Present Status of Coalbed Methane Exploration
in China and Countermeasures

Wang Hongyan Zhang Jianbo Liu Honglin
(Petroleum Oil Exploration and Development 
Research Institute Langfang Branch)

Abstract China is rich in coal and coalbed methane resources and both have promising exploration and development prospects. Great headway has been gained over the recent years in coalbed methane experimental exploration and development. However, majority of the experimental zones has not yet produced coalbed methane flow of industrial significance. The present work carries out a systematic analysis of the major coalbed methane development enterprises and the major experimental zones on the basis of which the existing problems in the coalbed methane exploration and development are identified. The article also puts forward a set of prediction methods for favorable blocks and countermeasures for the next step of coalbed methane exploration.

Foreword
China is the largest coal producing country in the world as well as the largest coal consuming country in the world. The present total methane discharge into the atmosphere during coal mining process is registered at around 15.0 billion m3, accounting for 1/3 of the world’s total methane. As the world’s largest coal mining country, China is endowed with abundant coal resources and coalbed methane resources. Since the ’90s, experimental coalbed methane exploration activities have been carried out by domestic industrial sectors such as coal, petroleum oil and geological and mineral industries. Also participating in these activities are those foreign firms who are interested in this sector in China.

Coalbed Methane Resources Conditions 
Based on the data collected during the 3rd national coal field prediction, China’s total coalbed methane within the depth of 2000m is 5.57 trillion t, of which proved reserves are 1.02 trillion t and predicted resources are 4.55 trillion t. Coalbed methane resources within the depth of 1000m are 2.84 trillion t, accounting for 51% of the total. Of the total proven reserves are 1.02 trillion t, and predicted resources are 1.82 trillion t. 

The national coalbed methane resources are 22.52 trillion m3, of which 14.80 trillion m3 are in North China gas reservoir, 4.16 trillion m3 are in South China gas reservoir, 3.15 trillion m3 are in North-west gas reservoir and 0.42 trillion m3 are in North-east China gas reservoir. 

2. Review of Coalbed Methane Exploration Experiments
Coalbed methane exploration and development started in China in a later stage. As early as in the ’50s, coalbed mthane drainage was begun in underground coal mines for alleviate gas related accidents. By the end of the ’70s, experiments were carried out in several regions in this country on surface drilling for coalbed methane extraction. In the ’90s, coalbed methane resources evaluation and experimental exploration-development were carried out in the entire country. Decades long experiences in coalbed methane evaluation research and coalbed methane exploration and development can be divided into 3 stages as follows:

The first stage when it was designed to alleviate underground accidents connected 
with mine gas
This stage started in the ’50 and concluded by the end of ’70. Main purpose of underground mine gas drainage was to reduce mine gas related accidents.

Primary period of coalbed methane exploration and development experiments
The stage began in the end of ’70s and ended in the beginning of the ’90s. In this stage, purpose of the activities was still aiming at achieving better safety records in coal mines. In some coal mines, experimental coalbed methane development and utilization was practiced. It was in the ’70s that experimental surface drilling for coalbed methane extraction was tried out and a total of more than 40 surface wells were drilled in the Longfeng mine of Fushun, Liaoning Province, Yangquan coal mining area in Shanxi Province, Zhongmacun mine of Jiaozuo, Henan Province and Wangmiao mine in Hunan Province, etc. Hydraulic fracturing was also carried out in these surface wells.

The stage when all-round trial exploration and extraction of coalbed methane are 
carried out
Starting from the early years of the ’90s till now. Over the eight years, coalbed methane was regarded as a high quality energy and coalbed methane extraction experiments were carried out accordingly. As a result, great headway and progress have been gained. Oil sector, the land and mineral resources authorities, oil sector as well as some local governments actively participated in this work. Many foreign firms also invested in the new area for coalbed methane exploration trials. Since 1990, more than 30 coal bearing areas in China have undergone coalbed methane exploration drilling with a total of 150 surface bore-hole wells for exploration and trial extraction completed, obtaining a batch of coalbed methane reservoir exploration/testing parameters and productive parameters. While in coal bearing areas in Liulin and Jincheng in Shanxi Province, Dacheng in Hebei Province and Tiefa in Liaoning Province, coalbed methane gas flow of industrial scale has been obtained. 

3. Distribution of Coalbed Methane Experimental Areas in China
Coalbed methane experimental areas in China are mainly concentrated in the north China coalbed methane reservoir. Others are north-east China coalbed methane reservoir and south China coalbed methane reservoir. So far, coalbed methane experiments of different scale have been carried out in a total of 30 blocks that are widely distributed.

North China coalbed methane reservoir
A total of 21 coalbed methane experimental drilling areas are located in the north China coalbed methane reservoir. These account for 70% of the total of 30 blocks. These include Huainan and Huaibei areas in Anhui Province, Pingdingshan area in Henan Province, Jincheng, Yangquan and Liulin areas in Shanxi Province. 

South China coalbed methane reservoir
The South China coalbed methane reservoir covers the following experimental areas: a. Fengcheng Experimental Area; b. Lengshuijiang Experimental Areas and c. Lianshao Experimental Areas. These three experimental areas are located in the Mid-Hunan and Mid-Jiangxi coal bearing areas where simple geological structure is found and coalbed methane occurrence is better.

North-East China coalbed methane reservoir
The North-East China coalbed methane reservoir covers the following areas: a. Shanyang-North Experimental Area; b. Hongyang Experimental Areas; c. Tiefa Experimental areas; d. Hegang Experimental Areas; e. Fuxin Experimental Areas and f. Liaohe Experimental areas.

4. Brief Information about the Coalbed Methane Experimental Wells 
in China
Up to September 1998, a total of 142 coalbed methane wells were under construction of which 25 were constructed by the oil sector, 43 wells were constructed by coal sector, 14 were constructed in the capacity of UNDP programs, 22 were constructed by the Land and Resources authorities, 26 wells were constructed by China United Coalbed Methane Co., 11 wells were constructed by foreign firms and 1 well was constructed by local authorities. Of the total of 142 wells 79 wells are exploration data gaining wells and 63 wells are trial extraction wells. Of the total production wells, 11 wells have surpassed the extraction rate of 2000m3/d. 9 wells have surpassed the extraction rate of 3000m3/d. And 6 wells have surpassed the extraction rate of 4000m3/d. This category of wells include Jincheng No.1 Experimental well, Hancheng No.1 Experimental well and L7-3 well.


5. Inspirations from the Coalbed Methane Experimental Results 
Obtained in China
We can see from the analysis of the aforesaid major coalbed methane experimental areas in China that great headway has been obtained in the recent years in coalbed methane exploration and development trials. Primary breakthroughs were obtained in some certain areas and good results have been obtained in high coal rank areas such as the experimental areas of Liulin, Jincheng, Tunliu and Tiefa, etc. However, in most experimental areas, no breakthrough of industrial importance has been obtained. A primary analysis discloses the following problems:

Blindly selecting high gas out-burst prone areas with higher rate of gas in-rush as
experimental areas
The later stage tectonic movements in these areas brought about serious damages to coalbed methane. Although coalbed methane in these areas are of high concentration, the tectonic damages to the coal seams resulted in such facts that coal in some of the coal seams has become loose and broken or totally broken coal. This resulted in very low permeability of coal seam and very bad mechanical property of the coal seams. All these present difficulties to the development of coalbed methane. Such conditions are found in some experimental areas at the eastern foot of the Taihang Mountains.

Exploration are carried out far apart with distributions of coal seas not clearly 
disclosed
In some certain areas, coal exploration is scarce and coal seam distribution is not clear. Some companies including some foreign firms are eager to achieve their purposes and they carry out exploration activities sparsely. This practice results in the loss of coal seams and some of the necessary coefficients were not obtained. This makes it impossible to proceed with the evaluation work. In some extreme cases, no coal seam is found at all.

No attention was paid to the analysis and study of the conditions for maintaining 
coalbed methane
China is a country with very frequent tectonic movements. It is natural that geological structures in coalbed methane areas are also very complicated. In many areas, coal measures are lifted and serious erosion of the coal seams results in the loss of great quantity of coalbed methane. The Guhanshan Experimental Area in Jiaozuo, Henan Province and Tangshan Experimental Area in Hebei Province are just some examples. Besides, although coal rank is very high in some certain areas and coal seams are found much deeper, the effective thickness of over lying strata is thin and this makes the coal seams affected by the palaeo-methane weathering zone and coalbed methane concentration is very low in these areas. Dacheng Experimental Area is just an example. It is very important therefore to analyze the geological structure evolution and the conditions for maintaining coalbed methane.
Geological structures are not exactly localized
In some experimental areas, the well is located in high position of a geological structure, for example the Qin Can No.1 Well. This position makes it difficult to discharge water and the pressure in coalbed methane layer is hard to be lowered. This makes it difficult for coalbed methane to come out. In Tangshan Experimental Area for example coalbed methane exploration wells are mostly concentrated in the north-west wing of the area where complicated geological structure and much steeper inclination angles are found.

Overlooking the coalbed methane exploration and development work in high rank
coal areas
This is the result under the influence by the coalbed methane exploration and development carried out in the U.S.A. Once influenced by the U.S. experiences, people will blindly looking for medium coal rank areas for coalbed methane. Coalbed methane exploration and development in widely distributed anthracite coal areas in China has never been carried out so far. The fact is that coalbed methane concentration in anthracite coal areas is high while anthracite coal seams are generally shallow and anthracite coal is generally of higher hardness and its swelling property in water is low. China Oil & Gas Corp. has obtained a first breakthrough in an anthracite coal zone in Jincheng Experimental Area, Shanxi Province. China United CBM also obtained its successful results in a close-by area.

Fracturing technology is blindly applied
Undoubtedly, hydraulic fracturing process can help increase the yield of coalbed methane. We must know, however, that hydraulic fracturing process is by no means universal. Many coalbed methane wells can not stand hydraulic fracturing and therefore in some certain cases coal seam roof and floor area seriously damaged and penetrated if hydraulic fracturing is applied. This makes it difficult to discharge water and gas yield is lowered. The experimental bore-holes such as Liu 1 Well, An 2 Well, An 3 Well, Da 1 - 5 Well and Da 1 - 6 Well are all such examples. In fact, the fracturing property of coal seam relies on the mechanical property. If hydraulic fracturing is randomly carried out in some soft coal seams, the outcome could be just the opposite. The Wu Shi 1 Well is just such an example.

6. Favorable Coalbed Methane Exploration Area Forecasting Method
6.1 Conditions for coalbed methane high yield and high concentration
Many factors decide on if coalbed methane yield is high or low. These factors are mainly coal seam thickness, gas concentration, absorption conditions, permeability, hydrological conditions and pressure gradient. 

San Juan Basin is so far the most successful example in coalbed methane development in the U.S. and even in the world. Its success is the result of the following 6 conditions. 
San Juan Basin is late-cretaceous sediment basin. The Fruitland group in the coal bearing series is marine-continental alternating sediments formed during the sea regression. Thickness of coal seams is generally 9 - 12m and to the north-west part of the Basin the thickness of coal seams is over 21m. The thick coal seams are stretching in a SE - NW direction in the central part of the Basin.

b. Coalbed methane concentration is generally higher than 20m3/t.

c. Permeability is about 1 - 4md.

d. Gas saturation is 100%.

e. Pressurized water sealed coalbed methane reservoir in coal seams are formed due to 
the thin-out of water-bearing coal seams or the cutting effect caused by faults.

f. The mature drilling technology and well completing technology of the U.S. is an 
indispensable factor accounting for the success in the coalbed methane exploration 
and development in San Juan Basin. Coalbed methane exploration and 
development in Jincheng gas field in the southern part of Qinshui Basin, Shanxi 
Province also proves the necessity of a rationalized combination of the following 6 
aspects. Only when the 6 aspects perform functions in a unified manner could the 
basin turn out to be a high deposit and high production rate coalbed methane 
reservoir. 

6.2 Favorable exploration block predicting method
Different coal bearing areas or coal bearing basins have different geological conditions and have been exposed to different degrees of exploration. It is just because of this that predicting steps and predicting procedures are different. To cope with this situation, appropriate steps and methods have to be selected according to the actual conditions so as to find high CBM concentration and high yield areas as soon as possible with the smallest possible input. Here, predicting method for coalbed methane favorable exploration areas is divided into 2 steps. It is suggested to use one vote to veto system, i.e. where any one of the following conditions exists, the area can be regarded as having no exploration value hence it should be given up.

Geological background is unfavorable. This includes the sedimentation history, hydrocarbon formation history and tectonic history are not optimally matched and the tectonic structure conditions are unfavorable;

Coal contained in the coal seams are in the rank of lignite, long flame coal and anthracite coal I and II;

Total coal seam thickness is <3m and single coal seam thickness is <1m;

Coal seam depth is >1500m or <300m;

Coalbed methane concentration is <4m3/t.

6.2.1 The stage of comprehensive study of selected areas
Comprehensive analysis and study shall be based on the geological background, coal seam thickness, coal rank, coal seam depth and coalbed methane concentration, etc. 
On the basis of the above work, a comprehensive analysis and study was carried out from five aspects for the geological conditions of coalbed methane so as to identify the favorable areas. Due to the lack of some key parameters such as permeability, gas content, gas saturation and still some others, the evaluation is mainly qualitative and predictive in nature. 

6.2.2 The stage of high yield and high concentration area prediction
On the basis of selection of favorable areas, conditions for high yield and high concentration of coalbed methane was analyzed. In doing this, many different methods were adopted for a comprehensive analysis and prediction of highly permeable zones with fracture developed, making arrangement for evaluating well logging to obtain key parameters for the prediction of high yield and high concentration areas. The high yield and high concentration area shall satisfy the following specifications:

Single seam thickness >3m;

Coalbed methane content >15m3/t;

Coalbed methane saturation >80%, Langmuir pressure >3Mpa;
Permeability >0.5×10-3μm2;

The structure zone is favorably located in water pressure zone;

Gas bearing seams are not damaged without caving-in and connection with other seams. When conditions permit, daily gas production from a trial production well is > 3000m3/d.

7. Countermeasures for Coalbed Methane Exploration in China
7.1 Coalbed methane exploration to be carried out simultaneously in both 
eastern and western parts of the country
Coalbed methane fields in China are characterized of the complicated structure in the eastern part of China and comparatively simpler structure in the western part. Coalbed methane resources in the western part of this country (including that in Erduos, Inner Mongolia) account for 60% of the national total. With that of Erduos excluded, coalbed methane resources in the western part of China will account for 26% of the national total. It is clear that the gas resources in the west are huge. Combined with the simpler geological structure in the west, the huge coalbed methane resources in the west is much more favorable for development. However, population in the west is much sparse while industrial development in the eastern part is much stronger. It is of great significance to carry out a comprehensive consideration for a simultaneous development for the coalbed methane to gain a CBM development breakthrough of industrial significance.

7.2 Joint efforts called for 
Up to now, many domestic departments, enterprises and organizations participated in coalbed methane exploration. These include units such as Oil & Gas Group Corp, China Unitd CBM, Ministry of Land and Resources, the State Administration of Coal Industry as well as many local authorities. However, a widely spread practice is that many organizations adhere to a data blockage in that they are doing everything possible to prevent their technical data from being known by other institutions or companies. This has resulted in repeated investment hindering the process of coalbed methane exploration. It is of great significance to carry out cooperative R&D activities among different organizations, governmental authorities and companies to share the data and information available by carrying out technical exchanges and seminars for example to reduce redundant expenditures. Only in this way could the pace of coalbed methane exploration be speeded up and hence the sooner realization of the breakthrough of industrial significance.

7.3 The U.S. experiences are of value
Research and study on coalbed methane started in the U.S. as early as in the ’50s. Efforts by the U.S. industries and research institutions over the past 3 decades have resulted in great success. At the present moment, the U.S. annual coalbed methane production is registered at around 30.0 billion m3. We think that the U.S. experiences are of value to us i.e. resources shall be pooled together and strive hard for breakthroughs in coalbed methane exploration and development. All these shall be done around the goal of gaining economic profits. Once industrial gas flow is obtained, work can be then separated between different parties. This practice will surely reduce, to a large extent, the blindness in separated jobs before any success is obtained. This practice will also benign to an overall and true-to-the-fact evaluation of the coalbed methane industry of this nation. Of course, this practice will also help minimize the loss which may otherwise occur during the course of the research, exploration and development of coalbed methane.