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Gas production from Coal Bed Methane
08.03.2017

Gas production from Coal Bed Methane

AGR’s Reservoir Management team has been working on analyzing and evaluating the possibilities to develop a coal bed methane prospect in the North-East part of Kazakhstan.
Coal Bed Methane (CBM) exploration and production is similar and different from conventional types of hydrocarbon reservoirs. In CBM, the source for the gas is the same as the reservoir and sands are considered for the most part as thief zones.

AGR's Chief Geoscientist from our Reservoir Management team explains in more detail:

Introduction

Coals start out as peat, which is composed of plant debris - leaves, sticks and trees which become buried. Through the burial process the peat is compressed and Lignite - a brown coal -  is formed. With further burial and heat, the lignite is compressed and becomes sub-bituminous - a light grey colored coal. This coal may be further buried, heated and compressed, to form bituminous coal and with further burial and heating the hard black coal, called Anthracite, is formed. Through this burial, increased pressure and heating converts the coal from a low grade to a high grade coal, with anthracite being a high grade coal.

With any coal, there are four parameters of importance:

- carbon content
- moisture content
- ash content and
- volatiles.

Table 1. Coal reservoir parameters
 
Coal Type Carbon Content Moisture Content Ash Content Volatiles
Lignite 25-35% < 66% 6-19% High
Sub-Bituminous 35-60% 15-30% <7 – 19% Not measured
Bituminous 60-80% <12% <7% Not measured
Anthracite 73-85% 5-15% 12-20% 2-10%

Lignites have a very high volatiles content, not measured but used for producing oil and gas from the material. The challenge is the potential for self combustion making it difficult to produce oil and gas. But for CBM projects, Lignite is not very attractive.

In CBM case the gas is adsorbed onto the surface of the coal, thus the more fractures there, the higher the number of surfaces for the gas to adsorb to.

Anthracite is highly fractured and thus has a greater number of surfaces for adsorption. The adsorbed gas quantity follows the Langmuir isotherm. The higher the quality of the coal, the greater the volume of gas adsorbed, thus anthracite has the greatest volume of adsorbed gas.

Coals and Their Evaluation

No two coal beds are the same. The coal beds may contain varying amounts of non organic material, sand and shale for example. The source of the coal is deposited in bogs, depressions on the flood plain, delta plain swamps and marshes and back beach barrier island swamps. Due the different types of depositional environments where coal is found, it is understood that the thickness and quality of the coals can vary.

Thus, in the commercial evaluation process of the prospect, cores, core plugs and well logging is required.

Experience indicates that first a standard petrophysical analysis needs to be done followed by a process to evaluate the coals. This can be challenging as the standard petrophysical analysis ignores the coal. In addition to the laboratory analysis of the cores, a detailed description should be done calibrating the petrophysical logs to the results from the core analysis and core plug analysis. In the core plug analysis for the coals, the parameters listed in the table above are measures at different pressures during pyrolysis.

From the measurements, the quality of the coal can be determined along with the prediction of the volume of gas that could be produced by producing the Langmuir isotherm plot.

This laboratory analysis approach was developed by the US Bureau of Mines and published in 1967. Others have developed tools for analyzing the well logs. 

Coal Bed Methane Production

In ordinary oil and gas fields, increased production comes from the increase in reservoir pressure. The opposite is true with CBM.

The pressure needs to be reduced to cause the gas desorption from the coal surfaces and into the fractures and pore space. Gas from coal beds is produced from roughly 200 m to 1,500 m, theoretically 2,000 m. Below 1,500 m the difficulty in pressure reduction is greater.

Pressure reduction is achieved through the production of the water in the coal.

In the figure below, with the CBM well, the initial gas production is small while the water is being produced.


cbm
Fig. 1 Generalized production profiles for a CBM well as well as a conventional gas well. Note the difference in the water production

In CBM production, three stages are defined:

1) Dewatering Stage
2) Stable Production Stages and
3) the Decline Stage.

In the example shown in the figure 1, the conventional gas well begins to decline immediately if there is no pressure support and in this case there is increased water production towards the end of the wells’ life.

The added challenge in CBM work is water treatment, as quite large volumes of water are first produced. 

Saransky License in Kazakhstan

The Saransky license lies to the west of the Karaganda city center. The license covers nearly 98 km2 and the license overlays the Saranskaya coal mine, which lie along the northern part of the CBM license. 

cbm 2
Fig. 2 Saransky license (red polygon). Taldykuduk license (magenta polygon)


Coal exploration drilling of the license was done during the Soviet Era. During this time, 1,551 wells were drilled. All the wells were logged and some of the coal beds were cored and tested. 

However, not all of the wells were tested to the degree needed to confirm the total gas content. The previous operator of the field drilled five wells with encouraging test results from one well. The license requirement prior to production start is for two wells having been logged and tested.

The data from the Soviet Era drilling campaign is all analog and thus needs to be digitised to perform modern analysis. Some of the data has become available digitally and maps such as Fig 3 can be produced. 

cbm 3
Fig. 3 Structure map of one of the coal beds in the Saransky license area. Note the white dots indicate the location of Soviet era coal exploration wells


40 coal seams have been identified with the 1,551 wells, of these 14 coal seams have been high graded to being producible. These 14 coal seams have been grouped into three major groups. Prior to AGR’s involvement only the tops in the wells have been digitised leading to inaccurate maps that can be used for production planning. The Soviet Era geologists produced detailed correlations and cross sections indicating a fair amount of thrusting. Thus there are a number of potential production locations where the best coal seam, identified as K-12, could be penetrated twice.

The task for our Reservoir team was to rebuild the models to incorporate the thrust interpretations as indicated in the detailed cross sections. 

cbm 4
Fig. 4 Comparison of the digital cross section and the cross section from the image of paper copy. Note the lack of faulting in the digital cross section. The solid horizon colors from the paper cross section  correspond to the top of the horizons in the digital model. The dashed green horizon on the right corresponds to the basal green in the “From Digital Model”


In addition to the correction of the structural model, AGR evaluated the results from wells drilled by the previous operator. Based on the test results and utilising the current inaccurate structural maps, the volumes are still significant.

The initial production from a single coal seam for one well was 3,500 m3/day; this increased to 5,000 m3/day after recompletion 3 months later. With a 1,500 m cut off depth and producing from all primary seams (14), then the GIP is approximately 73 Bsm3 or 2.6 TCF and producing from only the best coal seams a GIP of 54 Bsm3 or about 1.9 TCF. These numbers are significant. Recovery factors for CBM range from 36% to 90%, with the average being 50%. With a 50% recovery the production ranges from just under 1 TCF to nearly 1.5 TCF for Saransky.

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