Geng Meng, 1 Sun Fenjin, 1 Li Guizhong, 1 Liu Ping, 1 Liang Li, 1 Lilindin, 2
Fund project: supported by the National Major Science and Technology Project 37 "Development of CBM Completion and Efficient Stimulation Technology and Equipment" (28ZX537).
About the author: Geng Meng, male, born in 1981, master's degree, graduated from China Shiyou University (Beijing) in 29, engaged in geological evaluation of coalbed methane. Address: (657) No.44 CBM Institute, Wanzhuang, Guangyang District, Langfang City, Hebei Province. Tel: (1)69213236,1358188333. E-mail:gengmengxi@petrochina.com.cn。
(1) Langfang Branch of China Petroleum Exploration and Development Research Institute; 2. China Petrochemical Petroleum Exploration and Development Research Institute.)
Abstract: As an important unconventional natural gas energy, coalbed methane is quite different from conventional natural gas reservoirs in terms of reservoir-forming methods, reservoir types and development means. Coal seam is not only the gas producing layer of coalbed methane, but also the gas storage layer, and coalbed methane mostly exists in coal seam in adsorption state. Therefore, the damage of fracturing fluid to coal reservoir in the process of fracturing construction of coalbed methane wells is not only reflected in the macroscopic seepage capacity damage, but also mainly reflected in the impact on the adsorption-desorption damage of coalbed methane adsorbed on the coal surface. In this paper, the influencing factors of adsorption-desorption of coalbed methane are comprehensively analyzed and evaluated, and the effects of coal composition and chemical element composition in coal on adsorption-desorption of coalbed methane are analyzed in detail. The experimental evaluation method of coalbed methane adsorption-desorption damage is determined. It is proposed that the wettability of fracturing fluid and coal seam is a measure parameter to evaluate the damage degree of fracturing fluid to coalbed methane desorption. Using this evaluation model, the influence of fracturing fluid and active water containing clay anti-swelling agent on desorption damage of coalbed methane was evaluated for two samples with different coal characteristics. The research results have theoretical guidance for the selection of fracturing fluid in the fracturing construction of coalbed methane wells.
Keywords: wetting angle damage mechanism of coalbed methane adsorption-desorption fracturing fluid
Research on the mechanism of coal bed methane desorption damages by fracturing fluid
Geng Meng 1, Sun Fenji N1, LI Guizhong1,LIU Ping1,LIANG Li1, LI Lindi2
(1.Langfang Branch, PetroChina Petroleum Exploration and Development Research Institute, Lang-fang, Hebei 657, China; 2.Sinopec Petroleum Exploration & Production Research Institute, Beijing 183, China.)
Abstract: Coal-bed methane is an important unconventional natural energy resource.Compared to convention- al gas reservoir, it has greater difference with the ways of reservoir modes and storage types and exploration meth- ods.Coal seam is the generation and storage of the gas which prefers to exist with adsorption behavior.Therefore, the damage caused by fracturing fluid during the fracture treatment not only displayed on the harm to filtration ca- pability. moreover the influence on the adsorption & desorption of the gas being on the coal surface.This article makes synthetic analysis and appraisal of the coal bed methane absorption& desorption affecting factors.It analyzes the influence of the coal component and chemical elements composition to coal·bed methane absorption-desorptio n, establishes the coal-bed methane absorption & desorption damage experimental evaluation methods, proposes that fracturing fluid and coal seam wettability are the measuring parameters for evaluating the damage degree of the frac- turing fluid to co al-bed methane desorption.It evaluates the damages of the fracturing liquid and active water con- taining clay antiswelling agent with two samples of d ifferent coal quality features.The result has theoretical guid- ance on choosing fracturing liquid during coal-bed methane fracturing operation.
Keywords: coal-bed methane; adsorption & desorption; fracturing fluid; wetting angle; Damage mecha-nism
1 Foreword
As an important unconventional natural gas resource, coalbed methane has been paid more and more attention by countries all over the world. In 21, the annual output of coalbed methane in the United States has exceeded 56 billion cubic meters, reaching half of the conventional natural gas output; China is rich in coalbed methane reserves, and the industrialization process of exploration, development and utilization of coalbed methane is also proceeding rapidly. The development technology of coalbed methane has been continuously broken through, but due to the special properties of coal reservoirs, fracturing construction has become an important means to obtain industrial airflow, and coalbed methane mostly exists in coal seams in adsorption state, which greatly increases the damage factors to coal reservoirs during fracturing construction, and the compatibility between fracturing fluid and coal reservoirs is particularly important.
2 adsorption-desorption mechanism of coalbed methane
coalbed methane mainly exists in adsorbed state, free state and water-soluble state in coal. Coal is a dual-pore structure with fracture system and matrix pores, which controls the gas storage and migration in it. Coal seam is mainly adsorbed in the pores of coal, which is affected by temperature and pressure, resulting in the change of thermal movement ability, thus realizing adsorption and desorption on the coal surface [1].
The adsorption and desorption of coalbed methane are mainly different from the following four aspects: (1) Action process. Adsorption is a spontaneous thermal evolution process of hydrocarbon generation and expulsion; Desorption is a passive artificial drainage and depressurization process. (2) Action time. The adsorption process will take a long time, and it will take millions of years. The desorption process is very short, only taking a few minutes or hours. (3) the type of action. Adsorption includes two forms: physical adsorption and chemical adsorption. Chemical adsorption is ionic bond adsorption, which requires a lot of energy, but accounts for a small proportion of adsorption gas. Physical adsorption has the characteristics of low heat energy, high speed, reversibility and non-selectivity. The desorption process is a single physical process. (4) Action conditions. Adsorption is realized by gradual dehydration and pressurization during coal evolution; Desorption is a relatively constant temperature process [2].
through the analysis of the principle of adsorption and desorption of coalbed methane, it is known that the adsorption-desorption effect of fracturing fluid on coalbed methane mainly occurs in the desorption process.
3 Analysis of influencing factors on desorption of coalbed methane
The gas adsorption capacity of coal is influenced by many factors, usually the main influencing factors are pressure, temperature, mineral content, moisture content, coal rank, lithology and gas composition [3]. In this study, the same batch of coal and rock samples in the same area were used, and the isothermal adsorption experiment was carried out by using pure methane gas indoors. In order to avoid the influence of the above conventional factors on coalbed methane desorption, we can compare the influence of various fracturing fluid formulations on coalbed methane desorption under the same standard.
the influence of fracturing fluid on desorption of coalbed methane is mainly reflected in the fact that it is different from the wettability of gas on the coal surface, which leads to the difference in promoting desorption of coalbed methane, which reduces the decrease of desorption gas caused by pore blockage, and the injection of individual fracturing fluid formulas even increases the desorption amount of coalbed methane. Wettability of fracturing fluid and coal can be measured by contact angle. The smaller the contact angle, the better the wettability and the greater the promotion of desorption of coalbed methane.
4 Influence of coal quality characteristics on wettability
4.1 Moisture
The occurrence state of water in coal seam includes external water, internal water and part of crystal water. The water content involved in this study refers to the internal water content, at which time the internal water exists in the coal sample in the form of physical adsorption; However, the crystal water in coal samples is chemically combined with minerals in coal, and its content is very small, so its influence can be ignored. As can be seen from Figure 1, the contact angle between coal sample and water decreases with the increase of air-drying moisture content of coal sample, which indicates that the easier the coal sample is wetted by water, the better the wettability of the coal sample.
fig. 1 relationship between water content and contact angle of coal sample
fig. 2 relationship between ash content and contact angle of coal sample
4.2 ash
The ash of coal refers to the residue left after all combustible materials in coal are completely burned and minerals in coal produce a series of complex reactions such as decomposition and combination at a certain temperature. All the ash in coal comes from minerals in coal, but the composition and weight of ash are not exactly the same as the mineral content in coal, which is not an inherent component in coal and is usually called ash yield. The main mineral components in coal are kaolinite, pyrite, Shi Ying and calcite.
as shown in figure 2, the greater the ash content in the coal sample, the smaller the contact angle between the coal sample and water, and the better the wettability of the coal sample.
4.3 volatiles
Although moisture and mineral content play a certain role in the wettability of coal, they are both inorganic substances, not the main components of coal, while volatiles are organic components in coal, which are related to the genesis, maceral and coalification degree of coal, so the deterioration degree of coal can be roughly judged by the yield of volatiles. Because the volatiles are mainly released by gas and water adsorbed in the pores of coal samples and functional groups at the periphery of coal samples with the increase of temperature, in which water and polar functional groups are hydrophilic, and gas and nonpolar functional groups are not hydrophilic, it is difficult to judge the wettability of coal samples by volatile yield. As shown in Figure 3, there is a poor correlation between the yield of volatile matter and the contact angle between coal sample and water, which proves that there is no obvious correlation between volatile matter and wettability of coal sample.
Figure 3 Relationship between volatile content and contact angle of coal samples
4.4 Fixed carbon
Fixed carbon, like volatile, belongs to organic components in coal, and the content of dry ash-free fixed carbon in coal samples increases with the increase of coalification degree, so some countries (or regions) take it as the classification standard of coal.
in fact, fixed carbon is not only the content of carbon in coal, but also includes oxygen, nitrogen, sulfur and other elements. Fixed carbon is not an inherent component in coal, but a product of thermal decomposition. Because coal is a macromolecular structure composed of several structural units with similar structures connected by active bridges, its core structure is aromatic nucleus, and there are certain active groups at the edge. With the increase of fixed carbon content, the degree of coalification deepens and the stability of coal molecules increases, resulting in poor wettability. It can be seen from Figure 4 that with the increase of fixed carbon content in coal, the contact angle between coal sample and water gradually increases, and the wettability becomes worse.
Figure 4 Relationship between fixed carbon content and contact angle of coal samples
5 Analysis of the influence of fracturing fluid on coalbed methane desorption
5.1 Analysis of the influence of fracturing fluid containing clay anti-swelling agent on coalbed methane desorption
From the above analysis, it can be seen that moisture, ash and fixed carbon are all related to the wettability of coal and water, and the increase of moisture and ash content will reduce the contact angle between water and coal and improve the wettability of coal; The increase of fixed carbon content will increase the contact angle between water and coal and reduce the wettability of coal. As can be seen from the industrial analysis data in Table 1, the fixed carbon content of the target coal seam is much greater than the moisture and ash content, exceeding 75%, so the wettability of the target coal seam is poor.
Table 1 Industrial analysis results of target coal seam
In this experiment, distilled water is first used as the contrast fluid, and groundwater is selected as the base fluid, and KCl with different concentrations is added for comparison. Because the composition of fracturing fluid formula is much more complicated than distilled water, the wettability of each groundwater fracturing fluid with different concentrations of KCl is quite different from that of coal seam.
as shown in table 2, the contact angle of groundwater fracturing fluid with 1%KCl added to No.3 coal seam is the smallest, while the contact angle of groundwater fracturing fluid with 2%KCl is the largest. At the same time, for the 5# target coal seam, the contact angle between the groundwater fracturing fluid added with 2% and 6%KCl and the coal seam is small, while the contact angle between the groundwater fracturing fluid added with 1% and 4%KCl and the coal seam is large. The above theory completely verifies the characteristics of coal seam desorption curve after groundwater fracturing fluid pollution with different concentrations of KCl clay anti-swelling agent.
Table 2 Comparison of contact angles between different concentrations of anti-swelling agents and 3# coal samples in Hancheng area
For example,