(China Geo University (Beijing) School of Energy Beijing 100083)
Studying the unique single-phase water flow stage in the development process of under-saturated coalbed methane reservoir is helpful to deepen the understanding of the early drainage characteristics of this kind of coalbed methane reservoir and its indication for the potential productivity of gas wells. Taking the unsaturated coalbed methane reservoir in Qinnan coalbed methane field as an example, this paper focuses on the characteristics of single-phase drainage gas production in the early stage of this kind of gas reservoir development, reveals its relationship with gas well productivity in the later stage, and analyzes its significance in predicting the potential productivity of gas wells. The results show that the single-phase drainage characteristics of gas wells in southern Qin are greatly affected by faults, and the drainage time is exponentially related to cumulative production. Gas wells with drainage time of 50 ~ 140 d and cumulative water production less than 500 m3 show good gas production capacity.
Keywords: Indicative significance of production characteristics of single-phase drainage in unsaturated coalbed methane reservoir
Fund Project: National Major Science and Technology Project for the Development of Large Oil and Gas Fields and Coalbed Methane (201kloc-0/zx05034-001); National key basic research development planning project (973) (2009cb219600); Special funds for basic scientific research business expenses of central universities (2011py0211)
Author's brief introduction: Lv Yumin, male, from Ji 'an, Jiangxi, 1985, Ph.D. candidate, now engaged in the research of coalbed methane geology and development. Address: Energy College of China Geo University (Beijing), No.29 Xueyuan Road, Haidian District, Beijing. Tel: 010 82322011. Email: Yale1210 @163. com
Characteristics and indication of single-phase water flow in early development of coalbed methane —— Taking southern Qinshui Basin as an example
Lv Yumin, Tang Dazhen, Xu Hao, Tao Shu, Zhang Biao
(School of Energy, China Geo University, Beijing, 100083)
Abstract: Studying the unique characteristics of single-phase water flow in the development of under-saturated reservoirs is helpful to obtain early pumping characteristics and predict gas well productivity. Taking the unsaturated coalbed methane reservoir in the south of Qinshui Basin as an example, this paper focuses on the characteristics of single-phase pumping in the early stage of the development of unsaturated coalbed methane reservoir, reveals the relationship between single-phase pumping performance and gas well productivity, and analyzes its directive function to the potential productivity of gas wells. The results show that the flow characteristics of single-phase water in the southern Qinshui basin are mainly controlled by faults, and the single-phase pumping time is exponentially related to the cumulative water production. In addition, wells with pumping time of 50 ~ 140 days and cumulative water production less than 500 m3 show good gas production performance.
Key words: undersaturated coalbed methane reservoir; Single phase water; Pumping characteristics; indicate
As an important unconventional natural gas reservoir, coalbed methane reservoir has attracted more and more attention from domestic scholars. In recent years, a large number of domestic scholars have studied the reservoir physical properties (Chen et al., 2007), hydrogeological conditions (et al., 2001; Wang Bo et al., 2007), Boundary and Closure Mechanism (Su Xianbo et al., 2005; Song Yan et al., 2009) and reservoir-forming evolution (Song Yan et al., 2009; Zhao Qun et al., 2007; Zhao et al., 2005) and other aspects have carried out a lot of research work and achieved certain results. However, compared with foreign countries, the basic research on coalbed methane reservoirs in China started late, and the geological environment and formation mechanism of coal reservoirs, the adsorption characteristics and description model of coal under high temperature and high pressure, and the geological control factors of coal adsorption performance need to be strengthened and deepened (Song Yan et al., 2005). At present, there is a lack of systematic understanding of coalbed methane reservoir development in China, especially the lack of understanding of single-phase drainage characteristics in the initial stage of unsaturated coalbed methane reservoir development and its relationship with gas well productivity, which restricts the rational development and deployment of gas fields. Because of the obvious stress sensitivity of coal reservoir, unreasonable single-phase drainage measures in the early stage of unsaturated coalbed methane reservoir development will greatly damage the absolute permeability of the reservoir, reduce the potential gas production capacity of gas wells, and even affect the later development deployment and development effect of the whole coalbed methane field.
Water production characteristics of 1 undersaturated coalbed methane reservoir
Strong adsorption capacity is one of the remarkable characteristics of coal reservoirs, and the gas adsorbed in coal seams can generally reach more than 80% (Su Xianbo et al., 1999). This special occurrence mechanism, which is different from conventional natural gas reservoirs, determines the uniqueness of coalbed methane production mechanism. Coalbed methane mining is a process of drainage → depressurization → desorption → diffusion → seepage → mining (Feng, 2009). In this process, the gas water production mechanism of coalbed methane reservoir is influenced by its gas saturation, that is to say, the gas water production curve of coalbed methane well varies with the gas saturation of coalbed methane reservoir (Su Xianbo et al., 200 1).
Gas production characteristics of 1. 1 supersaturated/saturated coalbed methane reservoir
Supersaturated coalbed methane reservoir refers to coalbed methane reservoir with gas saturation greater than 100%, which is characterized in that part of coalbed methane exists in the pore fracture system of coal reservoir in a free state. When the gas well is drained and depressurized, the coalbed methane rapidly desorbs and diffuses, and is produced together with the free coalbed methane (Figure 1a). Therefore, when developing this kind of gas reservoir, coalbed methane will be produced immediately after the gas well is drained, and will basically enter the gas-water two-phase flow stage without going through the unsaturated single-phase water flow stage (as shown in Figure 1, Phase III).
Figure 1 water production characteristic curve of coalbed methane reservoirs with different gas saturation
Saturated coalbed methane reservoir refers to coalbed methane reservoir with gas saturation equal to 100%. When the gas well begins to drain and depressurize, the coalbed methane will desorb and diffuse immediately. With the progress of desorption and diffusion, the saturation of free gas in pores and fractures of coal seam gradually increases, and it is not until it is greater than the saturation of residual gas that gas wells begin to produce coalbed methane (figure 1b). Therefore, in order to develop this kind of coalbed methane reservoir, gas wells have to go through a short unsaturated single-phase water flow stage before producing coalbed methane (as shown in the second phase of 1).
1.2 Gas production characteristics of unsaturated coalbed methane reservoir
Unsaturated coalbed methane reservoir refers to coalbed methane reservoir with gas saturation below 100%. When the gas well is drained and depressurized, the coalbed methane has not been desorbed until the reservoir pressure is lower than the critical desorption pressure. At this time, the gas well has not yet produced coalbed methane. Only when the saturation of free gas in pores and fractures of coal seam is greater than that of residual gas, gas wells will begin to produce coalbed methane (Figure 1c). Therefore, in order to develop this kind of coalbed methane reservoir, gas wells have experienced saturated single-phase water flow and unsaturated single-phase water flow successively (Phase I and Phase II in Figure 1), and then began to produce coalbed methane.
It takes a long time to produce gas by single-phase drainage in the initial stage of unsaturated coalbed methane reservoir development, ranging from 1 ~ 2 months to several years. The gas-water drainage characteristics formed in the long-term single-phase drainage period are an important basis for understanding the reservoir characteristics of gas reservoirs and studying the potential productivity of gas wells.
2 Production Characteristics of Single-phase Drainage in Unsaturated Coalbed Methane Reservoir at the Initial Stage of Development
There are two main quantitative parameters to characterize the characteristics of single-phase drainage in the initial stage of unsaturated coalbed methane reservoir development: single-phase drainage time and single-phase water cumulative production.
2. 1 single-phase drainage time
Single-phase drainage time refers to the time when the unsaturated coalbed methane reservoir is developed, and the coalbed methane well only produces water in the early stage and does not produce gas. Long-term single-phase drainage time will inevitably increase the development cost of coalbed methane wells. Therefore, the single-phase drainage time directly affects the development cost of gas field and is an important parameter to evaluate the development economy of coalbed methane field.
2.2 Cumulative output of single-phase water
The cumulative output of single-phase water refers to the cumulative total output of groundwater in the early stage of coalbed methane wells when unsaturated coalbed methane reservoirs are developed. Because most of the produced water has the characteristics of high salt content and high salinity, it does not meet the national discharge standards, and it must be treated before it can be discharged, so as not to pollute the surface water system and groundwater (Pan et al.,1998; Wang Zhichao et al., 2009). The treatment of produced water undoubtedly increases the development cost of coalbed methane, so the cumulative output of single-phase water affects the development cost of gas field and is an important parameter to evaluate the development economy of coalbed methane field.
2.3 The relationship between single-phase drainage time and cumulative output of single-phase water
The drainage time of single-phase water and the cumulative production of single-phase water in the initial stage of unsaturated coalbed methane reservoir development are influenced by many similar factors such as geology, engineering and human factors, and there must be a certain relationship between them. From the diagram of the relationship between single-phase drainage time and single-phase water cumulative production of coalbed methane wells in Qinnan coalbed methane field (Figure 2), it can be seen that there is a strong linear relationship between single-phase drainage time and single-phase water cumulative production when it is less than 250d; When the drainage time of single-phase water is longer than 250d, its correlation with the cumulative output of single-phase water is poor and exponential. Generally speaking, there is an exponential relationship between them:
y= 144.37exp(0.0069x)
Where: x is the single-phase drainage time, d; Y is the cumulative output of single-phase water, m3. The R2 value of the fitting function reaches 0.8323, which shows that the function can well describe the relationship between the drainage time of single-phase water and the cumulative output of single-phase water in this area.
2.4 Influencing factors and production characteristics of single-phase drainage
There are many factors affecting the drainage characteristics of single-phase water, including gas reservoir proportion, drainage speed, tectonic geological conditions and hydrogeological conditions.
Under the same drainage rate, the higher the gas saturation of coalbed methane reservoir, the greater the critical desorption pressure, which means that the pressure drop required for gas production in gas wells is smaller, so the shorter the drainage time of single-phase water, the smaller the cumulative water production.
Fig. 2 Relationship between single-phase drainage time and cumulative production of single-phase water.
Under the condition of similar temporary storage ratio, the faster the drainage rate of coalbed methane wells, the faster the reservoir depressurization, the shorter the time to realize gas production (that is, the shorter the drainage time of single-phase water), and the smaller the cumulative water production, such as J7 and J 1 with gas saturation of about 82.8% in Table1.
Table 1 Relationship between single-phase drainage characteristics and faults
Tectonic and hydrogeological conditions have great influence on single-phase drainage and production characteristics. Different reservoir permeability, water content and groundwater activity in different structural parts and different hydrogeological conditions lead to different single-phase drainage and production characteristics of gas wells. There are many normal faults in coalbed methane fields in southern Qin (2005). The hydrogeological conditions near these faults are complex, which is not conducive to drainage and pressure reduction. Single-phase drainage takes a long time and the cumulative output is large (table 1).
3 Relationship between single-phase drainage and production characteristics and gas well productivity
For stress-sensitive coal reservoirs, unreasonable single-phase drainage measures (too fast or too slow drainage) in the initial stage of unsaturated coalbed methane reservoir development will inevitably damage the reservoir permeability and reduce the drainage and exhaust capacity of gas wells in the later stage. Studying and discussing the relationship between single-phase drainage characteristic parameters and gas well productivity can provide guidance for formulating reasonable single-phase drainage scheme, predicting coalbed methane well productivity in advance and taking necessary measures to improve reservoir production.
At present, the coalbed methane field in Qinnan is in the initial stage of development, and most coalbed methane wells have short drainage time. The gas saturation of coal reservoirs in Fan Zhuang, panzhuang and Block in this area is generally 80% ~ 90%, which belongs to undersaturated coalbed methane reservoirs (Yao et al., 2009). In order to scientifically evaluate the relationship between single-phase drainage characteristics and gas well productivity, the average gas production and maximum gas production formed after continuous drainage 1 year are selected as gas well productivity indicators.
3. 1 Relationship between single-phase drainage time and gas well productivity
Too fast drainage of gas wells and too short drainage time of single-phase water will often cause irreversible stress damage to reservoirs, reduce permeability and affect productivity; At the same time, the single-phase drainage time is too long, and the reservoir water is large (or connected with aquifer), which is not conducive to the formation of good productivity of gas wells.
Figure 3 shows the relationship between single-phase drainage time and gas production in southern Qin. It can be clearly seen from the figure that there are four obvious characteristics between single-phase drainage time of gas wells and 1 annual gas production: 1) The average gas production of coalbed methane wells with single-phase drainage time greater than 140d is basically less than 3,000m3/d, and the maximum gas production is less than 6,000m3/d; 2) The average gas production of coalbed methane wells with single-phase drainage time less than 50 days is basically less than 3000m3/d, and the maximum gas production is less than 6000m3/d; 3) High-yield coalbed methane wells (average gas production is greater than 3000m3/d and maximum gas production is greater than 6000m3/d), and the single-phase drainage time is between 50 ~ 140 d; 4) The productivity of some CBM wells with single-phase drainage time of 50 ~ 140 d is low. This shows that too long/too short single-phase drainage time is not conducive to the formation of high production of coalbed methane wells.
Fig. 3 Relationship between single-phase drainage time and gas well productivity
In Qinnan area, where the gas saturation of coal reservoirs is similar and groundwater is generally inactive, the single-phase drainage time of some wells is long, which means that the wells are connected with active water layers, which makes it difficult for gas wells to depressurize and limit gas production. The short single-phase drainage time indicates that the drainage speed of gas well is too fast, and the reservoir permeability is irreversibly damaged to varying degrees, which is not conducive to gas production. Therefore, coalbed methane wells with single-phase drainage time longer than 140d or less than 50d in southern Qin indicate that their productivity is generally low; However, the coalbed methane wells between 50 ~ 140 d are more conducive to the formation of higher productivity.
3.2 Relationship between cumulative production of single-phase water and gas well productivity
The cumulative output of single-phase water often reflects the regional hydrogeological characteristics. Under the same hydrogeological background, some gas wells discharge a large amount of single-phase water for a long time, which may indicate that the reservoir is connected with the aquifer, which is not conducive to drainage and production, and it is difficult to form good productivity.
Figure 4 shows the relationship between single-phase water cumulative production and natural gas production in southern Qin. It can be clearly seen from the figure that there are three obvious characteristics between the single-phase drainage time of gas wells and the gas production within 1 year: 1) The average gas production of coalbed methane wells with cumulative single-phase water production greater than 500m3 is basically less than 2000 m3/d, and the maximum gas production is less than 4000m3/d; 2) High-capacity coalbed methane wells (average gas production is more than 2000 m3/d, maximum gas production is more than 4000 m3/d, and cumulative single-phase water production is less than 500m33) have lower productivity.
Fig. 4 Relationship between cumulative production of single-phase water and gas well productivity
According to the table 1, most of the coalbed methane wells with high single-phase production in southern Qin are located near normal faults. In the process of coalbed methane accumulation, the normal fault is used as the escape channel of coalbed methane most of the time, which leads to poor preservation conditions of coalbed methane near the normal fault and low gas saturation of coal reservoir, which increases the drainage time and cumulative water production in the single-phase drainage stage. At the same time, the normal fault is connected with the nearby aquifer, which makes it difficult to depressurize the single-phase water for a long time, prolong the drainage time and increase the water production of gas wells. Therefore, the cumulative single-phase water production of coalbed methane wells in southern Qin area is above 500m3, indicating that its productivity is generally low. The CBM wells smaller than 500m3 are more conducive to improving productivity.
4 conclusion
(1) The gas water production characteristics of supersaturated, saturated and undersaturated coalbed methane reservoirs are different, and the gas water production characteristics of undersaturated coalbed methane reservoir are the most typical. The most remarkable feature of gas water production in unsaturated coalbed methane reservoir is that there is a long-term single-phase drainage stage in its initial development stage.
(2) Single-phase drainage time and cumulative single-phase water production are two important parameters to describe the characteristics of single-phase drainage in the initial stage of unsaturated coalbed methane reservoir development. The characteristics of single-phase drainage are greatly influenced by faults. The single-phase drainage time of gas wells in coalbed methane fields in southern Qin is exponentially related to the cumulative production of single-phase water.
(3) The coalbed methane wells with good productivity in the coalbed methane field in southern Qin have a single-phase drainage time of 50 ~ 140 d, and the coalbed methane wells with a cumulative single-phase water production of less than 500m3, a single-phase drainage time of more than 140d or a cumulative single-phase water production of more than 500m3 have generally low productivity.
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