Entombed in the salty bed of an ancient sea that once covered West Texas there lie thousands of forgotten oil wells in so-called “dead fields.” Drained long ago of their black gold, many of these hidden holes are now suddenly coming back to life in geyser-like eruptions spewing hypersaline brine across eerily snowlike landscapes or in slow, incessant waves of poisonous waters that engulf the surrounding desert. The unpredictable reanimation of these “zombie wells” can seem like something out of a horror movie. But, for growing numbers of landowners the nightmare is all too real.
Raymond Straub is a foremost expert on zombie wells. Headquartered in the Permian Basin, he leads the Zombie Well Investigative Unit, “a small team of unique industry professionals … that investigate wells that have lost integrity,” explains Straub. “The work is fascinating, complex. Nothing simple. If it’s simple, we don’t do it.” During a recent presentation titled “Anatomy of a Zombie Well,” Straub held the entire room in thrall to his dark tales from the underworld.
The birth of a zombie well “starts with the original design” and the “formations into which it penetrates,” says Straub. These rock and clay formations hold enormous volumes of water in addition to their world-famous hydrocarbons. The wells drilled to tap oil and gas are typically designed with a “finite lifespan” because drillers assume that “the resource is going to play out long before the integrity of the well gives out.” This failure to understand the “full exposure lifespan of a well” gives rise to a “pretty specific set of circumstances,” asserts Straub, ultimately culminating in a spectacular explosion or worse.
Wells are boreholes lined with casing, or steel pipe, that is installed in sections of decreasing diameter and welded together to form casing strings. Cement is then poured between the casing strings to provide a sealing mechanism between the strings themselves and the formations through which they pass. Casing and cement are “the only protection you have between your production and your aquifers,” explains Straub. But “cement has a finite life cycle. It’s not permanent. And even the best casing has a finite lifespan,” especially in wells drilled cheaply and quickly in the early 20th century. Back then, wells were commonly drilled through many formations of rock, groundwater, and hydrocarbons with no cement around the casing, a condition Straub calls “open hole behind pipe.” With no cement as a barrier, the steel is exposed to salts, sulfates, and chlorides that eat tiny holes into the casing, resulting in “microporosity” and eventual failure of the steel’s integrity.
Once a well stops producing hydrocarbons, cement is also used to plug it. But “a lot of times they won’t plug it according to best practices,” says Straub, or they don’t plug it at all. Issues that can compromise cement integrity include too little or too much water in the mixture, improper curing, and exposure to “gaseous environments, where gases can become entrained in the cement.”
In some instances, these “gaseous environments” are intensified through the degasification of reservoir fluids, a chain of events that was poorly understood until recently. “Gas can be dissolved into water much like CO2 is dissolved into cola to create a soft drink,” explains Straub. But when the water in reservoir fluids is exposed to the abundant salts residing in the subsurface, “the gas it contains is disassociated from the water proportionate to the concentration of the salt. In other words, the more salt that is dissolved into the water, the less gas can stay in solution.” The liberation of this gas, mostly nitrogen, “increases the volume in the reservoir and drives the fluid up vertically.” When this occurs in an improperly plugged or compromised well, the fluids can “penetrate to surface” under extreme pressure (up to 500 psi) and “can shoot 150 feet into the air like a jet engine roaring out there,” reports Straub. “That’s when we know we have a problem.” An even scarier, unseen upshot of this degasification involves subsurface cross flows between formations, where “fluid moves from lower formations into a … well casing, then … exits through a hole higher up the casing and into a surrounding formation” or aquifer, explains Straub. “The well bore is now a conduit” that can direct toxic fluids throughout the subsurface, cross-contaminating aquifers along the way.
Recent efforts by the Railroad Commission and the Texas Legislature have focused on increasing the funding available to plug more of these problematic wells. But in light of the complexity of the zombie phenomenon, this approach seems destined to address only the disasters that are visible, while ignoring the occult dangers that lurk beneath. According to Straub, “It’s the ones you don’t see that are the most egregious.”
