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Challenges and Opportunities of Coalbed Methane Development in China Hon Chung Lau, Hangyu Li, and Shan Huang Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 28, 2017

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Energy & Fuels

Challenges and Opportunities of Coalbed Methane Development in China Hon Chung Lau1, Hangyu Li2,* and Shan Huang2 1

2

National University of Singapore, Singapore

Shell International Exploration and Production Inc., Houston, USA

Abstract

Coalbed methane (CBM) resource in China has been estimated to exceed 36 Tcm. As of 2014, there are about 9300 producing CBM wells in China with annual production of about 4.4 Bcm. To satisfy its need for energy and to transition to a low-carbon economy, China has a big need to accelerate CBM development. This paper gives an overview of the status of CBM development in China, identifies key technical challenges, and proposes solutions to overcome them. Our review of the literature has revealed that current CBM development in China faces several technical challenges. Current projects are focused on high-rank coals in the Qinshui and Ordos basins which have major geological and engineering challenge. The former includes low permeability, sub-hydrostatic reservoir pressure and a lack of understanding of the connectivity of coal seams which leads to difficulty in sweet spot indication. The latter includes difficulty in hydraulic fracturing in vertical wells due to the ductile nature of the coal seams in the Qinshui basin and borehole instability and formation damage during drilling of horizontal wells. To remedy this situation, we propose a refocus on the more abundant high-permeability low-rank coals in China and detailed coal seam characterization using current industry best practices of static and dynamic modeling of CBM reservoirs. This refocus on low-rank coal may lessen the need for hydraulic fracturing in vertical wells. For

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horizontal wells, research should focus on non-formation-damaging drilling fluids, incorporating geomechanics studies to reduce the risk of borehole collapse during drilling and new horizontal well designs that minimizes the risk of collapse of the mother bore.

1. Introduction China has the world’s third largest CBM resources after Russia and Canada. Furthermore, of the three unconventional gas resources, China has more CBM resource than shale and tight gas. Commercial development of CBM in China has lasted for twelve years. However, CBM production has lagged behind the government proposed target. The objective of this paper is to review the published literature in CBM production in China and identify the technical challenges facing the industry. We then propose opportunities to overcome these challenges.

2. CBM Development in China Based on a recent study, CBM resource volume in China is 36.81 Tcm in coal seams with a buried depth less than 2000 m [1]. This is the third in the world following Russia and Canada. Of this, over 24 Tcm or 68% of total resource volume are found in buried depth shallower than 1500 m according to China’s National Development and Reform Commission [2]. The history of coal gas production in China is shown in Fig. 1, which includes both CBM production from surface wells and coal mine methane (CMM) production from coal mines. The production for CMM increased steadily since 1990. By contrast, CBM production started from around 2004 and saw significant increase only after 2008. In December 2011, China’s National Energy Administration (NEA) released the 12th Five Year Plan (FYP) for the period between 2011 and 2015 for the development and utilization of CBM and CMM [3]. It outlined China’s plan to produce annually


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30 Bcm of coal gas with 16 Bcm and 14 Bcm from CMM and CBM, respectively, by the end of 2015.

However, in April 2016, the Ministry of Land and Resources [4] released the most updated volumes for CBM reserves. It stated that CBM total production rose 24.8% to 4.43 Bcm in 2015 which was significantly lower than the target in the 12th FYP. Based on the same report, China’s proven CBM reserve is 306.3 Bcm [4]. Note that the definition of proven reserve in China is different from and can be significantly lower than the SEC definition. A detailed discussion on reserve category in China has been given by [5] and [6].

Figure 1. CBM and CMM production history in China (modified from [7]).

In February 2015, China’s NEA released the new Exploration and Production Activity Plan for CBM and CMM which set the target for CBM production for 2016 to 2020. It outlined that China’s annual CBM production should exceed 20 Bcm. To incentivize this, the government also


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boosts the subsidy for CBM by 50%, from 0.2 RMB/Sm3 to 0.3 RMB/Sm3 while simultaneously reducing the subsidy of shale gas to 0.3 RMB/Sm3 and eventually to 0.2 RMB/Sm3. All these indicate a refocus of the government from shale gas to CBM production.

3. Overview of Qinshui and Ordos Basins Most of CBM resources are located in several major basins, including Qinshui, Ordos, Junggar and Turpan-Hami (Tu-ha) as shown in Figure 2. The basins are color coded according to coal rank.

Figure 2. Distribution of China’s CBM basins (modified from Qiu 2008). Dashed-red marks the ocean border.

The Qinshui basin is located in the southeastern Shanxi Province in central China covering an area of over 2.3x104 km2, and is bounded by Wutaishan (to the north), Taihangshan (to the east),


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Zhongtiaoshan (to the south) and Huoshan (to the west) uplifts [8]. It is over 330 km long and it is generally aligned along the NE to SW direction [9]. The CBM resource with buried depth less than 2000 m is around 4 Tcm, which is over 10% of China’s total CBM resource [10].

In the Qinshui basin, the well count increased dramatically from around 100 in 2001 to over 10,500 by the end of 2014. During this period, the CBM discovered geological reserves increased from 75.5 Bcm to 435 Bcm. The current CBM discovered geological reserve in Qinshui basin amounts to 70% of China’s total as shown in Figure 3 [11]. Note that geological reserve in China represents the hydrocarbon in-place volume. A detailed discussion on reserve categories in China has been given by [5] and [6].

Figure 3. Discovered geological reserves for Qinshui basin and China total (modified from [11]).

CBM resource in the Ordos basin is about twice that of the Qinshui basin, amounting 20% of China’s total CBM resources. However, CBM development there lagged behind that in the


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Qinshui basin. As of the end of 2014, the total CBM discovered geological reserve for the Ordos basin is around 150 Bcm based on data published by the MLR. This represents about 23% of China’s total CBM discovered geological reserves [12].

In total, the Qinshui and Ordos basins contain over 30% of China’s total CBM resources and 93% of China’s CBM discovered geological reserves. In this paper we will focus our discussion on these two basins. The Qinshui basin is a part of the North China basin and is filled with multiple formations [13]: Pennsylvanian Benxi and Taiyuan, Permian Shanxi, Xiashihezi, Shangshihezi and Shiqianfeng formations, and Triassic deposits. The basin has coal-bearing sequences deposited extensively during the Carboniferous-Permian ages, mainly in the shallower Shanxi and the deeper Taiyuan formations. The Shanxi formation comprises of fluvial deposits of mudstone, siltstone, and sandstone, and the Yaiyuan formation mudstone, sandstone, and limestone. The average thickness of the two formations is around 150 m, and total net coal thickness varies from 6 to 15 m [13-14]. Figure 4 shows the stratigraphy and well logs of the two formations and coal No. 3 and No. 15 [14].


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Figure 4. Stratigraphy of southeast Qinshui basin (modified from [14]). The Qinshui basin was formed during uplifting of the late Paleozoic platform, associated with orogeny in the Triassic Indosinian and Jurassic to Cretaceous Yanshanian. Surrounded by many mountain uplifts, the basin is a complex syncline striking from NNE to SSW [15]. Although it has few major faults, many folds have developed, thus introducing geological complexities. Study of the Fanzhuang block focuses on the Shanxi formation [16]. It was reported that although coal measures No. 3 and No. 15 have stable distribution across the southeast Qinshui area, each coal measure has many internal small faults and fractures, with average distance between fractures less than 20 m.


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The sedimentary Ordos basin is famous for its large natural gas resources, including CBM. Figure 5 shows the stratigraphy of the Ordos basin. The basin, surrounded by mountain uplifts, can be subdivided into six sub-structures tectonically [17]. Gas accumulation exists in the middle part of the Yishan Slope. The basin has experienced four stages of evolution starting from early Paleozoic to Cenozoic. Hydrocarbon generation started in Mesozoic and terminated in the late Mesozoic uplift, and the present basin framework is delineated by subsidence in the Cenozoic. The evolutional history resulted in coal-bearing reservoirs with poor physical properties, abnormal pressure, significant heterogeneity, and lack of well-defined gas-water contacts.


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Figure 5. Upper Paleozoic chrono-lithology and gas-bearing layers in the Ordos basin (Reprinted with permission from [17]).


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4. Geological Challenges 4.1 Coal Rank Coal rank is the classification of coals based on volatile content, which is determined by the buried history. It provides insights into many coal properties such as gas content and permeability loss due to effective stress changes. Vitrinite reflectance (Ro) is a common indicator of coal rank, because it has a semi-linear anti-correlation with respect to volatile content as shown in Figure 6 [18]. Figure 7 compares the Ro of four basins, Qinshui and Ordos in China, San Juan in US and Surat in Australia. It can be seen that the coal in Qinshui basin is of semi-Anthracite to Anthracite rank. Coal from the Ordos basin has a wider range of Ro that runs up to Anthracite. Conversely, maximum Ro value measured in coal samples from the US San Juan basin is in Medium Volatile Bituminous level, and typical coal rank is lower than High Volatile Bituminous level. The coals in the Surat basin are of the lowest rank among the four basins and it ranges from Sub-Bituminous to High Volatile Bituminous. We conclude therefore that coals in Quishui and Ordos are generally of higher rank than those in San Juan and Surat basins.

A higher rank coal has a higher methane content as shown in Figure 8 [19]. Consequently, high rank coal in Qinshui and Ordos basins has been the development target. However, permeability of high rank coal is highly susceptible to increase in effective stress. Experiments have shown that irreversible stress-induced permeability loss is 80% in average [20-21]. Well test data show that CBM basins in China have high tectonic stress in general, while Qinshui and Ordos basins have relatively lower stress [9][22]. Stress distribution is also highly heterogeneous inside these basins [9]. As a result, coal seams in Qinshui and Ordos have low permeability.


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Figure 6. Cross-plot of vitrinite reflectance and volatile matter percentage (modified from [18]).


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Figure 7. Coal rank comparison (left two columns modified from [23]; data from [13][17][2425]).

Figure 8. Relationship between rank, depth, and adsorptive capacity (modified from [19]).


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4.2 Coal Porosity and Permeability Total porosity values measured in coal samples from the Qinshui and Ordos basins have been published by many authors [26-32], and are summarized in Table 1.

Table 1. Summary of total porosity in Qinshui and Ordos basins Basin

Area

Range (pu)

Mean (pu)

Sources

-

0.2 – 13.3

5.1

[31]

-

[29]

No. 3 seam

coal

3.0 – 8.0

Qinshui No. 15 coal 2.0 – 7.0 seam -

0.3 – 7.5

3.8

[32]

Binchang

2.4 – 20.1

9.0

[28]

-

5.2 – 9.3

7.0

[26]

Yanchuannan

1.6 – 3.8

2.5

[30]

Eastern

2.6 – 8.4

4.8

[27]

Ordos

Figure 9 shows total porosity histograms of coal samples from the Qinshui and Ordos basins (data from [33-34] are also used). Averages of the reported total porosity of the two basins are similar: 5.2 pu (porosity unit) and 6.2 pu for Qinshui and Ordos, respectively. In comparison, coal samples from the San Juan basin [35] have porosity from 3 to 10 pu, which is not too different.


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Figure 9. Histograms of coal total porosity in Qinshui and Ordos basins. However, the pore size distribution of the Qinshui and Ordos baisin reveals an important difference. Zhang et al. [27] published a summary of pore size distribution measured using 58 core samples from Ordos coal seams. Their study shows that 46 to 89% of the pore space consists of small (radius < 100 nm) and micro (radius < 10 nm) pores. Cai et al. [30] showed similar results that over 70% of the porosity in their coal samples are micro and mesopores. Yao et al. [36] also published pore size percentages for samples from north, middle, and south Qinshui basins, showing that over 85% of the pores are small and micro pores. Pore size distribution measurements from Qinshui coal seams No. 3 and No. 15 also show on average 78% of the porosity contain small and micro pores [32]. This pore size distribution is an important clue the origin of low coal seam permeability in the two basins.

Table 2 gives a summary of core permeability measurements from multiple authors [2632][37]. Figure 10 also gives the histograms of permeability data from Qinshui and Ordos basins. It can be seen that over 50% of the coal samples in the two basins have permeability lower than 1


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mD. In contrast, coal permeability of the San Juan basin is typically one to two orders of magnitudes higher [38-41].

Table 2. Permeability summary for China’s CBM basins

Basin

Area

Range (mD)

Geometric Mean (mD)

Median (mD)

Sources

-

0.01 – 7.47

0.29

0.43

[31]

-

-

[29]

No. 3 seam

coal

0.15 – 2.0

Qinshui No. 15 coal 0.08 – 1.5 seam -

0.01 – 8.4

0.57

0.75

[32]

Binchang

0.04 – 45.1

1.05

0.43

[28]

-

0.14 – 1.64

0.61

0.51

[26]

Yanchuannan

0.016 – 5.52

0.19

0.12

[30]

Eastern

0.007 – 4.78

0.15

0.15

[27]

0.41

0.41

0.78

1.17

Ordos

0.014 – (Qinshui) Seven basins

-

13.3

0.09 – 11.2 (Ordos) 0.002 – Others)

15.8

(5 0.1

[37]

0.08


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Figure 10. Histograms of coal permeability in Qinshui and Ordos basins.

We also compare permeability-depth trends for additional insights. Figure 11 compares permeability versus true vertical depth in Qinshui and Ordos basins to those in three US basins [31][42]. Evidently, average permeability of coal seams at the buried depth of 300 to 1000 m is over 10 times lower than that of the US basins.


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10000

1000

Permeability (mD)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

100

10

1

0.1

0.01 10

100

1000 Depth (m)

Qinshui

Ordos

San Juan

Black Warrior

Piceance

Figure 11. Permeability-depth comparison for CBM basins. Ayers and Kaiser [38] drew an important conclusion that permeability is the most critical parameter for CBM production, inferred from production data from over 500 CBM production wells in the San Juan basin. Similarly, low permeability of the Qinshui and Ordos basins suggests that limited flow capacity is one of the main reasons for low CBM production rate.

4.3 Gas Content Gas content is another important factor that impacts CBM production. It varies significantly between CBM basins, and often within the same basin. Even within the same coal seam, the gas content may exhibit large variation due to local hydrogeological and reservoir conditions [43]. In general, gas content increases with coal rank. Since coal rank (or thermal maturity) increases with buried depth, gas content also increases with buried depth.


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Table 3 compares the gas content of various CBM basins in China (Qinshu and Ordos), US (San Juan and Powder River) and Australia (Surat). It can be seen that the high-rank coals in Qinshui and Ordos basins have higher average gas content than the lower rank coals in the US and Australian basins. We may therefore conclude that low gas content is not the reason for low productivity in Qinshui and Ordos basins.

Table 3. Comparison of gas content of various CBM basins Gas Content (m3/t) Basin

Formation/Area Range

Depth (m)

Sources

Mean

Zhengzhuang No. 3 coal 20.3-31.5 seam

24.7

513-1336

[44]

Heshun No. 15 2.3-20.5 coal seam

>10

453-1178

[45]

Yanchuannan No. 2 coal 0.2-20.4 seam

9.2

463-1103

[30]

Weibei No. 5 2.69-16.5 coal seam

10.1

466-1215

[46]

San Juan

-

2.7-19.8

>11

390-990

[47]

Powder River

-

0.5-2.4

-

1.15)

-

[40]

[51] Qinshui [9] [29] Ordos San Juan


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4.5 Coal Connectivity Since coal seams often pinch out, split or merge, understanding coal seam connectivity is a key to CBM field development. During the initial phase of field development, lack of well control and scarcity of geological data over large geographic distances present a particular problem in constructing a static model to estimate in-place volume and locating potential sweet spots. To overcome this, detailed geological modeling that integrates all available data such as petrophysical logs, seismic, core samples and topology is needed to construct an accurate static model which needs to be properly upscaled for reservoir simulation. We found that this type of integrated static and dynamic modeling of CBM reservoirs lacking for the Qinshui and Ordos basins.

Some authors have reported on various aspects of geological modeling. Zhao et al. [52] recently briefly introduced how 3D seismic was used in exploration in Qinshui basin. Tang et al. [17] showed applications of 2D seismic cross-section for evaluation of gas content in Qinshui basin CBM area. Wang et al. [54] used 2D seismic to study fault-sealing and its influence on methane distribution in southern Qinshui basin. Yan et al. [55] incorporated 2D seismic to study the tectonic events that controls gas migration and accumulation in Weibei CBM field in the Ordos basin. Cai et al. [30] also evaluated gas content in Ordos basin No. 2 coal seam by incorporating 2D geophysical data into geo-modeling.

However, integrated reservoir modeling that includes both static and dynamic modeling and history matching of production data to verify the connectivity of coal seams is lacking for the


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Qinshui and Ordos basins. Zhou et al. [56] constructed a relatively simple model and matched CBM total production rate from 43 wells in southeast Qinshui basin. Single well history has not been reported.

This contrasts sharply with the many geo-modeling and history matching studies published for CBM reservoirs in US and Australia. Weber et al. [57] built a detailed 3D reservoir model and studied the impact of geological complexity of the Fruitland formation in San Juan basin on pilot CBM production. Moore et al. [58] conducted CBM well production history matching for studying permeability increase with production. Karacan [41] showed reservoir modeling and history matching results for determination of reservoir properties CBM reservoirs in several basins. Zhang et al. [59] published their best practices for CBM reservoir modeling using an example from Bowen basin in Australia, which integrates geological, geophysical, petrophysical, and production data. Similar work has also been done in [60] for Surat basin in Australia. The findings are summarized in Table 6. Table 6. Summary of reported geo-modeling study Basin

Qinshui

Purpose

Data/Methodology

Sources

Exploration

3D seismic

[52]

Gas content evaluation

2D seismic

[53]

Fault-sealing study

2D seismic

[54]

ECBM study

Geo-modeling with field [56] production history matching

Tectonic events study

2D seismic

[55]

Gas content evaluation

2D seismic

[30]

Ordos San Juan Geological study for 3D reservoir model

[57]


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ECBM Permeability dependence production Reservoir study

on -

[58]

properties 3D reservoir model, history [41] matching Geological, geophysical, petrophysical, and production [59] data

Bowen

Reservoir modeling

static

Surat

Reservoir modeling

static Geological, petrophysical

geophysical,

[60]

5. Engineering Challenges In 2014 there were 6,300 producing wells in the Qinshui basin targeting high-rank coal seams, including 110 horizontal wells [7]. The total CBM production was 2.65 Bcm, which corresponded to an average well rate of 1,150 Sm3/d. In the same year, the Ordos basin had 2,959 producing wells, including 106 horizontal wells, with annual CBM production of 0.85 Bcm, giving an average well rate of 787 Sm3/d. Table 7 compares the daily gas rate for CBM basins in US, Australia and China. It can be clearly seen that CBM basins in US and Australia have much higher gas rates than the Qinshui and Ordos basins.

Table 7. Gas production rate for various CBM basins

Country

Basin

Well count

Cum gas Gas rate produced Year (Sm3/d) (Bcm)

2032

7.65

1.00x104

1991

[61]

US

San Juan Black

3474

3.29

2.60x103

2002

[61]

Sources


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Warrior Uinta

1255

2.14

4.70x103

2002

[62]

Bowen

974

3.3

9.30x103

2015

[63]

3786

14.9

1.10x104

2015

[63]

Qinshui 6300

2.65

1.20x103

2014

[7]

Ordos

0.85

7.80x102

2014

[7]

Australia Surat China

2959

Due to the low permeability of CBM reservoirs in the Qinshui and Ordos basins, two well types are commonly used to achieve commercial production rates. The first is vertical well with hydraulic fracturing. The second is multilateral horizontal wells. Unfortunately, neither well type has a high success rate.

5.1 Hydraulic fracturing issues in vertical wells 5.1.1 Brittleness Vertical wells in the Qinshui and Ordos basins require hydraulic fracturing to produce at commercial rates. Unfortunately the mechanical properties of coal seams in these basins are not ideal for hydraulic fracturing due to the low Young’s modulus which renders the coal seams less brittle.

Multiple authors have come up with definitions of rock brittleness [64-67]. We follow the well-known method by Rickman et al. [66] and calculate brittleness as: ‫ = ܤ‬7.14‫ ܧ‬− 200ߥ + 72.9 (1)

where B is brittleness, E is Young’s modulus in GPa, and ν is Poisson’s ratio.


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Energy & Fuels

Table 8 shows the Young’s modulus and Poisson’s ratio of coal samples from the Qinshui and Ordos basins measured by various authors. It can be seen that the brittleness of coal samples from the Qinshui basin is in general less than that of the US basins. Brittle rocks, even those with high strength, absorb less energy prior to fracture initiation than ductile rocks, as Figure 13 shows [68]. Therefore fractures in ductile rocks are difficult to initiate. As a result of their low elasticity, coal seams in Qinshui basin are more ductile than those in the US basins, thus hampering effective hydraulic fracturing treatment. Brittleness of Ordos coals is in general higher than that of Qinshui, suggesting that they may be easier to fracture.

Table 8. Comparison of mechanical properties (brittlenesses were calculated based on the averages) Young’s Modulus (GPa)

Poisson’s Ratio

Range

Average

Range

0.21 – 1.63

0.91

0.28 0.33

–

0.55 – 2.08

1.26

0.27 0.33

–

4.05 – 4.48

4.27

0.35 0.36

–

1.36 – 2.99

2.37

0.124 0.33

– 0.225

2.38 – 4.53

3.7

0.12 0.43

– 0.28

1.66 – 4.29

2.83

-

2.07 – 4.83

3.38

0.26 0.40

-

3.6

-

Basin

Qinshui

Ordos

Brittleness* (%) Resources

San Juan

Average

–

0.31

17.4

[69]

0.31

19.9

[70]

0.35

33.4

[71]

44.8

[72]

43.3

[73]

-

-

[70]

0.296

37.8

[74]

0.21

56.6

[75]


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Page 26 of 48

-

4.5

-

0.32

41.0

[39]

Piceance -

2.4

-

0.31

28.0

[76]

Figure 13. Strain vs. stress diagram comparing brittle (red) and ductile (blue) curves (modified from [68]). Area under the stress-strain curve represents energy required to initiate fracture in the rock. In addition to fracture initiation, fracture length and width are also related to rock mechanical properties [77]. Rocks with low Young’s modulus tend to have wide fractures with low fracture height and length, resulting in low fracture surface area. Therefore, hydraulic fractures in coal seams in the Qinshui basin tend to have low overall surface area, thus reducing the contact area with the reservoir.

5.1.2 Fracturing fluids The most widely used fracturing fluid in China CBM wells is “activated water” which is a mixture of surface water, anti-swelling agent (typically KCl) and cleanup additives (typically surfactants). It is chosen mainly due to its low cost and ease of preparation. The biggest


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advantage of “activated water” is preservation of coal permeability. Cui et al. [78] investigated the permeability reduction for coal samples before and after applying a variety of fracturing fluid. Results showed that permeability reduction is less than 10% with “activated water” which is significantly lower than that with gelled fracturing fluid. However, there are disadvantages of “activated water.” First, it has low proppant carrying capacity due to low viscosity. Therefore, to achieve effective fracturing, large volume of fluid needs to be injected which generates massive coal fines. These coal fines can settle along the induced or natural fractures to reduce fracture conductivity [79]. The alternative is to reduce the proppant concentration, but this may lead to proppant embedment due to the relatively low strength of coal seam. In addition, high friction loss can limit the injection rate of “activated water” [80].

Due to its low viscosity, “activated water” has low proppant carrying capacity. Hence, it is difficult to carry the sand into formation far away from the wells. It is reported that the effective fracture half-length for Qinshui’s CBM wells is less than 40 m [81], which is inadequate for the average well spacing of about 300 m. Therefore, the development of low-cost, light-weighted proppant is another challenge in China’s CBM well fracturing process.

Other types of fracturing fluids have also been tested and applied in Qinshui and Ordos. Table 9 shows a few examples of wells fractured using gelled fluid, VES-based fluid and nitrogenbased foam. The results demonstrated that wells fractured with combined nitrogen and “activated water” typically have the best performance (even better than using nitrogen foam). Gelled fluids and VES-based fluids are not recommended due to high costs and formation damage [82]. Nevertheless, these “unconventional” fracturing fluids are not widely used in China.


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Table 9. Other CBM well fracturing fluids used in China

Basin

Area

well count

Fracturing fluids used

Sources

Panhe

2

Nitrogen foam with proppant

[79]

Panzhuang

2

Cross-linked gel with proppant

[83]

Yuwu

10

3 with activated water; 4 with nitrogen and activated water; 3 with VES-based clean [82] fluid

Tunliu

16

3 with activated water and nitrogen; 12 [82] with nitrogen foam

Licun

4

3 with activated water and nitrogen; 1 with [82] activated water

-

1


[84]

Hancheng

3

VES-based fracturing fluid with proppant

[78]

DaningJixian

4


[85]

Qinshui

Ordos

5.2 Drilling issues in horizontal wells Horizontal wells have been used to develop CBM reservoirs in China for the last decade. CNPC started the commercial development of CBM with horizontal drilling since 2006 in Fanzhuang block in the south Qinshui basin [86]. Until 2010, a total of 45 horizontal wells have been drilled and put in production. From 2010 to 2014, the horizontal wells in the Qinshui basin increased very slowly to only around 110 [7]. The main reason for such a slow pace was because of unsatisfactory single well production rate. Table 10 summarizes the production rates for 109 horizontal CBM wells in the south Qinshui basin. Of the 109 horizontal wells, 29 or 27% of them have zero gas production rate. In addition, 32 wells produced with average gas rate of only


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Energy & Fuels

456 Sm3/d. Only 11 horizontal wells produced at the rate higher than 10,000 Sm3/d (comparable to those in US and Australia). The average production rate for the 109 wells is 3,314 Sm3/d. This clearly indicates that the under-performance for horizontal wells, which has been attributed to issues in the drilling operations (for both underbalanced and overbalanced drilling).

Table 10. Production rates of horizontal CBM wells in south Qinshui basin [87] Block F1-C1

Block Z2-2

Block Z3

Gas rate Gas 3 Well (Sm /d) rate count (Sm3/d)

Gas Well rate count (Sm3/d)



0

13

-

-

-

14

-

2

-

1−1000

17

438

-

-

14

449

1

862

1000−2000

3

1232

-

-

7

1325

-

-

2000−5000

11

3328

1

4348

7

3220

-

-

5000−10000 5

7205

-

-

3

6212

-

-

>10000

7

16903

3

27114

1

15804

-

-

Mean

-

3609

-

21423

-

1577

-

287

Block F1-2

Xia et al. [88] discussed three under-balanced drilling technologies used in China. They are gas drilling, aerated liquid drilling and circulated micro-bubble drilling methods. Among them, the typical one used in China is aerated liquid drilling with surface water as the drilling fluid. Its advantages include low cost and abundance of easily accessible river water. However, there are serious disadvantages impacting well performance. For example, experiments by [89] on coal samples taken from south Qinshui basin showed that clay swelling by surface water can destabilize the wellbore significantly even at small clay concentrations. According to [90-91],


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another major reason for borehole collapse is the instable annulus pressure during the aerated under-balanced drilling process. Borehole collapse of the horizontal sections is commonly seen when drilling complex CBM wells. Similar issue was discussed by [92], who reported that during the drilling of the six horizontal wells in Zhengzhuang block in Qinshui basin in 2011, a total of five major incidents happened (e.g., stuck drill bit) due to borehole collapse. CNPC also reported 39.3% of the multilateral wells experienced stuck drill bit incidents [93].

Due to the borehole stability issue with under-balanced drilling, balanced drilling or slightly over-balanced drilling is also attempted. In this situation, surface water drilling fluid may enter the coal seam and cause formation damage. Swelling of clay in coal seam will further reduce the already low permeability in the near wellbore area [94]. Meanwhile, surface water drilling fluid is not compatible with the formation water in the reservoir. This may lead to the scaling of mineral around the well [95], which again affects well productivity. In addition, drilling fluid entering the micro pores through imbibition may cause water blockage [94] resulting in reduced gas relative permeability due to a high water saturation.

6. Opportunities for Future Development 6.1 Development of moderate to low-rank coal Current CBM projects in China focus on the medium and high-rank coals in the south Qinshui basin and east Ordos basin. However, due to the aforementioned geological and engineering issues associated with developing high rank coals, a refocus on low-rank coal may yield better results in China. The success of CBM development in low-rank coals in Powder River, Uinta and Raton basins in US and in the Surat basin in Australia are especially encouraging.


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China has abundant CBM resource volumes in low-rank coals. The overall national CBM resource in low-rank coals is 14.7 Tcm [96], which accounts for about 40% of China’s total CBM resource volumes. In fact, most of China’s CBM basins contain low rank coals. Representative basins include Junggar and Tu-ha basins in northwestern China, and Halaer and Erlian basins in northeastern China (Figure 2). In addition, a significant portion of the Ordos basin also contains low-rank coal, for example, the Binchang area in the southwestern Ordos basin.

Junggar basin is located in Xinjiang Uyghur autonomous region and it has CBM resource volumes of 2.2-3.8 Tcm which ranks the third after Ordos and Qinshui basins [97]. The coalbearing strata are found in two formations, Xishanyao and Badaowan, formed in the middle and early Jurassic periods. The vitrinite reflectance ranges from 0.45% to 0.76%, which indicates low rank coal. The total thickness is from 8 m to over 200 m, which is significantly thicker than coal seams in Qinshui and eastern Ordos basins. For Section 4 in Junggar basin, the coal seam total thickness ranges from 23 m to over 200 m. The buried depth for coal seams is shallow (200-500 m as shown in Table 11) which favors the CBM development. The average porosity is over 7.1% and the average permeability is as high as 11 md. Both parameters are better than those of highrank coals, though the gas contents are slightly lower. These reservoir properties are favorable for CBM development. In the Fukang area, southern Junggar basin, a total of 70 CBM wells have been drilled and put in production [12]. The average gas rate varies from 1,000 Sm3/d to 2,800 Sm3/d with the peak rate of 15,000 Sm3/d. Horizontal well can reach a peak gas rate of over 30,000 Sm3/d.


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Another example of low-rank CBM exploration is in Binchang, southwestern Ordos basin [28][98]. The main coal-bearing sequences occur in the middle Jurassic Yanan formation. The development target is the Yanan formation No. 4 coal seam, which has a thickness of 1.7 m to 21.3 m (about 9.2 m on average) with a buried depth ranging from 300 m to 1300 m. Porosity and permeability are generally better than coals in Junggar basin, though the gas contents are lower (Table 11). The vitrinite reflectance varies from 0.46% to 0.73%, which indicates the low rank coals. A variety of pilot CBM wells have been drilled in this area, from vertical to multilateral horizontal well. A peak gas rate of over 16,000 Sm3/d has been achieved for a horizontal well. Vertical well is able to achieve a gas rate of over 2,100 Sm3/d.

Additional low-rank CBM exploration and production activity in China includes the Fuxin basin (in northeastern China), in which a total of 26 CBM wells are put in production with a total gas rate of 60,000 Sm3/d [99]. In addition, the gas rate in Hunchun basin (in northeastern China) also reached a stable production of 1,500-2,200 Sm3/d [99]. CBM explorations are also conducted in southwestern China. In the western of Guizhou Province, 90 CBM wells have been drilled with a peak rate of 5,000 Sm3/d [12].

Table 11. Properties of low rank coals in China Vitrinite Buried Gas Thickness Porosity Permeability reflectance depth content Sources (m) (%) (mD) (%) (m) (m3/t)

Basin

Formation

Junggar

Xishanyao and 0.45-0.76 Badaowan

200500

8-206

0.2-16.4 0.22-23.2

2.4315.63

[97]


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Energy & Fuels

Yanan (No. Ordos (southwestern) coal seam) Fuxin

Fuxin

4

0.46-0.73

3001300

1.7-21.3

2.7-20.1 3.1-5.7

1.196.35

[28][98]

0.5-0.8

8001200

26.1-42.7

4.7-7.4

6-10

[6]

0.32-0.47

6.2 Understanding coal connectivity through improved reservoir characterization Due to variability in coal connectivity, success in CBM field development hinges on the identification of sweet spots for well placement. This is especially important in the initial phase of field development where well control and geological data over large geographical area are lacking. To solve this problem, detailed geological modeling that integrates all available data including well logs, cores, seismic and topology is needed to assess in-place volumes, conduct sensitivity studies and quantify coal connectivity. Furthermore, static models need to be properly upscaled before reservoir simulations can be carried out to forecast production. To properly constrain the reservoir model, history matching of field wide and single wells production will also be needed. This type of integrated reservoir modeling will be crucial for sweet spot identification and field development planning.

Zhang et al. [59-60] have proposed a best practice workflow for static modeling of a CBM field. The first step is to collect data from all sources. The second step is to normalize the well logs and use to them pick and correlate coal plies and delineate ply surfaces based on well-towell correlation, well-to-seismic correlation and well-seismic-graphic information system correlation. Simultaneously, core data are analyzed to obtain the probability distribution of reservoir properties such as gas content, relative density, permeability, ash content, Langmuir pressure and volume etc. for uncertainty analysis. Sensitivity analysis, which varies one reservoir


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Page 34 of 48

parameter at a time, is then performed to rank the impact of reservoir parameter on in-place volume. Uncertainty analysis, which varies all parameters simultaneously, is then conducted to obtain the in-place volume distribution from which the P10, P50 and P90 in-place-volumes are obtained. We believe such an endeavor will be beneficial for CBM development in China.

6.3 Hydraulic fracturing design Experience from the Surat basin in Australia and Black Warrior basin in US has shown that vertical wells drilled in high permeability low-rank coal seams may not require hydraulic fracturing to produce at commercial rates. Similarly, low-rank coals in China may have less need of hydraulic fracturing than higher-rank coals in the Qinshui basin. It is therefore worthwhile to collect data on the mechanical properties of lower-rank coals from various Chinese basins to determine their need of hydraulic fracturing.

In situations where hydraulic fracturing is needed, use of cross-linked fracturing fluid yield better results than linear gels or slick water in more ductile formations. This may be relevant for the relatively ductile coal seams in the Qinshui basin. Therefore, it may be worthwhile to investigate the use of cross-linked hydraulic fracturing fluid for this basin.

6.4 Horizontal well design Our review has revealed that horizontal well drilling in China suffers from borehole collapse and drilling fluid induced formation damage. Therefore, research into the choice of a nonformation damaging fluid which is compatible with the formation clays will be needed.


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Energy & Fuels

Furthermore, geomechanical studies will be useful to understand the conditions to avoid wellbore collapse during drilling and production of horizontal wells.

To overcome the problems in borehole stability during underbalanced drilling, a novel multilateral horizontal well design was developed recent by CNPC in Qinshui basin, which is named as tree-like horizontal well [100]. The major difference compared with traditional horizontal well is that the major hole of this well type is drilled in the stable formation either above or below the coal bed to prevent bore-hole collapse. The lateral wells and sub-lateral wells are then drilled from the major hole into the coal bed to extract water and CBM as shown in Figure 14. A pilot well has been drilled in Qinshui Basin and it has 1 major hole, 13 laterals and 26 sub-laterals with a total drilling footage of over 12000 meter, a total coal bed footage of over 9500 m. Though the production data is not available yet for this well, high gas rate is expected.

Fig. 14. Tree-like horizontal well design piloted in Qinshui Basin (Yang et al. 2014). 7. Suggestions for Future Research Areas


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Future research should focus on three main areas for low-rank coal in China: property characterization, geo-modelling and engineering studies.

Coal property characterization should begin with the collection and measurement of properties of low-rank coals in major CBM basins in China from all available sources. Laboratory measurement of coal properties such as permeability, porosity, gas content, Langmuir volume and pressure, ash and moisture content, among others, as a function of buried depth is fundamental. Second is the collection of well log data, 2D and 3D seismic data, topographical and geographic information system (GIS) data.

Geo-modelling studies should start with performing rigorous quality control on data collected from the previous step. Then detailed coal ply division is done based on well-to-well, well-toseismic and well-seismic-GIS correlations. Structure and property models are then built. Sensitivity and uncertainty analysis should then be done on reservoir properties to establish the P10, P50 and P90 volumes.

Engineering studies should involve three parts: reservoir, drilling and completion. Reservoir study includes upscaling of the geo-model to do history matching of production history, if any. Then the reservoir model can be used in predictive mode to do field development planning. Drilling research should focus on the development of non-formation damaging drilling fluids (water based, foam based) and methodology (underbalanced, balanced or overbalanced). Completion studies should focus on the best well type (vertical, dual horizontal, multi-lateral, hydraulically fractured or not) to ensure well longevity and optimal production.


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Energy & Fuels

8. Summary Our review of the literature has revealed that current CBM development in China faces focuses on high-rank coal in the Qinshui and Ordos basins which have major geological and engineering challenges. Geological challenges include low permeability, sub-hydrostatic reservoir pressure and a lack of understanding of the connectivity of coal seams which leads to difficulty in sweet spot indication. Engineering challenges include difficulty in hydraulic fracturing in vertical wells due to the ductile nature of the coal seams in the Qinshui basin and wellbore instability and formation damage during drilling of horizontal wells. To remedy this situation, we propose a refocus on the more abundant high permeability low-rank coals in China and detailed coal seam characterization using current industry best practices in static and dynamic modeling. This may reduce the need for hydraulic fracturing in vertical wells. For horizontal wells, research should focus on the use of non-formation damaging drilling fluids, and use of geomechanics to reduce borehole instability. In addition, a new horizontal well design that minimizes the collapse of the mother bore seems encouraging.

Acknowledgements The authors wish to thank the management of Shell International Exploration and Production for permission to publish this paper. The view presented in this paper are those of the authors and does not reflect that of Shell.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Abbreviations 2D, Two-dimensional; 3D, Three-dimensional; CBM, Coalbed Methane; Bcm, Billion Cubic Meter; CMM, Coal Mine Methane; CNPC, China National Petroleum Corporation; ECBM, Enhanced CBM Recovery; FYP, Five Year Plan; g/cc, Gram per Cubic Centimeter; GIS, Geographic Information System; GPa, Giga-pascal; m, Meter; m3/t, Cubit Meter per Ton; mD, Milli-Darcy; NEA, National Energy Administration; Pu, Porosity Unit; RMB, Ren Min Bi; Ro, Vitrinite Reflectance; Sm3, Standard Cubic Meter; Sm3/d, Standard Cubic Meter per Day; Tcm, Trillion Cubic Meter.

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