(Langfang Branch of China Petroleum Exploration and Development Research Institute, Langfang, Hebei 065007)
The following problems are found in the field test of coal seam gas content: ① When slow desorption method is used to measure the gas content of low-rank coal seam, the small residual gas content may lead to the failure of conventional methods to obtain results or large errors; (2) When the gas content of coalbed methane is determined by fast desorption method, the method of testing residual gas by crushing coal samples may cause a small amount of coalbed methane loss, and the result of residual gas is low. Therefore, it is necessary to establish a numerical calculation method for residual gas prediction, strengthen the comparison between measured and numerical results, and improve the accuracy and reliability of gas content testing. Refer to Langmuir formula describing adsorption proces, taking desorption amount corresponding to adsorption amount and desorption time corresponding to adsorption pressure, and combine with experimental analysis data, a new numerical calculation method for predicting residual gas is proposed. Compared with the measured data, it is considered that this method has high accuracy and good stability, and can accurately obtain the residual gas results under the condition of low gas content, which effectively improves the efficiency of on-site gas content testing.
Keywords: calculation method of residual gas content in coal seam Langmuir curve fitting method
Fund project: Evaluation and optimization of favorable coalbed methane blocks in China (No.:2008ZX05033-005), a sub-project of the national major science and technology project "Large-scale oil and gas fields and coalbed methane development" 33 "Study on enrichment law of coalbed methane and prediction and evaluation of favorable blocks".
About the author: Liu Pingtong, born in 1957, female, senior engineer, mainly engaged in experimental research on coalbed methane. E-mail: liuping69 @ petrochi-na.com.cn. Tel: (0 10) 692 13353.
A new numerical calculation method for predicting residual gas
Sun Fen Jin Chen Deng Zegeng Meng Zenghe
(Langfang Branch of China Petroleum Exploration and Development Research Institute, Langfang, Hebei 065007)
Abstract: The following problems were found in the field test of coal seam gas content: (1) When measuring coal seam gas content by slow desorption method, a small amount of residual gas may lead to the failure of the conventional method or large deviation; (2) When fast desorption method is used to measure the coal seam gas content, the residual gas measured by broken coal samples will lead to the loss of a small amount of coal seam gas, resulting in a low residual gas result. Therefore, a numerical calculation method for predicting residual gas should be established to strengthen the comparison between measured results and numerical calculation results and improve the accuracy and reliability of gas content testing. Based on Langmuir-Lamb equation describing desorption process, a new numerical calculation method for predicting residual gas is proposed by comparing desorption amount with adsorption amount, desorption time with adsorption pressure and combining with experimental analysis data. By comparing with the measured data, it is concluded that this method has high accuracy and good stability, and can obtain the residual gas results with low gas content more accurately, thus improving the working efficiency of on-site gas content testing.
Keywords: coalbed methane content; Residual gas; Calculation method; Langmuir curve fitting method
introduce
The content of coalbed methane is one of the key parameters to characterize the characteristics of coalbed methane reservoir, and accurate acquisition of coalbed methane content is of great significance to the exploration and development of coalbed methane resources and the prevention and control of coal mine gas disasters. During the test, the gas content in coal seam is divided into three parts: lost gas quantity, desorbed gas quantity and residual gas quantity. The loss gas volume is calculated by numerical regression, and the desorption gas volume and residual gas volume are obtained by actual desorption test (Qian Kai et al., 1996, Wu Xiyan et al., 2005). In general, residual gas can be obtained by selecting and desorbing residual samples and crushing them, but in special cases, direct testing can not meet the requirements of residual gas testing. In view of the above problems, this paper will discuss the causes of this special situation in detail, and put forward a new method of residual gas prediction based on Langmuir formula for the first time.
1 Special problems in residual gas testing
Domestic scholars have done a lot of research on the measurement and calculation methods of coal seam gas content. Zhou and Xu Chengfa (1995, 2002 and 2005) found that the desorption characteristic curve of coal samples in the whole process was asymmetric S-shaped, and they thought that the gas desorption amount in the initial stage of desorption was linear with the square root of desorption time, which needed to be corrected. Zhang Qun et al. (2009) found that the measured simulated gas loss is much higher than the gas loss estimated by the direct method of the US Bureau of Mines. Deng Ze et al. (2008) proposed a curve fitting method to calculate the lost gas volume by analyzing the effects of desorption temperature and loss time in the experiment. Gao (1999), Fu et al. (1999), Dong Hong et al. (200 1), et al. (20 10), according to the logging physical response of gas-bearing coal seam, the physical characteristics of gas-bearing coal rock and the time difference of density, gamma and acoustic wave are analyzed. Influenced by factors such as coal rank, ash content and coal sample particle size, the higher the coal rank and ash content, the higher the residual gas content. Liu Honglin et al. (2000) pointed out that parameters such as coal rank, ash content, temperature, maceral type, cleavage development degree and coal sample particle size are important factors affecting adsorption time and determining the proportion of residual gas. Previous studies mainly focused on the simulation calculation of lost gas, the direct or indirect prediction of total gas content and the analysis of influencing factors of residual gas specific gravity, but did not discuss the calculation method of residual gas in detail.
At present, the commonly used gas content testing methods are slow desorption method and fast desorption method. There are some problems in field operation and residual gas testing, which are mainly manifested as follows: (1) When measuring the gas content in low rank coal seam with slow desorption method, the conventional method may not be able to directly measure the residual gas because the gas content is generally small and the residual gas is low, or the measured value may increase the error if it is too low; (2) When the gas content of coalbed methane is determined by fast desorption method, a small amount of coalbed methane may be lost due to artificial termination of natural desorption and residual gas testing of broken coal samples, resulting in the residual gas test result being smaller than the actual value and the total gas content being smaller. On the other hand, because there are few desorption records, it is impossible to correctly reflect the desorption law of coal and rock, and it is impossible to obtain key parameters such as adsorption time and diffusion capacity. In view of the above problems, this paper puts forward a new numerical calculation method of residual gas, that is, Langmuir curve fitting method, in an attempt to discuss residual gas from the perspective of numerical calculation and solve the existing problems.
The influencing factors of residual gas specific gravity and Langmuir curve fitting method are put forward.
2. Characteristics of1coalbed methane desorption curve
Figure 1 shows the desorption curves of high-order and low-order samples. As can be seen from the figure, the desorption gas volume of the two samples increases with time, showing a curve shape of first steep and then slow. The starting point of desorption record is the moment when the coal sample is sealed to the desorption tank. At this time, because the desorption pressure is atmospheric pressure (far below the critical pressure), the coalbed methane adsorbed on the surface of large and medium pores first desorbs through favorable paths, resulting in a steep curve in the initial stage of desorption. However, after the adsorption time (63.2%), with the conventional desorption test, the gas concentration in the coal matrix gradually decreased, and the driving force of diffusion, namely the concentration gradient, also decreased. More and more gases can't overcome the diffusion resistance caused by micropores and can't be desorbed from coal (Zhou, 2002), and the curve gradually tends to be flat. At this time, the desorbed coalbed methane is mainly the gas remaining on the surface of micropores in coal matrix.
Fig. 65438 +0 desorption curve of high (a) and low (b) coal rank
2.2 Analysis of Influencing Factors of Residual Gas Specific Gravity
The specific gravity of residual gas refers to the percentage of residual gas in the total gas content. The influencing factors mainly include coal rank, coal sample particle size and ash content. Different coal ranks have different rock gap structures. Low-rank coal is dominated by large and medium pores, which is beneficial to desorption and diffusion. At the same time, the proportion of micropores is small, and the ability to retain residual gas is limited, that is, the specific gravity of residual gas is small. On the other hand, the micropores of high rank coal are developed, and the gas needs to overcome large diffusion resistance, which makes relatively more coalbed methane residue at the end of natural desorption; China coal is in between. The particle size of coal sample has certain influence on desorption rate. Generally speaking, the desorption rate of pulverized coal, coal chips (drill cuttings) and coal cores (lump samples) decreases in turn, the adsorption time increases, and the residual gas retention increases (Xu Chengfa et al., 2005). The more broken the coal sample is, the shorter the desorption distance is, and the lower the diffusion resistance is, so that some gases that cannot be desorbed in columnar and massive coal samples are desorbed. Therefore, the smaller the particle size of coal sample, the smaller the specific gravity of residual gas. In addition, with the increase of ash content in coal, the residual gas content gradually increases, and there is a good positive correlation between them. Through the study of coal petrology and scanning electron microscope, it is preliminarily considered that this is because fine minerals such as clay minerals in coal are filled in the pores of coal, which hinders the migration channel of gas to varying degrees, weakens the diffusion and migration ability of gas in coal, and is not conducive to gas desorption from coal. In addition, the composition of coal and rock, test temperature and so on also have some influence on the specific gravity of residual gas.
2.3 Langmuir curve fitting method
Langmuir formula is based on the dynamic equilibrium principle of vaporization and condensation. Its equation is simple and practical, and has been widely used in the adsorption of gas by coal and other adsorbents. At the same time, according to the assumption of dynamic equilibrium, the equation can also describe the desorption process of coalbed methane. The adsorption and desorption of coalbed methane are generally considered as a reversible process, but it is worth studying whether Langmuir formula suitable for coalbed methane adsorption can describe the shape of desorption curve well. Therefore, based on Langmuir formula, a new method for predicting residual gas content is proposed through parameter meaning transformation, and whether it is suitable for desorption process is judged through fitting test.
The standard Langmuir formula is
Technical progress of coalbed methane in China: 20 1 1 Proceedings of the symposium on coalbed methane.
Where: V is the adsorption capacity, m3/t; P is adsorption pressure, MPaVL is Langmuir volume, that is, theoretical maximum adsorption capacity, m3/t; PL is Langmuir pressure, that is, when the volume reaches 0.5VL, the corresponding adsorption pressure, MPa. It can be seen that the adsorption capacity increases with the increase of pressure. When the pressure is close to infinity, the adsorption capacity is infinitely close to the maximum adsorption capacity, and the desorption capacity also increases with the increase of desorption time. When the desorption time approaches infinity, the desorption gas volume also approaches the maximum and tends to be stable, showing a curve similar to the adsorption curve. Therefore, the letter meaning of Langmuir formula changes, and the desorption amount corresponds to the adsorption amount, and the desorption time corresponds to the adsorption pressure, that is, according to the adsorption and desorption curves.
Technical progress of coalbed methane in China: 20 1 1 Proceedings of the symposium on coalbed methane.
Where: g is the measured desorption amount, m3/t; T is the measured desorption time, h; GL is the limit solution inspiratory capacity, m3/t; TL is the desorption time measured when the desorption amount reaches 0.5GL, h, and the formula (2) is converted to obtain
Technical progress of coalbed methane in China: 20 1 1 Proceedings of the symposium on coalbed methane.
According to the measured desorption data, the corresponding diagram of T/G and t is obtained by referring to formula (3), and the limit desorption gas content GL is obtained by fitting. Since GL is the sum of the measured desorption gas amount Q2 and the residual gas amount Q3, the residual gas amount can be obtained by the following formula.
Technical progress of coalbed methane in China: 20 1 1 Proceedings of the symposium on coalbed methane.
3 field application
The calculation of residual gas by Langmuir curve fitting method is mainly based on on-site desorption data, and the reliability of the results is mainly limited by desorption time. As shown in Figure 2, the longer the desorption time, the smoother the desorption curve and the more reliable the predicted value.
In the test of a coalbed methane well in Turpan-Hami basin, it is found that a large number of low-rank coal samples have the problem that the residual gas is too low to be measured directly or the error is too large. Taking sample A as an example, the Langmuir curve fitting method proposed in this paper is used to calculate the residual gas in low rank coal seam, and satisfactory results are obtained. As shown in Figure 3, the predicted limit desorption gas volume is 1.26m3/t, and the residual gas volume is 0.02m3/t according to Q2= 1.24m3/t measured by desorption, and the correlation coefficient is above 0.99. At the same time, the specific gravity distribution map of residual gas in this area (Figure 4) is obtained, and the specific gravity of residual gas is 0. 10% ~ 4.35%, with an average of 0.94%.
In view of the fact that the measurement error of residual gas may increase under the condition of rapid desorption, Langmuir curve fitting method is used to fit and analyze the desorption data of a well 10 samples within 48 hours, and the residual gas value is obtained. It can be seen from Table 1 and Figure 5 that the predicted values are generally higher than the measured values, with an average of 16%. It shows that the improper regulation of entering the residual gas testing stage after 48 hours in the on-site fast desorption method leads to the loss of some coalbed methane during this period, which has a certain impact on the lost gas Q 1 and even the total gas content. It is suggested to extend the desorption time until the desorption curve is flat or the daily increase of desorption amount does not exceed 10%. In addition, the second sampling will also affect the accuracy of residual gas test. It is suggested to sample evenly on site as far as possible, repeat the test at least twice, and take the average value of two groups of similar data as the final residual gas.
Fig. 2 measured desorption curve of sample a
Fig. 3 fitting curve of sample a
Fig. 4 Specific gravity distribution of residual gas
Table 1 Well Sample Measurement Results
sequential
Fig. 5 Comparison of Residual Gas Results
4 conclusion
(1) Aiming at the main problems existing in residual gas testing, according to the reversibility law of coalbed methane adsorption and desorption process, a residual gas prediction method similar to Langmuir formula is proposed for the first time. Verified by the field measured data, this method has high fitting degree and certain reliability.
(2) Under the condition of rapid desorption, the measured value of residual gas is generally low, so it is suggested to extend the desorption time until the desorption curve is flat or the daily desorption amount does not exceed 10%, and keep uniform sampling, repeat the test at least twice, and take the average value of two groups of similar data as the final residual gas value.
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