Coal bed methane
What is CBM?
In a traditional coal mining process, the pressure in the rock mass is reduced causing methane desorption and its release into mine headings, which results in enormous hazard for hard coal mines. The above is the main reason why methane is drained during advance works by the mine demethanation units. That part of methane which does not cause major risk during hard coal mining is emitted with ventilation air contributing strongly to the greenhouse effect. In this respect, methane’s impact is ca. 20 times stronger than the impact of carbon dioxide.
On the beginning of the nineties companies begin to introduce new technologies of getting out methane (independent from getting out the carbon), applies with the success in the USA, Australia and in Canada. The methane is treated as a main ore from the layers of the carbon obtained from the surface using vertical boreholes (now vertical and horizontal).
Coal Bed Methane occurs in two forms:
- Adsorbed methane – physically and chemically associated with the coal substance in coal deposits; it occurs in the form of gas molecules adsorbed on coal grains.
- Free methane – filling coal bed pores and fissures, porous waste rock, fault fissures, and the like.
Coal beds are deposits with very complex properties. Unlike in sandstone, where gas fills free pore spaces in the rock, methane is adsorbed in internal coal structures which are able to store considerable quantities of gas.
A coal bed is a naturally fractured, heterogeneous deposit with internal macropores (coal cleats) and micropores (coal-matrix).
CBM production depends on the size of the space occupied by the fractures and on interconnections between the fractures. If the coal cleat system in a coal bed is not sufficiently well developed, methane production may be very difficult. Small porosity and low permeability of micropores make it almost impossible for the gas to penetrate from the coal matrix to the coal cleat system. Initially, the macro- and micropore systems are in the state of balance. To reduce the coal bed pressure and consequently to release methane from the system, coal bed water needs to be pumped out. The dewatering process may take from a few days to a few months and depends on borehole configuration. Generally speaking, water production dies out before gas output reaches its peak. Reaching of the „time-to-peak-gas” is the decisive parameter as from this moment on, gas production level is significantly reduced.
Amount of gas
Methane resources contained in the coal beds in the areas of the Upper Silesia Coal Basin where no mining activity is carried out amount to ca. 300-350 billion m3. On the other hand, the coal beds of active mines contain further 150 billion m3 of gas. Methanic structure or the spatial distribution of in-situ gas content in coal in the Upper Silesia Coal Basin is a result of two geological processes resulting in two patterns of in-situ gas content in coal deposits.
The north and central pattern was formed by the elevation of the rock mass in the Upper Carboniferous over the period of ca. 250 million years. The area of the Upper Silesia Coal Basin was a land at that time. Methane from the upper strata of the Carboniferous had a relatively free way of escape to the atmosphere. The upper strata of the Carboniferous were degassed (aerated) in a natural way. The degassing depth varies depending mainly on the lithological composition of the interval and its permeability. An average, it is 500-1,000 m. Beneath the degassed zone, the in-situ gas content rises quickly to reach high values up to several m3 per tonne of pure coal).
The south pattern came to existence in the process of formation of the Miocene Carpathian Foredeep. The Carboniferous strata roof was lowered and covered with a thick impermeable overlay of Miocene clays with thickness ranging from 500m to 1,000m (with thickness increasing towards the south). As a result, good conditions for the accumulation of natural gas (methane) in the roof strata of the Carboniferous were created. The previously degassed coal deposits of these strata were resaturated with methane. Methane accumulated also in the permeable waste rocks.
The methane content tests carried out in active hard coal mines in boreholes examined from the surface show that saturation with methane of coal beds within the Rybnik Coal District and within the concession area is variable. In general, it is thought that the level of coal bed saturation with methane increases considerably towards the south of the Upper Silesia Coal Basin. The CBM resources within Olza – Czyżowice – Godów concession area are estimated at ca. 34 billion m3.
Horizontal and vertical wells
Using specialist technology, Pol-Tex Methane LLC will draw coal bed methane from surface gas wells. The process consists in preliminary drilling of vertical boreholes and subsequent drilling of directional boreholes. Next, the pressure in the coal bed is reduced by pumping out the coal bed water, which results in desorption and emission of methane.. The main benefits offered by horizontal drilling include control of shape, direction and location of the boreholes. Thus, it is possible to define and obtain precisely located boreholes, taking into account the main fairways.
The most fundamental question is: what shape of lateral boreholes should be chosen to enhance methane recovery (maximise amount and minimise time)?
In the area under concession (“Olza-Czyżowice-Godów” and “Wilchwy” Pol-Tex Methane LLC is planning to drill directional exploratory and production boreholes with the purpose to investigate and extract methane from coal bed deposits (CBM – Coal Bed Methane). All geological work will be carried out according to a Geological Work Plan developed by the Company in collaboration with experts from the Katowice Geological Company. The exact locations of boreholes to be drilled and the method of drilling will be determined by Company management before each drilling operation, on the basis of an analysis of results obtained from the previous drillings, from historical geological research and from research to be carried out on the basis of the planned boreholes. In establishing the drilling details we co-operate with the above-mentioned Katowice Geological Company and with a coal mine demethanation company from Rybnik. The experts employed at these companies have extensive knowledge and practical experience in CBM recovery.
Drilling operations will be carried out with own top-class drilling equipment purchased directly from two American companies: Gefco and Amkin. Moreover, the Company has purchased a mud system from Triflo and drill pipes from Sooner. During horizontal drilling operations, we will use the British MWD (measure while drilling) system from Sondex, which has been recommended to us by Maverick Directional Drilling. Maverick will provide the relevant training to the Polish personnel to ensure effective implementation of the system. The training will be split between the United States and the area of planned drilling under the “Olza – Czyżowice – Godów” and “Wilchwy” concessions. All the equipment will be authorised for use in EU countries. We assume that the demand for the drilling operations performed by Pol-Tex Methane LLC will ensure effective utilisation of the production capacity of the above-mentioned drilling rigs for the next dozen-or-so years. Simultaneuously, in 2008, company signed many years' contract on production and the sale of the gas.