(Langfang Branch of China Petroleum Exploration and Development Research Institute, Langfang, Hebei 065007)
Long-term exposure of low-rank coal and rock to the air leads to the oxidation and over-drying of organic matter and minerals, which leads to the deviation of gas content and adsorption capacity from the true value and affects the geological evaluation and calculation of coalbed methane resources. In order to quantitatively study the effects of oxidation and overdrying on the gas-bearing and adsorption characteristics of low-rank coal, four samples of low-rank fresh coal from a well in Turpan-Hami Basin were exposed to air for 1 day, 7 days, 15 days and 30 days respectively. The analysis shows that the longer the drying time of (1) low-rank coal samples, the greater the water loss, and the equilibrium water content can not be restored to the initial state, which leads to the larger calculation results of in-situ base gas content and in-situ base density and overestimates the resources. (2) Excessive drying and oxidation lead to the redistribution of the original pore structure of coal and rock, and the ratio of mesopores to macropores is large, which reduces the adsorption capacity of coal and rock, but at the same time, the reduction of moisture enhances the adsorption capacity of coal and rock. The comprehensive action of the two makes the adsorption capacity of coal and rock larger as a whole, which leads to the underestimation of adsorption saturation and depressurization desorption prospect. Based on this, two suggestions are put forward: (1) For each process of coring and testing in low coal rank exploration, strict standards for sample treatment, preparation and storage should be formulated to resolutely prevent coal and rock from oxidation and over-drying; (2) When evaluating low-order geology and calculating coalbed methane resources, we should fully understand the whole process of coal and rock testing, distinguish the authenticity and reliability of data, and carefully select values.
Key words: low-rank coal and rock test, geological evaluation of oxidation and overdrying, calculation of coalbed methane resources.
Fund Project: The National Science and Technology Major Project No.33, Study on Coalbed Methane Enrichment Law and Prediction and Evaluation of Favorable Blocks (Project No.201kloc-0/zx05033) is a sub-project No.05, Suggestions on Coalbed Methane Evaluation and Exploration Deployment in China (Project No.201zx05033-003).
About the author: Deng Ze, born in 1982, male, engineer, from Yuncheng, Shanxi Province, obtained a master's degree in geological engineering from China Shiyou University in 2008, mainly engaged in geological evaluation and experimental research of coalbed methane. E-mail: Deng Ze @ PetroChina. com。 cn。 Tel: (0 10) 692 13353.
Uncertainty of low rank coal testing and resource evaluation —— A case study of Turpan-Hami Basin
Deng Zesun Fenjin Chen Gengping Zeng Liu Ping
(Petroleum Exploration and Development Research Institute of China Petroleum Langfang Branch, Lanfang, Hebei, 065007)
Low rank coal is more sensitive to drying and oxidation, which may affect the accuracy of approximate, isothermal adsorption, surface area test and other test results, resulting in a serious misunderstanding of coalbed methane resources. In order to quantitatively study this effect, four kinds of low rank coals from Turpan-Hami basin in China were air-dried for 24 hours, 7 days, 65 05 days and 30 days respectively. The results show that (1) when the moisture loss of low rank coal exceeds the minimum value (equilibrium moisture), it is impossible to return to the initial value, which leads to the increase of in-situ gas content and in-situ density, the decrease of coal surface area and the increase of uncertainty in industrial analysis and elemental analysis. (b) In a long time without protection procedures, resources will be overestimated, and desorption capacity will be underestimated due to higher adsorption capacity and lower saturation estimation. These results remind us that strict coal protection procedures should be adopted in sampling, preparation and preservation of low-rank coal, and data should be carefully selected in resource evaluation.
Keywords: low rank; Drying; Oxidation; Resource evaluation
1 preface
At present, the development of coalbed methane in China has entered the stage of small-scale commercial production from the experimental stage, but most of the exploration and development work is concentrated in the middle and high rank areas in Qinshui Basin and the eastern edge of Ordos, and the exploration and development in the low rank areas is still in its infancy. However, the characteristics of thick coal seam, good permeability, large resources and the occurrence of conventional oil and gas and coalbed methane in low-rank areas, coupled with the successful exploration and development experience of foreign low-rank basins such as Fenhe Basin in the United States and surat Basin in Australia, make the development prospect of low-rank coalbed methane broader, which is expected to become the focus of coalbed methane exploration and development in China in the next step [1 ~ 4]. Correctly understanding the characteristics of low rank coal reservoirs and coalbed methane resources is the premise and basis for the breakthrough of low rank coal. Because the low-rank coal and rock have the characteristics of low thermal evolution, high water content and easy oxidation, the conventional testing process may lead to the oxidation and over-drying of organic matter and minerals, which will lead to the deviation of gas content and adsorption capacity from the true value and affect the geological evaluation and calculation of coalbed methane resources [5]. At present, people are generally aware of the particularity of low-rank coal and rock testing, but there are few reports about the uncertainty of test results. Therefore, it is urgent to quantitatively study and evaluate the effects of oxidation and overdrying on the gas-bearing capacity and adsorption capacity of low-rank coals, and establish strict experimental testing procedures and specifications on this basis to improve the accuracy of low-rank coals testing.
2 experiment
2. 1 experimental scheme
The experimental sample is taken from the desorption sample of a well in Shaerhu area of Turpan-Hami basin, and the coal rank is lignite (see table 1). After the natural desorption is completed, the sample is divided into 5 parts, of which 1 part is used as the reference sample (sample number is CS3A-0H), and dried in the laboratory inert gas dryer until no free water can be observed on the surface by naked eyes; The other four samples were air-dried in the laboratory environment for 1 day, 7 days, 15 days and 30 days respectively (sample numbers are CS3A-24h, CS3A-7d, CS3A- 15d, CS3A-30d), and then the coal and rock were tested and analyzed. By comparing the test results, the influence degree of oxidation and overdrying on key parameters such as gas content, density, pore structure, calorific value and adsorption capacity is quantitatively analyzed, which provides a basis for establishing a more perfect low-rank coal and rock test process and system.
2.2 experimental results
(1) equilibrium humidity Em (maximum internal humidity)
Equilibrium moisture Em (maximum internal moisture) refers to the internal moisture maintained by the coal sample when it is in equilibrium with the ambient atmosphere at a temperature of 30℃ and a relative humidity of 96%, and it is the key to transform the test data results under other benchmarks into in-situ benchmarks [6,7]. The high porosity and water content of low rank coal and the irreversibility of water loss and rebalancing determine the sensitivity and instability of equilibrium water test. As shown in figure 1, the equilibrium moisture Em is negatively correlated with the drying time. Compared with the reference sample, with the extension of continuous air drying time, the Em test result of equilibrium moisture is gradually lower than the theoretical value, and the change rate, that is, the experimental test error, is 5.07% ~ 18.82%.
Table 1 CS3A-0h Detailed parameters
* ad: air-dried base; Daf: dry ash-free base; Mmmf: water-bearing mineral-free base; In situ: In situ group.
Figure 1 Effects of oxidation and overdrying on maximum internal humidity
(2) Gas content
Gas content data is one of the core data of coalbed methane test, and the determination of coalbed methane gas content is obtained by drilling coring, rope coring coal sample or logging coal chip test. Testing methods can be divided into two categories: direct method and indirect method. In this experiment, USBM direct method proposed by US Bureau of Mines is adopted. The uncertainty of the influence of oxidation and overdrying on gas content is mainly reflected in the change of coal sample quality under the combined action of water evaporation and saturated maximum internal water. As can be seen from Figure 2, the longer the air-drying time, the greater the air content of air-drying base and in-situ base. The change rate of in-situ gas content is 1.36% ~ 5.03%, which shows a trend of fast at first and then slow, indicating that the gas content changes greatly in the initial stage of air drying, and the error increases rapidly. Reasonable control of the initial stage of air drying can effectively improve the data accuracy.
(3) True apparent density
According to different determination methods, density can be divided into apparent density and true density. When measuring apparent density, volume includes internal capillary and fissure volume, while true density is the opposite. Apparent density is widely used in practical work, for example, it must be used when calculating coal and coalbed methane reserves. The formula (1) can be used to convert the air-dried basic density into the field basic density. As shown in fig. 3,
Effects of oxidation and overdrying on the contents of different reference gases.
Technical progress of coalbed methane in China: 20 1 1 Proceedings of the symposium on coalbed methane.
Where ρ in-situ is ρ in-situ basic density, g/cm3; ρad is the basic density of air drying, g/cm3; Mad is air-dried moisture; Em is the equilibrium moisture.
As can be seen from Figure 3, the longer the air-drying time, the lower the apparent density of air-drying due to the continuous evaporation of water, while the higher the apparent density of in-situ air-drying, which is mainly caused by the increasing difference between the air-drying base water and the equilibrium water Em. The apparent density change rate of in-situ foundation is 1.70% ~ 4.23%, which shows a logarithmic growth trend.
Fig. 3 Effect of oxidation and overdrying on density
(4) In situ resources
The volume method is used to calculate the in-situ resources, and the formula is as follows:
Technical progress of coalbed methane in China: 20 1 1 Proceedings of the symposium on coalbed methane.
In which: GIP is in-situ resource,108m3; A is the area, km2h is the thickness of coal seam, m; Is the average apparent density, g/cm3; Is the average gas content, m3/t.
It should be noted that each parameter of Formula (2) should be selected or converted into test data under the same benchmark conditions, such as air-dried basis, dry ash-free basis or in-situ basis. In order to facilitate comparison, field data are selected to participate in the calculation. As can be seen from Figure 4, dry oxidation has a great influence on the calculation of coalbed methane resources in low coal rank, and the change rate of resources shows a logarithmic growth trend. Assuming that the block area is 1000km2, the thickness of coal seam is 20m, and the density and gas content are as above, the calculated variation range of resources is 3.65,438+0% ~ 9.5%, which is the combination of uncertainty of density and gas content.
Fig. 4 Effects of oxidation and overdrying on resources.
(5) Adsorption characteristics
The isothermal adsorption line of coal and rock is the basis for evaluating adsorption capacity, adsorption saturation and critical desorption pressure. It is usually characterized by Langmuir model, and its main influencing factors include coal quality characteristics, maceral, pore structure, temperature and pressure. The overdrying and oxidation of low rank coal and rock discussed in this paper mainly affect the adsorption characteristics of coal and rock by changing the equilibrium moisture content and pore structure. As shown in fig. 5, the longer the peroxide oxidation time, the greater the adsorption capacity, and the change rate is 12% ~ 72%. The corresponding adsorption saturation decreases, and the change rate is 7% ~ 22%; The critical desorption pressure decreases, and the change rate is 1 1% ~ 22%.
Fig. 5 Effects of oxidation and overdrying on adsorption capacity
3 discussion
3. 1 overdrying is the main factor affecting the measurement of low-rank coal and rock, followed by oxidation.
Low-rank coal and rock exposed to air for a long time will be oxidized and overdried, which makes the test results have a large error. Among them, overdrying is the main influencing factor, and oxidation only has a great influence on the results of elemental analysis. Low-rank coal is dominated by mesopores and macropores, and a large number of micropores can adsorb and condense high moisture. The longer the air drying time, the greater the evaporation of water, and the greater the deviation between the measured value of equilibrium water and the theoretical value, which leads to the larger test results of air content and density of in-situ foundation and the optimistic calculation results of air content.
Low-rank coal and rock easily react with oxygen in the air. A large number of chemical experiments have confirmed that the non-aromatic structure of coal molecules is first destroyed in the process of coal contacting with oxygen. Non-aromatic structures mainly include bridge bonds and side chains, in addition to cycloalkanes and heterocycles. According to the theory of organic chemistry, the non-aromatic structure of coal molecules is analyzed, which shows that cycloalkanes and heterocyclic compounds have stable chemical properties and are not easy to react with oxygen in the air at room temperature and pressure. Compared with bridge bonds, side chains are more easily oxidized than side chains, because bridge bonds are greatly influenced by aromatic rings and other groups or structures. It is inferred that oxidation has a great influence on the maceral and calorific value of low-rank coal, but the influence of oxidation as a single factor is not involved in this study and needs further study in the future.
3.2 The change of equilibrium moisture is the main factor affecting its adsorption capacity, followed by pore structure.
It is generally believed that the adsorption capacity of coal and rock is inversely proportional to the equilibrium moisture content and directly proportional to the specific surface area without considering other factors. In the process of drying and oxidation, the synergistic effect of water loss in pores and oxidation reaction changes the pore structure and morphology of coal and rock, and makes the specific surface area decrease continuously.
Table 2 Specific surface test results of comparative samples
Fig. 6 Effect of oxidation and overdrying on pore size distribution
4 conclusion
Overdrying and oxidation have great influence on the gas-bearing property and adsorption capacity of low-rank coal, mainly in the following aspects:
(1) Some alkyl side chains, bridge bonds and oxygen-containing functional groups with non-aromatic structure in coal surface molecules are easy to generate oxidation heat with oxygen in the air, which destroys the original structure of coal molecules and changes the coal quality characteristics and pore characteristics of coal.
(2) The longer the drying time of low-rank coal samples, the greater the water loss, which leads to the greater the calculation results of in-situ base gas content and in-situ base density, and the overestimated resources.
(3) Excessive drying and oxidation lead to the redistribution of the original pore structure of coal and rock, and the ratio of mesopores to macropores is large, which reduces the adsorption capacity of coal and rock, but at the same time, the reduction of moisture enhances the adsorption capacity of coal and rock. The comprehensive action of the two makes the adsorption capacity of coal and rock larger as a whole, which leads to the underestimation of adsorption saturation and depressurization desorption prospect.
In order to improve the accuracy of low-rank coal and rock testing, it is suggested that:
(1) For each process of coring and testing in low coal rank exploration, strict sample handling, preparation and storage specifications should be formulated to resolutely prevent coal and rock from oxidation and over-drying.
(2) When evaluating low-order geology and calculating coalbed methane resources, we should fully understand the whole process of coal and rock testing, distinguish the authenticity and reliability of data, and carefully select values.
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