Coal is contained within about 2.1x103 km2 in Huainan
mining area, where rich coal resource and coalbed methane (CBM) can be found
(Shuxun et al., 2001). However, because of the
complex tectonic condition, the reservoir permeability of this mining area is
low and absorption ability is high (Yuan, 2009). With
the increase of mining depth, the gas disasters of Huainan mine get more serious
(Yuan, 2006; Wei et al.,
2009). Among the 11 coal producing mines, there are 7 gas outburst mines,
2 high gas mines and 2 low gas mines. The production of gas outburst and high
gas mines accounted for 91.4% of the total output. Nearly 200 coal and gas outburst
accidents had happened since the mines been built. According to the geological
survey data and gas information of producing mines, the gas content is fairly
high in each coal seam, average content is 6.23 m3 t-1
and the maximum content can reach 17.91 m3 t-1. The gas
is mainly accumulation in the B and C groups of middle coal measure while in
the lower A groups, upper D and E groups, the gas content is relatively low,
gas emission quantity of producing mines are relatively high. By the end of
2009, there were 146 coal and gas outburst dynamic phenomena happened in the
mines of Huainan Mining Group which includes 90 times in the South Huai River
mining area and 56 times in PanXie mining area. There are 143 outburst phenomena
occurred in the central of B and C group coal beds and 2 times belong to the
lower A groups (one is in A1 coal seam of Xinzhuang Zi mine and other
is A3 coal seam of PanEr mine), the upper D and E coal seams have
one time (D17 coal seam of PanEr mine) (Table 1).
In South Huai River mining area, the gas content increase with burial depth
first then begin to decrease with maximum is 28.33 m3 t-1
in C13 coal seam. In PanXie mine the gas content has same trend with
the increase of burial depth, however the peak is B4 coal seam which
is 33.01 m3 t-1 (Table 1). Deeply study
about the geological controlling factors and characteristics of gas distribution
for mining area has great significance for the gas control, mine construction
and promotion of coal mining safely and efficiently (Tang,
Objective of the study is to find out gas composition and distribution characteristics and analyze the chief geological controlling factors of high gas content in Huainan mining area. The research results will provide support to efficient develop of coalbed gas and adoption of reasonable gas control measures to ensure mine safety.
|| Characteristics of gas content and outburst of coal and gas
STATISTICAL CHARACTERISTICS OF GAS DISTRIBUTION
Among the gas composition of Huainan mining, the hydrocarbon accounted for 78% in average, N2 is 18% and CO2 is 4%. For the Xieyi mine, CH4 of each coal seam (C13, B11b, B10, B9 b, B7, B6, B4b) accounted for 50.84~95.16% and the average is 75.74%, N2 is 0.91~40.32% with 18.03% as average, CO2 is 0.51~21.01% with 5.30% as average. In this mine, the gas content is between 0.08~21.71 m3 t-1 and average is 8.71 m3 t-1, CO2 content is between 0.04~17.54 m3 t-1 and average is 2.47 m3 t-1. For the regional distribution, the CH4 content extend in N-W direction and NE area is higher than SW. The CO2 content in this mine performances that the south part is higher than north while the N2 content shows a feature that south is lower than north.
Among the gas composition of PanXie mine, the hydrocarbon accounted for 50.14~97.92%,
N2 is 0~44.63% and CO2 is 0~30.67%. Average percentage
of gas composition, hydrocarbon is 77.58%, N2 is 16.55%, CO2
is 5.87%. For the regional distribution, CH4 content (south wing
of TaoWang syncline) extend in NW direction generally and NE is lower than SW,
CO2 content is high in the hinge zone of PanJi anticline and low
in the two wings of anticline. N2 content is on the contrary.
GAS GEOLOGICAL CONTROLLING FACTORS
Factors which controlling gas distribution can be separated into two categories: One is original factors which include coal bearing property, coal thickness, surrounding rock lithology and so on, other is epigenetic factors, include geologic structure, burial depth of the coal bed, groundwater activity and so on.
Control effect of coal bearing property to gas: Coal bearing ratio of the Carboniferous Taiyuan formation in south Huai river mine is 3.98%. Maximum coal bearing ratio of Carboniferous coal measure is 14.91%, the average is 6.35%. Coal bearing ratio of XieJia Ji mine is the highest area, reaching 11.15% and its gas content is also high, coal and gas outburst is most serious. By contrast, Coal bearing ratio of LiZui Zi mine is relatively low, so coal and gas outburst has not happened up to now. Coal bearing ratio of PanXie mine is 8.70%, coal and gas outburst is also serious.
Control effect of tectonic to gas: In Huainan coal field the coal measures
are Carboniferous and Permian. Main structural framework is an EW ramp fault-fold
structural belt (Song et al., 2005). Influenced
by Tan-Lu fault and Qinling Mountains latitudinal structural belt, the structure
of south Huai River mining area is complicated and fracture structure is development.
The coal and gas associated structure styles include fault, fold and combination
of the two structures (Zhang et al., 2003).
Among them, fault is the main controlling factor of gas distribution. Gas accumulation
is primarily depended on the closure of fault and permeability of rock.
Open fault (tension, tension-torsion, water conductivity) or well permeability
rock contacting with coal seam will lead to gas content declining in nearby
area. Tension and tension-torsion fault are development in west and north of
PanXie mine, west of DingJi GuQiao mine which helpful for gas emission and result
in the low gas content. Although, the F1 fault of LiZui Zi mine is
compression and compresso-shear fault, coal seams of hanging side was raised
and been denuded under the thrust nappe role, due to the layers dip is
vertical and is cut by fault, coal seam will be disconnected by the large drop
fault and contact with the well permeability rock formation on other side.
|| Tectonic and hydrogeology map of Huainan mining area
In addition to groundwater activities, it is favor for gas emissions, so gas
content of these areas are low (Fig. 1).
Sealing fault (compression, compresso-shear, aquiclude) and bad permeability rock formation that connect with coal seams can prevent the gas emission. So, it is probable that forming gassy area in these area. Such as the brush structure region (F13-4~F13-8 and F12-8~F12-13) of Xieyi mine, the coal body structure is destructed violently in the brush structures convergence end where stress is concentration, this result in the gas concentration in the area. There are derived faults developed in or near the compresso-shear fault belts (F13-5-1~F13-5). So these mines are large gas content regions with in-situ stress concentration high methane pressure and content (Fig. 1).
Control effect of groundwater activity to gas: Ground water system controlled
the absorption and aggregation of gas by the reservoir pressure (Ye
et al., 2001; Zhang et al., 2005).
The water-richness of Permian coal system in south Huai River mine is feeble.
Units-inflow is ordinarily smaller than 0.1 L s.m-1, permeability
coefficient is smaller than 1.13 m day-1. The water discharge of
coal is about 1 m3 t-1. Water content of coal bed is about
1.5% and the salinity is above 750 mg L-1. These figures illustrate
that groundwater of coal series is retention and flow slowly. PanXie mine is
the subject of synclinorium (eastern and middle). Owing to the resistivity water
effects by the thrust fault located north and south wings (FuFeng fault and
ShangYao-Minglong Shan fault), bedrock aquifers were cut off from the water
supply of exposed areas and composes closed hydro-geological unit. Groundwater
in this area is detention. Therefore, aquifers of coal measures in Huainan mine
is relatively blocking and flow slowly. It is in favor of storage and enrichment
Control effect of roof lithologic character to gas: Gas occurrence in
coal seams. The permeability of roof can influence the gas content directly
(Lu, 2006). The statistics result of Fig.
2 shows that, gas content decreases with the lithologic particle coarsening.
The direct roof of upper and lower main coal seam are shale and clay, account
for 49.4%. Silty mudstone account for 32.8%, siltstone account for 11.5%, the
other rock types only account for 6.3% (Table 2). Therefore,
roof of this mining area is more compact and beneficial for gas enrichment.
Statistics to the gas outburst point of PanJi No. 3 mine shows that, surrounding
rock nearby the outburst point always have the characteristics of bad permeability,
harder and denser (Zhu et al., 2008).
Control effect of burial depth to gas: Burial depth is a key factor
that control gas content. It can influence the pressure and preservation conditions
of reservoir (Xu et al., 2002). The statistic
shows that, the gas weathering zone depth is located 100-200 m below bedrock.
|| Roof lithologic characters of each coal seam in south Huai
River mining area
||Gas content different caused by the lithology of roof
Thickness of Cenozoic is between 200-600 m (Liu et
al., 1999). The occurrence of gas in Huainan mining area was notably
controlled by burial depth (Fig. 3 and 4).
In the level of above -1000 m, gas content increases with the buried depth.
The gas content can increase from about 10-20 m3 t-1 when
burial depth increases from 550-950 m in South Huai River mining area. In PanXie
mining area, the gas content can increase from 2.5-20 m3 t-1
when burial depth increases from 350-820 m. On the same burial depth, the smaller
dip angle of coal seam, the higher gas content.
Control effect of coal thickness to gas: Coal reservoir is a highly
dense and low permeability rock formation itself. Middle stratification was
sealed strongly by the upper and lower parts stratifications. The more thickness
of coal reservoir, the longer travel that coalbed methane (CBM) diffusing from
middle stratification to roof.
||Relationship between level and gas content of C13
coal seam which located from F4-5(F17) to F13-5
in south Huai River mining area
||Relationship between level and gas content of 13-1
coal seam which located from the footwall of F5 fault to the
wall of F2 fault in south PanXie mining area
Diffuse resistance is great and it is helpful to storage of CBM (Wei,
1999; Qin et al., 2000). The gas content
of south Huai River mining area and PanXie mining area correlate positively
with the thickness of coal seams. When the thickness of coal seam further increasing,
the gas storage ability will be expressed to the maximum and the influence from
thickness to gas content will decrease gradually (Fig. 5 and
||Relationship between coal thickness and gas content of south
Huai River mining area
||Relationship between coal thickness and gas content of PanXie
CH4 content of south Huai River mining area extend in N-W direction and NE area is higher than SW. CO2 content in this area performances that south area is higher than north while the N2 content is on the contrary. CH4 content of PanXie mining area extend in NW direction totality and NE is lower than SW. CO2 content is high in the hinge zone of PanJi anticline and low in the two wings of anticline. N2 content is on the contrary.
Gas content of south Huai River mining area is controlled by coal bearing property, buried depth, coal thickness, geological structure and groundwater activity. Gas content was positively correlated with coal bearing property, buried depth and coal thickness. Open fault or well permeability rock connected with coal seam will lead to gas content declining in nearby area. While the fault connected with coal seam is sealing and bad permeability will result in gas enrichment. Gas content decreases with the lithologic particle coarsening. It will beneficial for accumulation of methane when the groundwater keeps retention state totally.
Thanks for the funding by National Science and Technology of major special projects of China (2011ZX05034), National "973" CBM project (2009CB219605), Natural Science Foundation of China (41272178), Qing Lan Project and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).