HOUSTON–Commodity pricing is driving the drill bit to oil and liquids-rich-gas shale plays that deliver higher Btu value. With the decline in natural gas prices, producers are seeking to capture the liquid component in all types of unconventional plays and squeeze out every revenue-producing drop of liquid hydrocarbons.

Along with increased oil and condensate production comes associated gas in a variety of well bore pressures. Some of these applications require compression equipment to assist producers in removing all potential liquids, as well as to provide fuel for the compression packages and other fuel gas users such as stabilizers, line heaters, and dehydration equipment.

While providing an attractive economic return, these plays present complications in the applying gas compression and other natural gas-burning equipment. An equipment provider also may look at wet gas plays as an opportunity to provide its customers with an additional revenue stream to offset operating expenses by utilizing ancillary equipment to recover process liquids upstream of compression equipment.

There are a number of challenges and a range of compressor dynamics associated with burning the variety of gases produced in shales and plays yielding high natural gas liquids, as well as equipment alternatives for treating the produced gas in the field to enhance its quality as a fuel for compressor engines and other production equipment.

Using raw and heavy shale gases as a fuel source for reciprocating-engine-driven compressors affects the dynamics of performance. With this heavy gas as fuel, most larger-horsepower machines are subject to a substantial derate. High levels of heavy hydrocarbon components lead to reciprocating gas engines predetonating, which requires derating the engines so they can maintain safe air-to-fuel ratio levels. In other words, the available horsepower is reduced, leaving less horsepower for the process usage, compared with the nameplate on the machine.

In addition, operators are finding it increasingly difficult to meet the emission levels specified for compressor stations. Heavy hydrocarbon-rich gas can damage or foul engine components, causing mechanical reliability issues and reduced compressor/engine efficiencies, possibly leading to an engine shutdown, which has an immediate negative impact on production flow until the damaged components are replaced or repaired.

Treatment Options

Several technologies are available to remove the heavier hydrocarbons that will condense before the engine can consume the gas. The first is fuel conditioning (Joules Thompson) systems, which utilize the cooling effect from two pressure reduction valves in conjunction with an exchanger to condense the heavy hydrocarbons from the gas stream. Fuel analyses and simulations reflect the correct pressure drop to achieve maximum recoveries.

Another option is a membrane system. Membrane systems utilize the different diffusion rates of the gases through the membrane media to separate the heavy hydrocarbons from the gas stream. In most cases, a membrane system requires a pressure drop across the membrane to function.

Refrigeration chiller systems also can be an effective option in the right applications. A refrigeration chiller cools the gas below the dew point of the heavy hydrocarbons. The liquid hydrocarbons then are separated from the gas stream, utilizing a high-efficiency filter coalescer.

This article focuses on fuel gas conditioning technology and natural gas-driven compressor packages. While the natural gas compressor has been the workhorse of the gas patch, the fuel gas conditioning skid could be considered the crescent wrench. These units are versatile, tough and reliable, and fit most applications with ease.

A fuel gas conditioning system takes raw, high-pressure gas through a Joules Thompson valve and utilizes liquid-to-gas exchangers to provide lean fuel gas and to maximize liquids recovery. A high-performance fuel conditioning skid is capable of achieving much lower temperatures than conventional Joules Thompson units (designed to reach -40 degrees Fahrenheit). This is far colder than the temperature any membrane or refrigeration unit can sustain without electricity for chillers or pumps. With basically no rotating parts, fuel conditioning skids have minimal maintenance requirements. The gas liquids are collected into a pressurized product tank.

Fuel Conditioning System

The advantages of a fuel conditioning system include the technology’s simple, passive design; low maintenance requirements with minimal operator attention; large turndown ratios; rapid startup and shutdown; and low capital and operating costs. In addition, no chemicals are required (such as methanol), and mobile skid-mounted units can be redeployed to other locations. The systems are lightweight, have small footprints, and can handle fluctuating feed gas compositions.

Treating fuel gas utilizing a fuel conditioning unit provides dry, lean fuel for combustion engines and also recovers valuable liquefied hydrocarbon components. This is a cost-effective method to offset the expense of field compression. Moreover, properly removing moisture and solid particles prior to the gas stream entering the combustion chamber improves the operation of the natural gas engine-driven equipment, and enables the most efficient use of horsepower and the lowest volatile organic compounds emissions.

By effectively reducing the heavy hydrocarbon content, fuel gas conditioning units reduce the volume of unburned VOCs emitted by incomplete combustion of hydrocarbons in the firing chamber. These units have been used to fix horsepower derating issues and high maintenance problems related to poor fuel gas quality for reciprocating engines, both of which help make the compressors economical in a low-gas-price environment, and allow them to load to their rated horsepower for maximum throughput.

Fuel gas conditioning units have been utilized in high liquids-yield resource plays across the country, including the Marcellus, Barnett and Eagle Ford shales as well as Permian Basin tight plays. As noted, the leading use of fuel gas conditioning skids is to reduce heavy contents in the fuel gas, thereby maximizing horsepower engine ratings and helping meet emissions requirements while producing an additional revenue stream.

Wolfcamp Case Study

The benefits of implementing a fuel conditioning skid are best demonstrated in a real-world case study of high-Btu and heavy hydrocarbon-rich fuel gas for natural gas-fired engines. An application in the Wolfcamp play (Figure 1) required compressors driven by natural gas-fired engines to compress gas from a low field pressure to a higher pipeline pressure in order to move gas into the sales pipeline. The remote location of the compressor site dictated that the only potential source of fuel gas for the natural gas engine-driven compressor packages was raw associated gas from the well bore.

FIGURE 1

Process Simulation of Wolfcamp Compressor Station

A gas analysis confirmed that the raw gas was extremely rich, containing more than 24 percent C2+ hydrocarbons and a low heating value (LHV) of 1,300 Btu. The significantly high levels of heavy hydrocarbons in the fuel gas present difficulties in operating the gas-fired compressor engines at this site, as well as for assuring compliance with the regulations for emitting nitrogen oxides, carbon monoxide and unburned hydrocarbons from the engine exhaust.

The compressor station consists of multiple engines where a slipstream is taken from the compressor discharge pipeline at 1,000 psi, and is passed across the feed side of the fuel conditioning system. Methane is retained preferentially on the feed side. The resulting low-pressure gas, enriched in heavy hydrocarbons, is recirculated to the suction side of the compressor.

The conditioned gas, stripped of these components, is used to fuel the engine. The fuel conditioning unit lowers the LHV of the feed gas from 1,314 to 1,050 Btus, which is much better suited to maximize horsepower.

A thorough economic analysis determined that installing a fuel gas conditioning unit to capture the recovered liquids would be a beneficial and a cost-effective solution for treating the heavy hydrocarbons and keeping emissions within the regulatory thresholds for the existing compressor packages. As an added benefit, the analysis showed that installing the fuel gas skid would generate an additional revenue stream to assist in offsetting the operating and maintenance costs of the compression equipment.

Additional equipment was provided as part of the fuel gas conditioning unit, such as a pressurized product tank to store the recovered liquids, along with minimal piping and valving. These capital costs were included in the economic analysis.

Through process simulations, it was determined that using the fuel gas conditioning unit would liberate 35-40 barrels of liquid hydrocarbons a day at normal operating rates. The Wolfcamp station began full operations in March, utilizing the fuel gas unit and other required equipment. Preliminary data after one month of operations indicated actual liquids recovery from the fuel conditioning unit was averaging 38 bbl/d.

In addition, the data indicate that the compression units experienced a 25 percent increase in horsepower, compared with the horsepower achieved using the heavy hydrocarbon fuel gas. This increase in horsepower availability corresponds to a potential 2.5 million cubic feet a day in throughput per location. The increased horsepower and leaner-burning fuel also has reined in emissions.

All indications are that liquids plays will continue to create profitable rates of return for producers for the foreseeable future, regardless of the additional challenges they present–whether down hole or on the surface. The case study in the Wolfcamp play was an opportunity for the producer and the equipment provider to utilize each company’s resources and squeeze every drop of revenue and subsequent profit out of their resources. And at today’s natural gas prices, all can agree that producers will take every drop they can get.