Oil wells in the Eagle Ford Shale: The Extraction Process

InsideClimate News
Published: February 18, 2014

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Details will vary based on the operator and local geology, but the basic steps are:

1. Site preparation: Operator constructs a well pad by clearing and leveling the land. This may involve building roads to the site and always involves constructing pipelines. Equipment is brought onto the pad by trucks and/or helicopters. Processing equipment may be constructed downstream of the well pad.

2. Drilling: Drilling mud – a mixture of a liquid base (water, oil or synthetic chemicals), clay and chemicals – is injected to facilitate the drilling. The mud is brought back to the surface and the rock cuttings it transports disposed of in landfills. The liquid portion can be disposed of in underground injection wells. Some wells, and sections of wells, are drilled using compressed air instead of drilling mud.

The drilling process can last from a few days to a couple of months, depending on the length of the well. In the Eagle Ford, where wells may be more than 10,000 feet long, drilling can easily take a month or more. Operators begin by drilling a vertical wellbore, before turning the wellbore towards the horizontal so it's sandwiched within the shale layer.

As the wellbore is being drilled, concentric layers of metal piping (casing) are cemented in place to keep the wellbore from collapsing and to prevent fluids in the rock from moving up the wellbore. It also prevents fluids that are pumped down the well from seeping into the surrounding layers of rock. Good well integrity is crucial for protecting aquifers and to control methane emissions.

3. Well completion: Explosives are detonated to perforate the horizontal part of the casing that's inside the shale. Fracking fluids are pumped into the well and flow out of these holes into the shale, opening up fissures in the rock. What comes back out of the well, called flowback, is a mixture of oil, gas, natural gas liquids and brine. In some wells, most of the injected fracking fluid will stay underground.

This mixture is sent to a separator unit where the gas, oil and other fluids will separate out due to their different densities. Flowback containing, fracking fluids, salts, heavy metals, and produced water from the shale, is collected in tanks. The oil is collected in storage tanks so it can be shipped to refineries.

The gases can be vented freely into the air or burned in a flare to reduce emissions. Flaring converts most of the methane to CO2, which is a much less powerful greenhouse gas.

Some oil well operators use green completion methods to collect the gases from flowback. When implemented correctly, green completions can reduce VOC emissions by up to 95%. The Environmental Protection Agency will require green completions at all new and re-fracked natural gas wells starting Jan. 2015 (the EPA defines a natural gas well as a well "drilled principally for production of natural gas"). But they are not required for oil wells, and will not affect most of the wells in the Eagle Ford.

4-5. Well production and processing: Well completion ends when the well has been cleared of most of the flowback. Now the well begins to produce large quantities of oil and gas.

The key is to separate the various fluids coming out of the well, and to strip the oil and gas of impurities. The associated gas can be flared or collected in pipelines and sold. Oil is then trucked offsite to refineries.

Virtually all natural gas produced in the U.S. requires some form of processing to remove impurities such as water, sulfur and other hydrocarbons so they don't degrade the pipelines. The processing can be done on or near the well pad, or at a central processing plant. Equipment used includes dehydrators, amine units, heater treaters and separators. Additional equipment is required to separate and purify natural gas liquids like propane, butane and pentane.

Once the gas is processed, the natural gas can be stored or sent via large transmission lines to market. There are large compressor stations along those pipelines to push the processed natural gas down the line. Some operators also use smaller compressor units near the well pads to move the gas from the well to centralized facilities.

The natural gas liquids are stored on site until they can be sold for use in manufacturing, refining or petrochemical plants.

Click here for a glossary of extraction-related terms.

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