenue stream, and could also result in significant emissions reduction credits under the new CAA rules.

TOCA Services is discussing a pilot program with a major oil and gas firm in Denver to explore the use of this technology strictly as production enhancement.