Chevron, one of several energy companies working in the energy-rich Piceance Basin, does at least one thing a bit differently than its fellow operators: It encases the entire length of its gas wells with cement, top to bottom.The company, known by the full name of Chevron North America Exploration & Production Co., was drilling for natural gas in the Piceance until about two years ago.As the recession hit in 2008, and the price of natural gas fell, Chevron pulled its drilling rigs out of Garfield County, but fully intends to return, according to spokeswoman Cary Baird.It had been working the Clear Creek and Skinner Ridge areas, which branch off from Roan Creek north of DeBeque, in the far western reaches of Garfield County.The company decided before it drilled a single well that it would be worth the extra effort and money to completely seal off the well bore from the surrounding rock formations.The continuous seal, according to company officials, is intended to prevent leaks and preserve the integrity of the metal pipe or "casing" through which the gas ultimately flows.Chevron's method differs from the practice of other operators in the industry, which is laid out in regulations from the Colorado Oil and Gas Conservation Commission.The COGCC requires that the top section of the well bore's metal casing pipes be enveloped in cement, to a depth below the source of drinking water for local residents and communities, typically about 500 feet deep.Then, under COGCC rules, drillers must encase the very bottom portion of the pipe in cement. Typically, that means from the point of extraction at the shallowest gas-producing zone (where gas enters the pipe) upward by 200 feet or so.This method leaves a gap in the cementing that equals at least hundreds, if not thousands, of vertical feet.Representatives of Chevron, as well as the others that do not practice top-to-bottom cementing, say that the partial cementing complies with the rules laid down by the Colorado Oil and Gas Conservation Commission."We are confident our cementing protects the aquifer," said Donna Gray, spokeswoman for Williams Production RMT, which does not practice the top-to-bottom cementing.Chevron's top-to-bottom method uses about twice as much as cement and is perhaps 12 percent more expensive than the "gap" method, according to Baird. She said the company "does not divulge actual dollar costs of its operations."
It was not the bosses at Chevron who made the call to do it differently, but engineers familiar with conditions in the field, according to Marty Knauss, a drilling superintendent with Chevron."This was not a corporate mandate," Knauss said in a telephone interview with the Post Independent. "We are allowed to make a certain amount of decisions ... about what we think is the right thing to do."He added that Chevron uses the top-to-bottom cementing methods in other places, as well, and that it makes the most sense in wells drilled at riverbed level, rather than on high mesa tops.The reasons, he said, are technical, and have to do with the amount of "supporting fluids" in the surrounding rock, which he said creates upward pressure that aids in pushing the cement back up to the surface level.
Chevron's method, Knauss said, calls for carefully cementing around the top section of the topmost metal casing, called the "first string," which gives drillers the ability to handle and control the well.Once that portion of cement is set, Knauss said, the procedure sets up a veritable layer-cake in the well casing. It starts with water, which is followed by a specially designed, ceramic "bottom plug," then the cement itself, which is pushed downward by a "top plug."The metal casing pipe is lubricated and cleansed on the way down by the water below the bottom plug. It prevents the cement from sticking to the interior of the casing and washes out any impurities, Knauss explained.Once the bottom plug reaches the end of the pipe, thousands of feet underground, the bottom plug breaks apart. That allows the cement, still under pressure, to flow out of the pipe and turn back upward toward the surface, filling in the space between the pipe and the rock.After the upward moving cement has reached the surface, forming a long, unbroken column bonded both to the pipe and to the surrounding rock, the drilling crews get to work on the subsequent phases of the process.This includes pressure testing the pipe to 6,500 pounds per square inch before starting the hydraulic fracturing procedures, said Kieth Hejl, a completion engineer for the company. He said the gas, once it gets flowing, typically runs between 500 and 1,500 pounds per square inch.
The company officials said all wells drilled by Chevron in the past four years have followed this method."Chevron takes this view that the rock and the pipe are completely sealed, which is a good practice," Knauss said.He said the decision to follow this procedure was made before 2007, well before the company began large scale drilling in the Piceance Basin region.In the four years since then, both Baird and Knauss said, there has been only one well that failed to perform as expected. Testing indicated either the casing or the cement had been compromised in some way.That well was shut down, Knauss said, and the drilling crews moved to another location."We have not completed that well to this day," Knauss said, primarily due to the high cost of completing just one well in a remote area. He said Chevron has no plans to drill in that area again soon.
Some other drilling companies, such as EnCana Oil and Gas (USA) use similar methods on occasion."First, we do not cement to surface on all our wells," said EnCana spokesman Doug Hock. "I would guess less than 10 percent of our wells do achieve that."In cases where they do, he continued, "The main concern is protecting the aquifer."But, he added, "Every well in each area is different," so much so that EnCana has no hard and fast rule concerning cementing.At Williams Production, the largest driller in Garfield County, spokeswoman Donna Gray said, "We cement from the surface to an average of 800 feet below. Then we cement from about 200 feet above the Mesa Verde formation - the gas-bearing zone - to the bottom of the bore hole."In general, Gray wrote in an e-mail, "We have drilled 3,500 wells in this area and are confident our cementing protects the aquifer."