Richard Payton, Joe Niederhofer
Taurus Exploration Inc.
Tuscaloosa, Ala.
Centralized compressors afford Taurus Exploration Inc.'s coalbed gas operations optimum control of gas production. Unlike satellite stations, the centralized system allows methane gas to be shifted from station to station via the interconnecting low-pressure pipeline network.
The operations area encompasses approximately 40,000 acres, about 40 miles southwest of Birmingham, Ala. The project includes about 250-miles of low-pressure gas flow lines to almost 400 wells.
Fig. 1 locates six of the compressor stations on the 33,760 acre area. The seventh station, not shown, is on another 6,240 acres.
Development of the projects was done at a rapid pace. Most of the drilling (Fig. 2), well completion, and facility construction was completed over a 20-month period beginning in June 1989.
Taurus Exploration operates this Black Warrior basin project from its office in Vance, Ala. The operation's goal is to maximize production at the lowest possible cost while maintaining environmental compliance. An important part of environmental compliance is meeting strict air-quality standards.
Twenty-three compressor sets are currently processing 61 MMcfd of coalbed methane.
The centralized system is less costly than a satellite station to build and operate. Unlike a satellite station that requires each compressor to have a complete set of ancillary equipment, the centralized system requires only one suction manifold, one dehydration setup, and one metering facility for every five compressor sets.
PRODUCTION CONTROL
Taurus began production in June 1989, and is currently the leading producer of coalbed methane in Alabama's Black Warrior basin. The projects at Vance are being developed in partnership with TECO Coalbed Methane and Chevron U.S.A. Inc.
Fig. 3 illustrates the general arrangement for a centralized compressor station. Coalbed methane gas enters the station via a suction manifold. From there, the gas stream enters high-speed compressor sets where it is compressed and cooled (Fig. 4). Gas is then processed through scrubbers for removal of free liquids before going through dehydrators for drying to meet pipeline specifications. The gas is metered prior to entering the sales pipeline.
The compressor set consists of an engine, compressor, cooler, control panel, scrubbers, and interconnecting piping. All of this equipment is mounted on a structural steel base.
Each compressor station is located near the center of a module that comprises between 50 and 60 wells. Methane enters the compressor station suction manifold from the flow line network at less than 1 psi pressure. From the suction manifold, gas is distributed to the individual compressor sets.
The compressors have three stages. In the first stage, gas is compressed from near 0 psi suction pressure to 50 psi. The second stage compresses the gas to 160 psi. The third stage takes the gas up to a maximum 550 psi discharge pressure.
To obtain maximum operating efficiency, the compressor station functions are monitored remotely.
Each compressor station is designed to process up to 18 MMcfd of methane gas with provisions for five compressor sets. Actual compression capacity is dictated by the number and ratings of compressor sets installed at each station.
Through speed control and compressor cylinder clearance adjustments, individual unit output can be varied from 2 to 3.8 MMcfd. As methane production increases, units are added to the existing compressor stations in increments that will maintain approximately 85% utilization of available horsepower.
With the ability to adjust the output of individual compressor sets and to shift gas from one compressor station to another via the pipeline network, Taurus can control production during scheduled maintenance periods and while unscheduled repairs are being made. This flexibility reduces gas loss and helps achieve the goal of 98% run time availability.
Continuous dewatering of the coal seams is key to coalbed methane production. Utilization of the designed excess capacity in Taurus' centralized compression system allows the wells to continue operating during compressor set maintenance. Without a centralized system, the alternatives are to shut wells down or flare excess production.
HIGH-SPEED ENGINES
The project has demonstrated to Taurus that a centralized compression station system costs less to own and operate than a satellite system because the central system requires less equipment and provides operational flexibility. However, Caterpillar Inc.'s high-speed engines also contribute to Taurus' ability to produce methane gas at a low cost.
The engines were specified for the compressor sets by Tidewater Compression Service Inc. Tidewater and Taurus worked together during the mid-1980s on the TEAM (Taurus/Energen/ Alabama/Methane) project in the Warrior basin. Of the 23 compressor sets used at Vance, Taurus and its partners own 8 and lease the remainder from Tidewater.
With compressor sets that use high-speed engines, Taurus has realized approximately 30% more horsepower per unit when compared to low-speed engines. This allows Taurus to economically meet production goals, while at the same time operate within strict exhaust emission standards.
Emission control was a key element in selecting the compression equipment. Because of the rapid development pace, Taurus designed the stations and selected equipment to maintain emissions below levels required by state exhaust emission permits.
To be below the permit level, each compressor station cannot exceed 250 tons of emissions/emission type/year. Emission types include CO, NO2, SO2, total Hydrocarbons, and particulates.
Gas-fueled Caterpillar G3516 turbocharged and aftercooled engines were selected to power three-stage reciprocating compressor sets. Each 4,210-cu in., 16-cylinder engine produces 1,085 hp at a continuous speed of 1,200 rpm and a maximum emission/emission type of 2 g/bhp-hr.
Each G3516 engine employs an open-chamber that enables the unit to operate with a 14:1 air/fuel mixture. The lean mixture reduces combustion temperatures, winch in turn reduces NO2.
An engine produces about 21 tons/year of each emission type. These emissions are based on actual average engine run times of 8,640 hr/engine, Downtime for routine maintenance is expected to be 120 hr/year.
A station with five compressor sets, operating at full load for a total of about 43,200 hr/year, exhausts about 105 tons/emission type/year. This amount of emissions is 95 tons below the level allowed by the permit.
A compression ratio of 9:1 contributes to the engine's ability to run at a lean air/fuel mixture. To achieve the high compression ratio, the engine features deep-cup, high-squish pistons. A deep cup on the top of each piston produces a high level of turbulence from compression. This, in turn, allows for complete mixing and quick, thorough burning of fuel.
High compression is also aided by a turbocharger that forces 50% more air into the combustion chamber when compared with a standard emission engine.
With a high-compression ratio, the density of the air/fuel mixture is increased. To create the spark in this dense mixture, high-energy coils deliver 18,000-20,000 v to elongated, high-voltage spark plugs. Electrodes on the spark plugs feature metal bars to accommodate the high voltage.
ENGINE DURABILITY
Engine durability was determined based on the fact that the high-speed engines employ the same block, crankshaft, main bearings, camshaft, and connecting rods as their diesel counterparts that have proven to be reliable in petroleum applications for decades.
Additionally, the gas-fueled engines operate at a 40% lower peak cylinder pressure than diesels.
The engines also feature detonation-sensitive timing (DST), that allows the engines to run at a high-compression ratio without incurring damage. When sensors detect detonation, a signal is processed by an electronic timing control and the ignition timing is retarded 6. If the engine is still in detonation, the DST system shuts down the engine. To date, the compressor sets have logged up to 10,000 hr each and have required only routine scheduled maintenance.
FUTURE DEVELOPMENT
Coalbed methane production levels are currently increasing at the Vance projects. Previously drilled wells continue to be added to the centralized compression system.
If production held as planned, yearly production was to be about 22 bcf by the end of last month.
Methane gas production at the Vance projects is expected to continue well into the next century. With its interconnected pipeline network; centralized compressor stations; and high-speed, low-emission compressor sets, Taurus is positioned to produce coalbed methane gas at low cost and still be within the strict air quality requirements.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
