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Marcellus Issues in West Virginia: An Introduction

Well Construction, part 2

While there should be two barriers to protect the environment when constructing a well, there is always the possibility of chemical spills taking place on the pad. These spills can be directly from bags or barrels of chemicals onto the ground or from leaks in pipes or hoses which transfer the chemicals to equipment or pumps. When the chemicals are under high pressure, such as during fracturing, if there is a leak, they will be sprayed a long distance and at a high volume.

Site construction should be such so that any spills that take place are retained on the pad. A berm around the pad is recommended; unfortunately it's not part of state regulation or guidance. The site entrance roadway should be constructed so that it doesn't provide an avenue for escaping pollution. Operators should have secondary containment around chemical and diesel storage and the secondary containment should include an impermeable liner. Again this isn't part of state regulation or guidance, except operators are required to construct sites so that pollution can't enter the environment.

The above ground barrier to protect the environment is variably the blowout preventer, fracture T, or production head. While these can fail (blowout preventers fail about 5% of the time if there is a blowout), failure isn't common.

It's the below ground barrier, casing and cementing the well, that is of major importance. It serves as a primary barrier but is in a location that makes it hard to maintain. This means that casing and cementing have to be done correctly and done according to a high standard.

Unfortunately, the state's regulations and the Office of Oil and Gas' Casing and Cementing Standards don't fulfill the minimum requirements of the American Petroleum Institute's (API's) Hydraulic Fracturing Operations -- Well Construction and Integrity Guidelines (HF1).

A typical well can have multiple strings of steel casing. The outermost and most shallow is the conductor casing. The next string of casing is the surface (or freshwater) casing whose purpose is to protect drinking water aquifers. There will be cement between the casing and the hole's edge (and close to the surface, the conductor) for the whole length of casing. Generally there is an intermediate string of casing (also called in West Virginia the coal protection casing) which is also usually cemented the whole length of the casing to the surface. In most areas the intermediate string of casing helps to provide a barrier preventing gas and fluid flow from gas producing formations between the freshwater aquifer and the major producing formation that is deeper underground. Drillers in West Virginia are not required to have intermediate casing in horizontal wells (see 35CSR8.9.2.g).

In some areas there might be several intermediate strings of casing, though this has not been noted in West Virginia. The final protective string of casing is the production casing which extends the whole length of the well. This is generally about 5 inches in diameter. Usually this is cemented only to above the major producing formation, leaving a gap between the casing and drilled hole (or, further up, the intermediate/surface casing string). Once the well is producing a smaller, thin walled, production tubing is installed. The produced gas travels up the production tubing -- there should be no flow in the annulus between the production tubing wall and the production casing wall. And there should be no flow in the annulus between the production casing wall and the edge of the drilled hole.

Leaks in the annulus between any string of casing cause Sustained Casing Pressure (SCP). In spite of all the effort to install casing and properly cement, SCP is a common problem in wells -- over 5% of new wells leak (an excellent article by Dr. Tony Ingraffea). As wells age if they don't already leak they begin to develop leaks. The Office of Oil and Gas has no quantified well monitoring system that operators must follow -- well self-monitoring by operators is solely by vague verbal descriptors.

What causes leaks in the primary barrier system? Casing and cementing a well properly is more an art than a science and a large number of factors can cause failures in the barrier. These can include an open annulus (such as normally exists between the production casing and intermediate casing) which encourages corrosion to the back of the casing. Failures in the cementing are not uncommon (an industry paper on the difficulties of properly cementing natural gas wells). The state doesn't require pressure testing to determine if the casing or cementing is adequate (see the API's Well Construction and Integrity Guidelines for a description of the tests that are necessary).

Casing itself can fail for a number of reasons, including improperly torquing sections during construction of a string. Pushing a long (possibly 2 mile long) string of production casing down a vertical hole and into a horizontal lateral stresses the joints and entire casing string. Dropping a casing string, even a couple of inches, so it lands hard can cause failure. Over pressuring casing during testing can cause it to burst.

Primary barrier leaks can be minor, or they can include fluid and/or gas leaks into groundwater, or they can be major blowouts. Described as uncontrolled fluid/gas migrations, blowouts would include leaks into groundwater and not just spectacular above-ground events.

A gas migration -- underground blowout -- occurred in Bainbridge county, Ohio that has been well documented. The state's report shows an operator, ignoring the fact that a well was improperly cemented, going ahead and fracturing a well anyway, polluting the area's groundwater with methane which caused a house to explode.

 

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