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Innovative techniques backed by solid technology are changing coalbed methane drilling and production from an art to a science.
North America is sitting on trillions of cubic feet of technically recoverable coalbed methane (CBM) reserves. This information is not new. But sitting on something is not the same as holding it in your hand, and for many years production of CBM could not be economically justified. The gas was adsorbed within coal cleats, which resemble small blocks, and held in place by tons of interstitial water. For years, coalbed methane was viewed as merely an impediment to coal production. Miners needed to degas their mines before they could safely work in them - a lesson many learned the hard way. The first coalbed wells were drilled prior to mining to lessen the risk of explosion. Later, entrepreneurs started thinking of ways they could capture and monetize the gas, and the race was on.
Traditionally, the technique for getting at the CBM was to drill a well into a coal seam and spend the next several months dewatering it. This reduced the hydrostatic head on the coal so the gas could desorb and make its way through the matrix to the cleat system and finally into the well. In many cases it was a gamble. Wells were drilled and thousands of dollars invested in dewatering only to find out that the gas volume was insufficient to support an economic producer. Now, thanks to a patented technique being applied by independent CDX Gas of Dallas, Texas, coalbed methane can be successfully drilled and produced - scientifically.
How much is there?
According to estimates by the Potential Gas Committee and others, the US has more than 703 tcf of coalbed methane, with 147.4 tcf being technically recoverable. Currently, almost 1.5 tcf are being produced per year - about 10% of total annual US natural gas production. Canada is like a sleeping giant with reserve estimates ranging from an ultra conservative 146 tcf to more than 3,000 tcf. Most of Canada's CBM (352 tcf) is found in the Alberta plains, with the foothills of British Columbia and Alberta combining to add another 179 tcf. A small amount can be found in the Maritime Provinces. All areas are potential targets for CDX' technique.
The most prevalent drilling method for CBM has been conventional vertical wells. Unfortunately, production from these wells is inefficient and hundreds are required to exploit a reservoir. Deviated wells that place long horizontal drain holes within the coal seam produce more efficiently, especially after hydraulic fracturing, but are often cost prohibitive. And after-the-fact techniques called GOB drilling seek to recover attic gas from abandoned mines by drilling into caverns formed when mine pillars are pulled as the last coal recovery effort before the mine is closed.
In each case, marginal economics put a damper on activities. Most reservoirs give up their coal gas slowly, and only after extensive and expensive dewatering. Surface locations are large and costly, and conventional well spacings fail to drain the reservoir adequately.
A new wrinkle
CDX Gas set the objective to develop game-changing technology to economically produce unconventional reservoirs. The key was higher rate gas recovery, uniform drainage and early production contemporaneous with dewatering activities. Impact to the environment had to be minimized, so a technique had to be developed that maximized production from a limited number of surface locations. To accomplish these objectives CDX formed a wholly-owned subsidiary company, CDX-DART Drilling & Technology, LLC (CDDT). They created a unique "Pinnate" drilling and drainage technique that involves drilling two wells. First, a vertical well is drilled to the target coal seam. If seams are stacked, it is possible to extend the vertical well downward to pierce additional seams.
A cavity is reamed in the vertical well at each seam to create an accumulation chamber or sump for water collection. Next, a directional well is drilled nearby and steered to intersect the cavity horizontally, then continuing to pierce the coal seam laterally. Branch laterals, totaling as much as 25,000 ft are drilled off the main lateral until a roughly square drainage pattern is made. Subsequently, as many as three additional pinnates can be drilled from a single surface location in a "quad-pinnate" pattern. This provides 360? of drainage, optimizes both dewatering and production, and replaces 16 standard 80-acre locations.
Most importantly, a horizontal pinnate well can produce gas immediately, while dewatering is taking place. Comparing a 500 MMcf horizontal pinnate well with a 500 MMcf fracture stimulated well, the pinnate well produces at a high initial rate and depletes in slightly more than 6 years, whereas the conventional well starts very slowly and takes 15 years to give up the same volume of gas.
Some advantages of pinnate drilling include recovery of up to 85% of gas in place within 36 months at flow rates ranging from 1.2 MMcf/d to 2.5 MMcf/d from low permeability reservoirs. A one-mile radius drainage pattern from each well can cover up to 1200 acres from a single location. And productivity tests to support business models are quickly acquired without waiting for a lengthy dewatering process. Most importantly, the horizontal pinnate technique creates minimum environmental impact, operating from small widely spaced locations and requiring few roads, surface facilities or gathering systems. And pinnates can extend under adverse land obstacles that would ordinarily preclude vertical well drilling.
Turning to the right
CDDT has created a fleet of purpose-built rigs to drill CBM wells around the country. Many leases are in mountainous country where road access and large drilling sites are limited, so the company set out to build modular, highly mobile rigs suited to the environment. Even though capable of draining up to 1,200 acres from a single site, CDDT wants to leave as small a footprint as possible. Their CBM rigs pull only singles or doubles, and get by with minimum-size drilling pads, often sited on the shoulders of switchback roads. Still, they have chalked up six world drilling records. They lead in single run footage with 4.75-in insert bits and PDC bits at 9,019 ft and 18,384 ft (2,750 m and 5,607 m) respectively. With the same bits they have set cumulative footage records of 10,016 ft and 48,180 ft (3,054 m and 14,694 m) respectively, and boast ROP records of 179 fph with the insert bit and 200 fph with the PDC bit. Among other areas, the company has drilled wells in the Arkoma Basin, San Juan Basin, Black Warrior Basin, and Appalacian Basin. They core the coal seam and use a special core analysis trailer onsite to determine desorption properties of the coal samples.
By turning the traditional art of coalbed methane drilling and production over to the technologists, CDX Gas has solved the perennial problem of economics that has doomed earlier CBM projects. Although natural gas prices have been strong recently, it will take efficient drilling and production to sustain the business through a downcycle. With built-in efficiency, a scientific reservoir drainage plan, and with early positive cash flow assured, the company is positioned to weather any storm.