This document discusses the exploration and production methods of coal bed methane (CBM). It describes 3D seismic prospecting techniques used to explore CBM, including predicting based on burial depth, impedance inversion, frequency spectrum decomposition, and seismic attributes. CBM production passes through three phases - a dewatering phase with high water and low gas, a stable production phase with decreasing water and increasing gas, and a declining phase with low water and declining gas.

1. EXPLORATION AND
PRODUCTION METHOD OF CBM
Submitted by-
Anubhav Talukdar (PE-10/12)
Bhaskar Jyoti Gogoi (PE-15/12)
Biki Saikia (PE- 16/12)
Submitted to
Mr. Prasenjit Talukdar
Asst. Professor
Dept. of petroleum Engineering
D.U.I.E.T, Dibrugarh Assam.

2. INTRODUCTION
Coal Bed Methane (CBM)- A generic term for the methane rich gas originating in coal seams.
CBM is classified in three categories :
1. Coal-mine methane(CMM)
2. Abandoned mine methane(AMM)
3. Virgin coal bed methane(VCBM)
Govt of india adopted CBM policy in july 1997.

3. EXPLORATION METHOD OF CBM
 3D seismic prospecting technology has been widely used in coalmine area as a structure
prospecting method.
The main seismic prospecting methods for coal bed methane :-
Predicting coal bed methane based on bury depth.
Predicting coal bed methane based on impedance inversion.
Predicting coal bed methane based on frequency spectrum decomposition.
Predicting coal bed methane based on seismic attributes.

4. Application of 3D seismic prospecting
technology -
• The concerned coal mine is located in Qinshui basin, North China.
• The area that buries coal seam is about 42000km2, and coal reserve is about
2700×108t.

5.  According to the relationship between gas content and bury depth, we can infer that red region has lower
gas content, and purple region has higher gas content.
 Based on the finely interpreted seismic data, using 3D seismic data, along with interpreted coal seam
reflection layer, faults data, well logging data, lithology data and histogram data, combining geological
data, a 3D post-stack impedance inversion under multi-well constraint was carried out in prospecting
area.
 By the fact that high gas content coal seam has lower impedance than low gas content coal seam, the low
impedance zone can be predicted as coal seam that has higher gas content.

6.  The impedance inversion section and the cross plot of impedance versus gas content in different
boreholes. In impedance inversion section, the low impedance area and high impedance has
distinct colour difference.
 From the colour map we can see that the brown zone represent low impedance value, and can be
predicted as coal bed methane accumulated area.
 The gas contents of boreholes in high impedance area are: CZ-81 2.43m3/t, CZ-66 5.39m3/t, CZ-
61 4.76m3/t. While the gas contents of boreholes in low impedance area are: CZ-20 8.61m3/t, CZ-
35 7.18m3/t.

7.  Do frequency spectrum decomposition on finely interpreted seismic data volume, we can get
different frequency components of the amplitude slice of target layer.
 when the frequency increases, the red region turns into yellow, while the yellow region turns into red.
 When the frequency increase to tune frequency 75Hzthe amplitude reaches its maximum.
 The red region in 35Hz amplitude slice represent coal bed methane accumulated area, while yellow
region represent lower gas contents area.

8. CBM PRODUCTION PHASES
Coal bed methane production passes through three phases during the
life time of the reservoir-
• Dewatering phase
• Stable production phase
• Declining phase

9. DEWATERING PHASE
• During the phase CBM wells experiment a constant water production with a very low or
negligible gas production.
• Initially most CBM wells are naturally water saturated because water liberation occurs during
the coalification process
• The water is occupying the principle cleat network and there is the need of removing the
water from the major fractures system in order to produce gas.
• The number of days of this dewatering process and the amount of produced water can vary
widely
• They are very difficult to estimate and their influence in the economics is very hard to predict.

10. STABLE PRODUCTION PHASE
• Phase 2 is described by a dramatically decrease in the water production and increase in the
gas production rate.
• The water relative permeability decrease the gas relative permeability increase
• The gas production has stabilized and starts to experiment a typical decline trend.

11. DECLINING PHASE
• Drilling phase 3, the well is considered to be dewatered . So the water production is in the
low level or negligible.
• The water and gas relative permeability do not change extensively.
• The steady state exist for the rest of producing life.
• The limit between phase 2 and 3 is determined by peak gas is reached.