Conjuring with coal
BP is one of the largest producers of coal bed methane in the world. Tom Seslar talks to BP's team in the prolific San Juan Basin about the technology and challenges of extracting gas from coal
'Thinking is a technology,' says Rusty Riese, a BP geoscience advisor who has spent 13 years at work in San Juan. 'It doesn't have to be a computer or a new facility design to be a technological tool. The technological tool that has emerged from all these years of work in the field of coal bed methane is an understanding of coal bed systems that differentiates BP from most of our competitors.'
'Eventually,' adds Frances Morris-Jones, vice president for renewal in BP's exploration and production business, 'the understandings gained here might grant BP a sharply competitive edge as it considers possible coal bed methane opportunities in parts of the world far distant from the USA.'
Coal bed methane (CBM) is present almost everywhere there is coal. Global CBM resources are estimated to amount to more than 30 trillion cubic metres and may be many times higher. Outside the USA, CBM is being produced in relatively small amounts in Canada and Australia. But the San Juan Basin has the size and production potential that everyone seeks as a CBM prize.
In the USA, CBM supplies about 10 per cent of the market for natural gas, a market gain that came between 1980 and 2002 when the US government encouraged CBM drilling through federal tax incentives. The country has to date produced about 600 billion cubic metres of CBM, and about two thirds of that total has come from San Juan, which has an estimated 550 billion cubic metres or more remaining in its coal beds. As an operator in San Juan, BP's share of total production from its numerous licence partnerships is around 30 per cent.
In January this year, BP announced it is to invest up to $2.4 billion to increase its share of ultimate recovery of CBM from the Colorado portion of the basin by an estimated 54 billion cubic metres net. The company anticipates a steady drilling and facility development programme that will increase current BP net production of 12 million cubic metres per day by more than 20 per cent and maintain production above present levels for more than a decade.
The methane that is produced from coal beds differs from the natural gas in a conventional gas reservoir in the way the gas resides in the subsurface structure. While conventional reservoirs hold their gas as a discrete gas phase in the rock's pore spaces, coal beds hold their gas adsorbed in their matrix - coal can store around six to seven times more methane than the equivalent volume of rock in a conventional gas reservoir.
Methane molecules generated by coal adhere to its surface. While most of the methane is stored within the molecular structure of the coal, some is stored in the fractures or 'cleats' within the coal, or dissolved in the water trapped in the fractures. The methane is kept in place by water pressure - when the hydrostatic pressure is reduced, the gas desorbs from the coal surface and is free to flow into the fracture system. By drilling wells into the coal, the gas and water can flow to the surface where they are processed to give clean and dry methane.
Roger Gierhart, a BP reservoir engineer, says that in a CBM reservoir, the contents can be visualised like champagne in a bottle.
'The coal in this case is the "liquid" and the gas is dissolved in the coal, "sorbed" onto molecular surfaces,' he explains. 'Dropping pressure draws the gas out of the rock in the same way that uncorking the champagne bottle allows the gas to emerge from the liquid.'
According to Neil McCleary, BP's vice president for the San Juan Basin asset, the CBM development has benefited from a combination of three key features in the region's coal beds.
Cleating is one of the best predictors of how CBM can be harvested, but is also one of the features most difficult to measure in a coal reservoir. Cleating refers to the network of multiple fractures creating molecular surfaces to which nearly all of the gas is sorbed. In conventional rock reservoirs, a core sample can be brought to the earth's surface to measure porosity of the rock. But in a coal reservoir, the same tests compress the coal and destroy the cleating pattern, thus preventing an accurate measurement of how much and how easily CBM can be produced.
To understand the properties of the coal in San Juan, BP has acquired extensive fracture data in different parts of the basin. The methods used for fracture characterisation include core description and palaeomagnetic analysis. Alex Karpov, a BP reservoir engineer, says the information gained has allowed BP to compile statistics on fracture and cleat azimuth, dip, density, mineralisation and aperture, and orientation of stresses - ultimately leading to a better understanding of the coal beds. This enhanced knowledge gives BP a competitive advantage in optimising field development through more effective selection of well locations and their orientation for gas production.
Now BP is using its reservoir characterisation know-how for innovation in the use of horizontal wells in developing its coal bed methane resource. For example, the company has successfully used hydraulic fracturing technology to stimulate flow into horizontal laterals - laterals are side branches coming from a vertical well that penetrate the coal seam and run nearly parallel with it. Though 'fracing' is commonly used in conventional oil and gas reservoirs, this is the first such use in coal beds. BP has employed this technique to frac across flow barriers into adjacent coal seams above and below the zone where the frac was initiated. These and other innovative approaches may allow horizontal drilling to become a key to more efficient development of CBM recovery in San Juan and elsewhere.
'BP has long been at the forefront in recognising and quantifying the unique characteristics of CBM reservoirs and production,' says Bill Pelzmann, a BP geologist. 'This understanding allowed the company to gain a leadership position in the San Juan at the time when most other companies still regarded CBM as a nuisance.'
Conventional hydrocarbon development of the San Juan Basin began in the 1930s, and until the 1990s primary gas production was from the sandstones that lie beneath the coal. For over 50 years, the Fruitland coals were considered a nuisance because water influx and high pressures required well casing to be set before drilling to the deeper reservoirs. More than 20,000 wells penetrated through the Fruitland coals before industry recognised, in the 1970's, the potential gas resource trapped in the coal.
There are important differences in producing methane from coal, compared with production from conventional reservoirs.
CBM wells also have long lives. A high-permeability conventional gas well may completely deplete in five or six years, but a CBM well might only be reaching peak production after that much time. And it might sustain high production for a number of years before beginning a decline that can last decades, making CBM production an attractive long-term prospect.
However, the presence of two phases of fluid in the reservoir - water and gas - renders CBM very challenging to manage, making the reservoirs some of the most complex that BP has developed. Understanding a well's potential generally means completing the well, attempting to stimulate it and then flowing the well for a substantial period so that its permeability can be determined. After this, dewatering can be evaluated and the drainage area can be measured.
When in production, the coals also swell and then shrink as pressures diminish and the gas they contain is released, so their permeabilities are constantly changing. And they can swell and completely seal if there is introduction of fluids which can also be sorbed.
Learning opportunities
Looking ahead, there are potential operational challenges which CBM development faces. The most significant is handling and disposing of the large amounts of water produced. BP manages water production by gathering this through hundreds of kilometres of pipelines, then reinjecting the water into deeper formations.Managing air emissions also presents challenges. Pressure in the wells is kept low to encourage flow, but gas compression is needed for subsequent transportation through long gas pipelines - and compressors can be a major source of atmospheric emissions. The coal itself also releases CO2 that, like the methane, is sorbed in the coal matrix and liberated during production. BP manages air emissions challenges by using electrically driven compressors and pumps wherever possible, and by using lean-burn technology on its gas-fired engines. Excess CO2 is currently vented to the atmosphere, but the company is evaluating reinjection as a means to sequester the CO2 in the reservoir - as a bonus this could displace even more methane from the coal.
Preserving the landscape in the local area is also important. Coal beds are discontinuous, low permeability reservoirs and lie at relatively shallow depths, generally less than around 1000m, features which mean more wells are necessary to efficiently develop the resource - and that in turn increases the environmental 'footprint'. BP is addressing this by drilling more deviated and horizontal wells - an established drilling technique that allows more wells to be drilled and produced from a single drilling pad, thus keeping the footprint as small as possible and reducing the surface impact of operations.
Interestingly, none of these challenges is regarded as insurmountable within BP's San Juan team.
Or, to update that old premise from the early years of the past century, innovation in the recovery of coal bed methane is most likely to be found in the minds of BP's men and women.
