Production characteristics and the key factors in high-rank coalbed methane fields: A case study on the Fanzhuang Block, Southern Qinshui Basin, China

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Abstract

The temporal and spatial production characteristics of coalbed methane (CBM) wells in the Fanzhuang Block of the Southern Qinshui Basin, the first commercial CBM-producing basin in China, were studied to determine the dominant factors. The study indicates that gas production differs significantly in different wells adjacent to or located in different tectonic areas. In combination with the production characteristics in the temporal and spatial variations, the impact of seven factors (burial depth, thickness of coal, gas content, porosity/permeability, effect of fracturing, structural setting, and hydrogeological conditions) on the gas production was analyzed based on bivariate correlation analysis and gray system theory, which are suitable for solving the complex interrelationships between multiple factors and variables. The results indicate that hydraulic fracturing is an effective guarantee of high-production CBM wells in the first several months during a high-rank CBM field development. However, the effect of hydraulic fracturing on enhancing the gas production of CBM wells decreases with time because the initial reservoir permeability, lower than the hydraulic fracture permeability, limits the adsorption, diffusion and flow of CBM. This limitation inevitably leads to a slow gas production rate when the adsorbed CBM from the initial reservoir becomes the main gas source instead of the fracturing-affected zone. Gas content and permeability are two key factors that dominate CBM well productivity, because they perform elementary roles in controlling the volume of gas sources and conductivity, respectively. The structural setting and hydrogeological condition are also two important factors that dominate CBM well productivity because they can influence the spatial distribution of permeability and gas content as well as other factors (such as aquifers).

Highlights

► CBM wells production performance in the Southern Qinshui Basin was introduced. ► GS theory and statistics were used to study the factors affecting gas production. ► Hydraulic fracturing determines the initial production performance of CBM wells. ► Gas content and permeability determine the long-run performance of CBM fields.

Introduction

At present, China is both the largest consumer and producer of coal in the world (Dai et al., 2012) and much attention has been focused on the origin, distribution, and production of coalbed methane (CBM). As China has gradually expanded its investment in the development of coalbed methane fields in recent years, the number of drilling and producing wells has increased markedly. From 1980 to 2004, only 287 CBM wells had been drilled (Sun, 2005). However, the number of drilled CBM wells by August 2010 had grown to 4657, among of which more than 3700 were producing wells (Sun et al., 2010). Due to the increase in the number of producing wells, China has gained a significant breakthrough in CBM production. In 2005, the annual CBM production from wells drilled from surface reached 1.5 × 108 m3 (Cai, 2008). By 2010, the annual production had grown to more than 15 × 108–20 × 108 m3 (Gao et al., 2010). Nonetheless, the goal that China's annual CBM production was expected to be 100 × 108–110 × 108 m3 by 2015 (Lin and Fu, 2004) will not be achieved without a substantial increase in CBM production in the Qinshui Basin, the largest CBM producing basin in China (Meng et al., 2011, Qian et al., 2009). The problems causing the lag in the development of CBM in the Qinshui Basin need to be analyzed and overcome for CBM reservoir to reach its true potential in production.

The gas and water production performance of the CBM wells in the Southern Qinshui Basin varies significantly from one well to another. However, the CBM production has been affected by a lack of knowledge about the temporal and spatial variations of well productivity and its various factors (depth, thickness, gas content, porosity/permeability, effect of fracturing, structural setting and hydrogeological conditions), which limits reservoir engineers to make practical decisions for the aim of increasing gas production. Therefore, understanding the temporal and spatial variations of production patterns in areas where gas and water production are influenced by structural features will further facilitate the effective management of CBM fields (Pashin and Groshong, 1998).

The goal of this study is to evaluate the impact of various factors on the temporal and spatial variations of productivity and to determine which factor has the greatest effect. The temporal and spatial production characteristics of numerous wells in the Southern Qinshui Basin, as well as the key factors regarding the characteristics of the temporal and spatial variations in CBM well production, are discussed based on correlation scatter diagrams, bivariate correlation analysis, and the gray correlation theory (Cao and Hu, 1993, Deng, 1985, Huang, 2007, Lian et al., 2007, Liu and Xiao, 2010).

Section snippets

Overview of the Qinshui Basin

The Qinshui Basin covers an area of 23.5 × 103 km2 and is bounded to the south by the Zhongtiaoshan Uplift, to the east by the Taihangshan Uplift, to the north by the Wutaishan Uplift, and to the west, the basin is separated from the Linfen Basin and the Lvliangshan Uplift by the Huoshan Uplift (Fig. 1; Cai et al., 2011, Liu et al., 2010, Ye, 2009, Zhang, 2004). The long axis of the basin is more than 330 km long and is generally aligned northeast-southwest and (Zhang, 2004). The basin is a large

Productivity characteristics of CBM wells in the Southern Qinshui Basin

The Fanzhuang Block, operated by PetroChina, is one of the major commercial CBM producing blocks in the Southern Qinshui Basin. By the end of 2010, more than 500 CBM wells had been drilled in this block, over 90% of which were vertical producing wells.

Gas and water rates as well as their production trends in CBM wells differ significantly in the different production stages. The productivity characteristics of CBM wells that initially produced at different times are also markedly varied (

Effects of factors on gas production in the Southern Qinshui Basin

Coalbed methane productivity is dominated by the complex interplay among six key controls: coal distribution, coal rank, gas content, permeability, ground-water flow, and depositional and structural setting (Kaiser et al., 1994). In this study, seven factors including burial depth and thickness of the coal, gas content, porosity and permeability, fracturing effect, structure and hydrogeological conditions, were analyzed to reveal which factor has the greatest effect on gas production in the

Gray correlation analysis and the effect of key factors on CBM well productivity

As mentioned above, the relationships between CBM well productivity and depth, thickness, gas content, porosity, and the effect of hydraulic fracturing have been analyzed. However, the predominant factors affecting CBM well productivity are yet to be determined. To determine the primary factors, gray correlation analysis was introduced to quantify them and to evaluate the influence of the magnitude of various factors on CBM well productivity.

Conclusions

Since 2004, the CBM field in the Southern Qinshui Basin has been highly productive and has become the first commercially developed CBM area in China as well as the first commercial development of high-rank CBM fields in the world. The current production performance shows that the CBM well productivity in this field varies significantly and is controlled by various factors (burial depth, thickness, gas content, porosity/permeability, hydraulic fracturing, structural setting, and hydrogeological

Acknowledgments

This work was supported by the National Basic Research Program of China (“973” Project) (Grant No. 2009CB219600), the Major National Science and Technology Special Projects (Grant Nos. 2011ZX05034-001, 2011ZX05038-001), the Fundamental Research Funds for the Central Universities (2011PY0211). We appreciate the constructive comments and suggestions from the reviewers and the editors. We are also grateful to Yuanyuan Kang, Jinli Song and Jakob Johansson for improving the manuscript.

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