Figure 4-7 Types of Mercury Injection Curve in Ordos Basin (Type A)
Generally, the adsorption capacity is a function of temperature and pressure. At a fixed temperature, the adsorption capacity only changes with pressure, which is the adsorption isotherm. There are two determination methods: static method and dynamic method. The former is more accurate and the latter is more time-saving. Static method is divided into volumetric method and heavy scale method. The former calculates the adsorption amount according to the changes of gas pressure and volume before and after adsorption, while the latter directly measures the weight gain of adsorbent after adsorbing gas by using sensitive time spring or precision balance. Previous experiments have found that there are many gas adsorption isotherms with different shapes in solids, but most of them can be classified into five types (Figure 4-9).
Figure 4-8 Types of Mercury Injection Curve in Ordos Basin (Type B and Type C)
Type I: p/p0 (p0 is the saturated vapor pressure of the gas at the adsorption temperature) increases rapidly when the pressure is very low, and increases slowly when the pressure continues to increase, and finally reaches the limit adsorption. Usually, this limited adsorption capacity is only regarded as the saturated adsorption capacity of a single layer. Actually, the problem is not that simple. In fact, such isothermal curves can often be obtained as long as the adsorbent is microporous. In this case, because the size of micropores is in the same order of magnitude as the size of adsorbed molecules, the limit adsorption is the result of filling micropores with adsorbed molecules, rather than the saturated adsorption capacity of a molecular layer covering the surface. Type ⅱ-ⅴ isotherms are all the results of multi-molecular layer adsorption. If the adsorbent is non-porous and the adsorption space is not limited, the obtained isotherm is of type II or III. When p/p0→ 1, the adsorption capacity increased sharply. If the adsorbent is porous, but not microporous or at least not completely microporous, the adsorption space can accommodate multi-layer adsorption, but it cannot be infinite, so the adsorption capacity tends to a saturation value at p/p0→ 1, which is equivalent to that the pores of the adsorbent are filled with adsorption liquid, and thus the pore volume of the adsorbent can be obtained. Another difference between them and Class II or III is that at moderate p/p0, the curve rises more steeply, which is the result of capillary condensation. As for Class Ⅱ and Class Ⅲ (or Class Ⅳ and Class Ⅴ), the difference lies in whether the slope of the starting point of isotherm changes from large to small (Class Ⅱ and Class Ⅲ) or from small to large (Class Ⅲ and Class Ⅴ), which is related to whether the adsorption heat of the first layer is greater or less.
Figure 4-9 Five Isothermal Curves of Physical Adsorption
The existence and size of pores in porous solids have obvious influence on the types of physical adsorption isotherms. According to BET theory and Polanyi theory, gas molecules condense in the pores of porous solids, and the study of capillary condensation can provide us with information about the pore structure of porous solids.
Now let's study the adsorption of porous solids. It is assumed that the holes are all cylinders with an opening radius of r at one end, the size of the cylinders is within the range of the middle holes, and the liquid completely wets the hole wall. In this way, the obtained adsorption isotherm should have the shape of Figure 4- 10. The AB line in Figure 4- 10 represents adsorption at low pressure, which can be expressed by Langmuir formula or BET formula. It is generally believed that the micropores of solid in AB area will be filled by adsorbed molecules. Capillary condensation occurs when the pressure reaches b, and the relationship between r and p obeys Kelvin formula. Because of the same pore size, the isotherm rises vertically along BC at point B; In fig. 4- 10, at c, all capillary holes are filled with liquid to meniscus a. Because the contact angle between the liquid surface and the hole wall is zero, the radius of curvature at a is R. As the pressure continues to increase, the curvature of the meniscus will decrease (dotted line B in Figure 4- 10). When p = P0, the curvature of the meniscus is zero, that is, a plane. The adsorption increment of CD cross-section in Figure 4- 10 includes the condensed liquid in the hole where A changes from B to plane in Figure 4- 10, and the adsorption increment of all external surfaces when the pressure increases from P' to p0 under normal conditions. In fact, the pores of porous solids usually have a certain distribution according to their size. Therefore, the BC line in Figure 4- 10 does not rise vertically, but gradually rises, which explains the type Ⅳ isotherm that is often encountered; The situation of ⅴ isotherm is similar, but the AB region is different.
Fig. 4- 10 adsorption isotherm of porous solid with cylindrical capillary with radius r.
When testing the physical adsorption of porous solids, it is often encountered that the adsorption isotherm obtained when the gas pressure increases gradually and the desorption isotherm obtained when the pressure decreases gradually after adsorption, which is the so-called adsorption lag phenomenon. If the surface is clean and there is no interference from chemical reaction or dissolution, this hysteresis phenomenon can be repeated. Fig. 4- 1 1 shows the hysteresis of type ⅳ isotherm. As can be seen from the figure, the desorption line is always at the upper left of the adsorption line, and the ring composed of adsorption line and desorption line is usually called hysteresis loop.
For an "ink bottle" capillary with a narrow mouth and a wide belly (as shown in Figure 4- 1 1), only when the pressure reaches the widest part of the hole (Ra in Figure 4- 1 1) will it condense and the hole will be filled with condensate; However, in the desorption process, the liquid in the hole will evaporate only when the pressure is reduced to be equivalent to the meniscus evaporation at the hole neck (Rb in the figure).
Foster thinks that the hysteresis phenomenon is caused by the delay of meniscus formation during adsorption. For example, the capillary is a cylinder with two open ends, and the radius of the cylinder is R. In the adsorption process, only a cylindrical liquid surface with two open ends can be formed at first, but a meniscus spherical surface cannot be formed. But once the pressure rises to the point where condensation can occur, the whole hole will be filled with liquid, because the deeper the cylindrical liquid surface, the smaller the radius of curvature and the lower the vapor pressure.
Fig. 4- Schematic diagram of adsorption-desorption hysteresis loop of "ink bottle" shaped hole +0 1
According to De Boer, the shape of hysteresis loop can be roughly divided into five categories, each of which corresponds to a certain type of hole structure.
Type A (Figure 4- 12) has steep adsorption and desorption lines, and both occur in the range of moderate relative pressure. The most typical hole structure corresponding to this hysteresis loop is a cylindrical hole with two ends open; In Figure 4- 12, some other hole structures with Class A hysteresis loops are drawn on the right. However, only in the case of cylindrical holes and regular polygonal cylindrical holes, the relationship between the pressure pa at the steepest rise of the adsorption line on the hysteresis loop and the pressure pd on the corresponding desorption line conforms to Cohan's law, that is, (pd/p0)2 can be larger or smaller than pd/p0 when the holes are of other shapes.
Fig. 4- 12A hysteresis loop and corresponding hole structure schematic diagram.
Fig. 4- 13b schematic diagram of hysteresis loop and corresponding hole structure.
Type B adsorption line (Figure 4- 13) rises gently and becomes steep only when the pressure approaches p0. The desorption line decreases slowly, and then decreases rapidly at moderate relative pressure. The typical pore structure corresponding to this hysteresis loop is a slit capillary composed of parallel plates. Because a meniscus cannot be formed during adsorption, condensation does not occur, and capillary condensation does not begin until the pressure approaches p0. In the desorption process, because the slit is full of liquid, the liquid will evaporate from the slit only when the pressure drops to meet the effective radius of meniscus corresponding to the slit width.
The adsorption line of type C (Figure 4- 14) is steep under moderate relative pressure, while the desorption line is very gentle. The corresponding typical pore structure is conical or double bond tubular capillary. When the pressure in the adsorption process reaches the value corresponding to the pore radius, capillary condensation occurs until the liquid fills the pores to the radius equivalent to R, and the pressure continues to increase, and the curve rises gently until the pores are completely filled with liquid. In the desorption process, with the decrease of pressure, the radius gradually evaporates from the big mouth to the small mouth, and the curve always decreases gently.
Fig. 4- 14C hysteresis ring and corresponding hole structure schematic diagram
Fig. 4- 15d hysteresis loop and corresponding hole structure schematic diagram.
The adsorption line of type D (Figure 4- 15) rises gently, and only rises rapidly when the pressure is close to p0. While the desorption line always drops gently. The corresponding pore structure is a capillary formed by mutually inclined flat plates. The mechanism of forming this hysteresis loop is similar to that of class B, except that the plates here are not parallel, so there is no sudden drop on the desorption line. If the gap between the narrow sides of the hole is very small, for example, only a few molecular diameters, the meniscus will form quickly, and the result will be like a V-shaped capillary. At this point, the adsorption and desorption lines coincide and the hysteresis loop will disappear.
Fig. 4- 16E schematic diagram of hysteresis loop and corresponding hole structure.
The adsorption line of E type (Figure 4- 16) rises slowly, and the desorption line drops sharply at moderate relative pressure. Bottle hole, narrow mouth and wide capillary are typical hole structures corresponding to this hysteresis loop. In the process of adsorption, the pores are gradually filled with condensate; However, when the pressure during desorption must be reduced to a value corresponding to the radius of the small mouth, the liquid in the hole will evaporate until all the condensed liquid is evaporated, resulting in a sudden drop in the desorption line.
(2) Characteristics of nitrogen adsorption pores in coal reservoirs of Junggar Basin at low temperature.
1. pore diameter distribution characteristics of coal reservoir
Because nitrogen is a chemically inert substance, it is not easy to adsorb chemically at the temperature of liquid nitrogen. Low temperature nitrogen adsorption method is the most commonly used method to determine the specific surface area and pore size distribution. The measured pore radius is in the range of 0.3 ~ 80nm, which partially overlaps with that measured by mercury replacement method. However, due to the qualitative differences in testing principles and methods, the pore content of the overlapping part of the same sample is not comparable. According to the research of Wang Changgui et al. (1998), with the increase of maturity, the number of micropores less than 2 nm in coal increases gradually, which can be increased from 20% to 50%, the volume percentage of transition pores between 2 and 25nm decreases gradually, and the mesopores above 25 nm do not change much.
According to nitrogen adsorption data at low temperature (Table 4-20), the specific surface area of coal reservoirs in Junggar Basin is 9.734 ~ 0. 17 1m2/g, the total pore volume is 0.00098 ~ 0.0 1873ml/g, and the average pore diameter is 8.888 ~/kloc. Transition pores account for 48.93% ~ 6 1.7 1%, and mesopores account for 2.97% ~ 28. 1%, mainly transition pores and micropores.
Table 4-20 Nitrogen Adsorption Characteristics of Coal in Junggar Basin at Low Temperature
2. Pore structure characteristics of coal reservoir
Types of nitrogen adsorption isotherms at (1) low temperature
By analyzing the curve types and comparing the characteristics of low-temperature nitrogen adsorption curves of coal samples with five typical curves, it is found that the low-temperature nitrogen adsorption isotherm of coal samples in Junggar Basin (Figure 4- 17) has only ⅲ adsorption isotherm, and there are no other types. This curve is a smooth parabola, the first half of the curve rises slowly or gently, and the second half rises sharply, indicating that there are pores at all levels in coal, and the surface of the curve has undergone multi-molecular layer adsorption and capillary condensation. For this curve, when the adsorbate is adsorbed on the adsorbent, the adsorption heat of the first layer is less than the liquefaction heat of the adsorbate, which means that the energy required for desorption of molecules above the first adsorption layer is greater than that required for desorption of matrix by the first adsorption layer, so it is relatively difficult.
Figure 4- 17 Typical curve of nitrogen adsorption at low temperature for coal reservoirs in Junggar Basin
The absence of type ⅰ adsorption isotherm indicates that the pore surface of coal is heterogeneous and the pore types are complex and diverse. There are no adsorption isotherms of Class Ⅱ, Ⅳ and Ⅴ, because the upper radius of pore size measured by liquid nitrogen adsorption method is only 85 nm, and there are still many pores in coal larger than this pore size range, which have not reached saturated adsorption, so the second half of adsorption isotherms have not tended to be flat.
According to the analysis of nitrogen adsorption curve of coal samples in Junggar Basin at low temperature (Figure 4- 17), the curve is a tubular pore-like model with two open ends, and the desorption line lags behind when the relative pressure is high, that is, the adsorption amount during desorption is greater than that during adsorption at the same relative pressure, indicating that the larger pores are open pores. Until the relative pressure is extremely low and close to zero, the desorption curves tend to overlap, indicating that the opening degree of smaller pores is also better. The analysis of the characteristics of the adsorption ring of the measured coal samples shows that some pores are tubular or plate-shaped capillary pores with good openings at both ends and large pore size range, and some are relatively uniform parallel plate-shaped pores. The development of open pores in this area is beneficial to the communication of pores in coal and the flow of coalbed methane. This pore structure facilitates the storage and migration of hydrocarbons, and is beneficial to the storage and exploitation of coalbed methane.
(2) Adsorption ring and pore structure
Pore structure types can directly reflect the difficulty of oil and gas accumulation and migration in pores. Among the above five common adsorption isotherms, except the first one, the other four adsorption isotherms often have the phenomenon of separation of adsorption branches and desorption branches, forming adsorption rings. Delport (1958) summarized five types of adsorption rings, A, B, C, D and E * * *, each of which reflected a certain type of pore structure. The five types of adsorption rings summarized by De Boer are typical idealized pore structure patterns, that is, these pores have relatively uniform shapes and sizes. If the shape and size of pores have a certain distribution law, they often present atypical rings, which are the superposition of several typical rings. In fact, the types of pore structure in coal are complex and diverse, so the pore structure reflected by adsorption ring is a comprehensive reflection of all levels of coal pore structure.
Comparing the adsorption rings (i.e. "desorption curve" or "desorption curve") of coal samples measured in this area with five typical adsorption rings in Delport (1958), it is found that the types of pore structures in coal are complex and diverse, and the coal reservoirs in Junggar Basin should be a variant of Class A curve, that is, there are separation of adsorption branches and desorption branches in the whole process, and both branches are steep and medium. It reflects that some pores are tubular or plate-shaped capillary pores with good openings at both ends and large pore size range, and some pores are relatively uniform parallel plate-shaped pores. This pore structure is beneficial to the storage and migration of oil and gas and to the storage and exploitation of coalbed methane.
Comparing the adsorption isotherms of two coal samples in the basin, it is found that the separation degree of desorption branch and adsorption branch of Changji Sulphur Ditch coal sample is smaller than that of Fukang Sangonghe coal sample, indicating that the small pore opening degree of Changji Sulphur Ditch coal sample is smaller than that of Fukang Sangonghe coal sample. In areas with moderate relative pressure, the situation is just the opposite, that is, Changji Sulphur Ditch coal sample is larger than Fukang Sangonghe coal sample, and the desorption of Changji Sulphur Ditch coal sample is steeper than Fukang Sangonghe coal sample when it is close to saturation pressure, which reflects that Changji Sulphur Ditch coal sample has a large opening.
(3) Characteristics of nitrogen adsorption pores at low temperature in coal reservoirs in Tarim Basin.
The experimental results of nitrogen adsorption at low temperature show that the BET specific surface area and BJH specific surface area of Jurassic coal seam in Yang Xia mining area in the northern margin of Tarim basin are 4.248 m2/g and 4.63 1 m2/g, indicating that the content of small pores in this coal seam is high. The BET specific surface area and BJH specific surface area of Jurassic coal seam in Hotan Ya Bu mining area in the southern margin of the basin are 0.806 m2/g and 0.873 m2/g, respectively, indicating that the content of small pores in this coal seam is low (Table 4-2 1).
Table 4-2 1 Experimental Results of Nitrogen Adsorption at Low Temperature for Coal Reservoirs in Tarim Basin
The statistical analysis of the experimental results of nitrogen adsorption at low temperature shows that the pore volume ratio of Jurassic coal seams in Yang Xia mining area is 1 1. 1%, the area ratio is 0.68%, and the pore volume ratio of 100 ~ > 100 nm is 40.5%. The corresponding pore volume ratios of Hotan Ya Bu mining area are 265,438+0.3%, 60.65,438+0% and 8.6% respectively, and the corresponding area ratios are 2.8%, 365,438+0.9% and 60.7% respectively (Table 4-22).
Table 4-22 Pore Characteristics of Coal Reservoir in Tarim Basin
The test results show that there are basically three different types of isothermal curves of liquid nitrogen adsorption for coal samples in Tarim Basin. One is represented by A 'ai Coal Mine in the northern margin of Tarim Basin, 1 coal seam in Yang Xia coal-producing area, and Ya Bu Coal Mine in Hotan, Tarnum. The first half of this curve rises gently and convexly, and the second half rises sharply, indicating that capillary condensation has taken place. The adsorption line is close to the desorption line, reflecting a capillary-like pore structure with a large size change at one end, and its pore size is 5 ~ 10 nm. The second type is represented by No.2 well in Yang Xia coal-producing area and Ohobulake coal mine on the northern edge of the tower, with a pore size of 5 ~ 100 nm. This curve rises gently, and the adsorption isotherm does not coincide with the desorption isotherm obtained when the pressure gradually decreases after adsorption, and it does not coincide until the relative pressure is low, which is the so-called adsorption lag phenomenon. The third type, represented by Ohobulake Coal Mine in the northern edge of the tower, shows that the adsorption curve rises gently, the desorption curve does not coincide with the adsorption curve, and there are many steps, which indicates that there are "ink bottle" pores with various pore sizes, which is not conducive to coalbed methane desorption.
(4) Pore characteristics of coal reservoirs in Turpan-Hami Basin by nitrogen adsorption at low temperature.
When a solid comes into contact with a gas, the molecules of the gas will constantly hit the surface of the solid, and some of them will immediately bounce back to the gas phase, while others will stay on the surface of the solid for a period of time and then return to the gas phase, resulting in adsorption. The retention of molecules on the solid surface is caused by the attractive force between the solid surface and the adsorbed molecules. This kind of gravity can be roughly divided into two categories. One is that molecules are adsorbed on the solid surface by van der Waals force, which is called "physical adsorption"; The other is that molecules, atoms and atomic groups are adsorbed on the solid surface through chemical bonds, which is called "chemical adsorption".
The determination of pore distribution of coal by low temperature nitrogen adsorption method is based on the fact that methane adsorption by coal belongs to physical adsorption process. Using nitrogen as adsorbent, the adsorption capacity of nitrogen under different pressures was measured under the condition of equilibrium temperature, and the distribution of different pore diameters could be obtained through drawing and calculation. Adsorption isotherm can be used to judge the pore distribution and pore characteristics of coal. According to a large number of experimental results, Brunauer and DeMIng divided the isothermal curves of gas adsorption into five categories, indicating that there are not only single molecule adsorption but also multi-layer adsorption in coal pores.
1. Pore characteristics
According to the nitrogen adsorption data at low temperature (Table 4-23), the BET specific surface area of coal reservoir is 0. 126 ~ 16.72m2/g, the total pore volume of BJH is 0.00066 ~ 0.0 1847ml/g, and the average pore diameter is 3.627 ~.
Table 4-23 Statistical Table of Low Temperature Nitrogen Adsorption Data of Coal Reservoir in Turpan-Hami Basin
2. Types of nitrogen adsorption isotherms at low temperature
The test results show that there are basically two isothermal curves for liquid nitrogen adsorption of coal in Turpan-Hami basin. One is represented by Aiweiergou Mine (Figure 4- 18). The first half of this curve rises gently and convexly, and the second half rises sharply, indicating that capillary condensation has taken place. The adsorption line and desorption line almost coincide, reflecting the capillary pore structure type with one end almost closed and large size change, and its pore size is 5 ~ 10 nm (Figure 4- 19). The other is Sandaoling Mine (Figure 4-20, Figure 4-2 1), with a pore size of 10 ~ 100 nm (Figure 4-20, Figure 4-2 1). This curve rises gently, and the adsorption isotherm does not coincide with the desorption isotherm obtained when the pressure decreases gradually after adsorption. Krae mer and mcBain believe that this is an "ink bottle" phenomenon, that is, only when the pressure reaches the widest part of the hole to condense, the hole will be filled with condensate; However, in the process of desorption, the pressure must be reduced to the point where the hole neck is in meniscus evaporation, and the liquid in the hole will evaporate. Low metamorphic coal in Turpan-Hami basin has high micro-pores, which may be due to the existence of many unfilled cavities in coal, which communicate with other pores and fractures through cell wall rupture or smaller pores, thus forming a large number of "ink bottle" pores, which are beneficial to the storage of coalbed methane, but not conducive to its desorption.
Fig. 4- 18 isothermal curve of nitrogen adsorption at low temperature in aiweiergou coal mine
Fig. 4- 19 micropore distribution map of aiweiergou coal mine
Figure 4-20 Nitrogen Adsorption Isothermal Curve of Sandaoling Coal at Low Temperature
Fig. 4-2 1 sandaoling coal mine coal micropore distribution map
(5) Characteristics of low temperature nitrogen adsorption pores of coal reservoirs in Yili Basin.
The characteristics of low-temperature nitrogen adsorption-desorption curve of coal show that the pores of coal in Yili basin are divided into closed pores and open pores. The desorption curves at higher and lower relative pressures lag, indicating that both larger and smaller pores are open. The desorption curve at higher relative pressure lags behind, indicating that the larger pores are open pores; The adsorption and desorption curves coincide at low relative pressure, indicating that the smaller pores are closed pores. The development of open pores is beneficial to the communication of pores in coal and the migration of coalbed methane. The experimental results also show that the Jurassic coal seams in Yili basin have a total specific surface area of 0.668 ~ 1. 175m2/g, a total pore volume of 0.00472 ~ 0.00793ml/g and an average pore diameter of 8.409 ~1.665. 66.82% ~ 7 1.33% of micropores are in the range of 100 ~ 10 nm, and 5.78% ~ 6.9 1% of micropores are smaller than 10 nm. It shows that the coal seam porosity in this area is absolutely dominant (Table 4-24).
Table 4-24 Low Temperature Nitrogen Adsorption Characteristics of Coal Reservoirs in Yili Basin
The isothermal adsorption experiment of coal shows that the saturated adsorption capacity (VL) of raw coal in Yili Basin is 1.34 m3/t, and the saturated adsorption capacity of combustible materials is 1.57 m3/t, which indicates that the gas storage capacity of coal reservoirs in this area is weak, which is extremely unfavorable to the development of coalbed methane. Langmuir pressure (PL) 1.79 MPa (Table 4-25). Under isothermal conditions, adsorption capacity is positively correlated with reservoir pressure. With the increase of pressure, the adsorption capacity increases. In the range of 0 ~ 4 MPa, the adsorption capacity increases approximately linearly with the increase of pressure, and then the growth rate gradually decreases until the adsorption capacity reaches saturation.
Table 4-25 Isothermal Adsorption Characteristics of Coal Reservoir in Yili Basin
(6) Characteristics of nitrogen adsorption pores in coal reservoirs of Qaidam Basin at low temperature.
According to the experimental results of nitrogen adsorption at low temperature, the specific surface area of Jurassic coal reservoirs in Qaidam Basin and Qilian Mountains is 0.0676~25.0 1 m2/g/g, and the specific surface area of coal samples in You Xiang mining area in the northern margin of Qaidam Basin is the highest, reaching 25.05438+0m2/g; The specific surface area of coal samples in Dameigou mine in the northern margin of Qaidam, Datong mining area and Moeller mining area in Qilian area is more than 2.099 m2/g; The specific surface areas of coal samples in Lvcaoshan mining area, Wanggaxiu mining area, Muli mining area and Haider mining area in Qilian area are all below 0.828 m2/g. ..
The total pore volume of BJH is 0.00042 ~ 0.0364 ml/g; The average pore size is 5.367 ~ 12.99 nm, in which micropores account for 0. 14% ~ 70. 17%, transition pores account for 26.62% ~ 76.33%, and mesopores account for 3.22% ~ 51.3. The experimental results of nitrogen adsorption at low temperature are shown in Table 4-26.
Table 4-26 Experimental Results of Nitrogen Adsorption in Coal Reservoirs in Qaidam Basin and Qilian Area at Low Temperature
Statistical analysis of the experimental results of nitrogen adsorption at low temperature shows that the pore volume ratios of Jurassic coal seams in You Xiang mining area in the northern margin of Qaidam Basin are 3.22%, 26.62% and 70. 17, respectively, and the pore size is greater than 100 nm, between 100 nm and < 100nm. The corresponding pore volume ratios in Dameigou mining area are 6.22% ~ 18.30%, 3 1.84% ~ 73. 19% and 8.5 1% ~ 6 1.94%, respectively, corresponding to. The pore volume ratios of Jurassic coal seams in DaOuyang mining area are 33.39%, 55.04% and 1 1.57% respectively, and the corresponding area ratios are 7.36%, 37.87% and 54.76% respectively. The pore volume ratios of Jurassic coal seams in Wanggaxiu mining area are 14.77%, 36.00% and 49.23% respectively, and the corresponding area ratios are 0.99%, 8.89% and 90. 1 1% respectively.
The pore volume ratios of Jurassic coal reservoirs in Muli mining area, a coal-bearing area in Qilian Mountains, are 565,438+0.365,438+0%, 48.55% and 0.65,438+0.04%, and the corresponding area ratios are 27.33%, 765,438+0.65,438+0.04 respectively. The pore volume ratios of Jurassic coal seams in Datong mining area are11.67% ~16.87%, 52.69% ~ 76.33% and 12.00% ~ 30.44%, respectively, and the corresponding area ratios are/kloc. The corresponding pore volume ratios in Haider mining area are 19. 13%, 57.38% and 23.49%, and the corresponding area ratios are 2. 18%, 28.30% and 69.52%, respectively. The corresponding pore volume ratios in Moeller mining area are 13.85%, 67.54% and 18.6 1%, and the corresponding area ratios are 1.80%, 37.34% and 60.85%, respectively (Table 4-27).
Table 4-27 Pore Characteristics of Coal Reservoirs in Qaidam Basin and Qilian Area
The experimental results show that there are basically three kinds of isothermal curves of liquid nitrogen adsorption in coal samples in Qaidam basin and Qilian area. The first category is represented by Lvcaoshan Coal Mine in the northern margin of Qaidam. The first half of this curve rises gently and convexly, and the second half rises sharply, indicating that capillary condensation has taken place. The adsorption line is close to the desorption line, reflecting a capillary-like pore structure with a large size change at one end, and its pore size is 5 ~ 10 nm. The second type is represented by Dameigou Coal Mine in the northern margin of Qaidam Basin, with a pore size of 5 ~ 100 nm. This curve rises gently, and the adsorption isotherm does not coincide with the desorption isotherm obtained when the pressure gradually decreases after adsorption, and it does not coincide until the relative pressure is low, which is the so-called "adsorption lag phenomenon". In the third type, represented by You Xiang coal mine in the northern margin of Qaidam Basin, the adsorption curve rises gently, the desorption curve does not coincide with the adsorption curve, and there are many steps, which shows that there are "ink bottle" pores with various pore sizes, which is not conducive to coalbed methane desorption.
(7) Pore characteristics of coal reservoirs in Ordos Basin adsorbed by nitrogen at low temperature.
1. Pore characteristics
Physical adsorption depends on van der Waals force which is ubiquitous between molecules. Therefore, the adsorption capacity mainly depends on the size of the surface area, rather than the special properties of the surface (such as chemical properties), and its adsorption capacity changes with the gas pressure.
According to the data of nitrogen adsorption at low temperature (Table 4-28), the specific surface area of coal reservoirs in Ordos Basin is 0.208 ~ 12.85 m2/g, in which the specific surface area of Carboniferous-Permian coal reservoirs is relatively small, such as the specific surface areas of coal seams of Shanxi Formation in Tongchuan Taoyuan and Wubao Wuyi Coal Mine are 0.208 m2/g and 2.748 m2/g respectively. The specific surface area of Jurassic coal seams in Yulin and Shenbei areas of Shaanxi Province is relatively high, for example, the specific surface area of Tiaogou coal in Yulin is 12.85m2/g ... while the specific surface area of low metamorphic coal and Rujigou high metamorphic coal in Dongsheng coalfield of Inner Mongolia is relatively low. The total pore volume of BJH is 0.0007 ~ 0.0 154 ml/g, the average pore diameter is 4.75 ~ 14.78 nm, the micropores account for 4.4 1% ~ 95.8%, and the transition pores account for 4.73% ~ 79./kloc-0.
Table 4-28 Low Temperature Nitrogen Test Results of Coal in Ordos Basin
Table 4-29 List of Pore Diameter Distribution of Low Temperature Nitrogen Test Coal in Ordos Basin
Figure 4-22 Pore Distribution of Coal Low Temperature Nitrogen Test in Ordos Basin
The adsorption capacity of solids to gases is determined by many factors, besides the types of solids and gases themselves, it also depends on the pressure and temperature of gases and the specific surface area of solids (referring to the total surface area of 1g solids), which in turn depends on the subdivision state of solids, especially the pore structure of solids. Unlike liquid, atoms on the solid surface cannot move freely, so the solid surface is always uneven. The surface area of a solid is a relative property related to the determination method. Measuring the specific surface area of a solid by gas adsorption is to measure the surface area of a solid with the adsorbed molecules as a ruler. Obviously, the results of this measurement will also change with the size and shape of the selected adsorption molecules. In fact, no matter what method is used, the result must be relative. On the basis of studying the pore structure of activated carbon (Figure 4-23), the pores are divided into micropores (pore radius less than 10 nm), micropores (pore radius between 10 ~ 100 nm) and mesopores (pore radius greater than 100nm). The experimental results show that most of the surface mainly comes from micropores. Due to the difference of coal rank and coal composition, the specific surface area and pore structure of coal reservoirs will be very different.
According to the distribution curve of specific surface area, the types of specific surface area curve of coal reservoirs in Ordos Basin can be divided into four categories. The first type, the specific surface area comes from micropores and pores, and the contribution is equivalent (Figure 4-24a). Second, the specific surface area mainly comes from small holes (Figure 4-24b); Third, the specific surface area mainly comes from micropores, and the mesopores have a certain contribution (Figure 4-24c); In the fourth category, the specific surface area is basically contributed by micropores with a diameter of 3 nm (Figure 4-24d). Among the tested samples, the distribution curve of specific surface area belongs to the fourth category, accounting for the majority, mainly middle and low rank coal reservoirs, such as Wuyi Mine in Fugu, Shaanxi Province, Huashi Mine and Caragana Tower Mine in Xinmin Mining Area of Shenbei, and Tiaogou Coal Mine in Yuheng Mining Area. The first type is mainly distributed in medium-high metamorphic coal reservoirs, such as Rujigou Coal Mine in Ningxia and Tongchuan Mining Area in Shaanxi. The second type is distributed in low rank coal reservoirs, such as Dongsheng coalfield in Inner Mongolia; The third kind is the high vitrinite coal reservoir with middle and low rank, such as HK80 sample in Huating Coal Mine, Gansu Province.
Figure 4-23 Micropore Volume Distribution of Activated Carbon
Figure 4-24 Specific Surface Curve Types of Coal Reservoirs in Ordos Basin
2. Types of nitrogen adsorption isotherms at low temperature
There are three types of isothermal adsorption curves of low temperature nitrogen in coal reservoirs in Ordos Basin.
The first type belongs to type B, represented by the low-rank Jurassic middle-low rank coal reservoir in Dongsheng coalfield, Inner Mongolia. Its adsorption line rises gently and becomes steep only when the pressure approaches p0 (Figure 4-25c). The desorption line decreases gently, and then decreases rapidly at moderate relative pressure. The pore structure is a slit capillary composed of parallel plates. The measurement results of micro-cracks also show that micro-cracks are very developed in coal reservoirs in this area.
The second type is Type D, represented by the coal rank of Wuyi Mine in Fugu Mining Area, Shaanxi Province (Figure 4-25b). The adsorption line rises slowly and rises rapidly only when the pressure is close to p0. However, the desorption line always falls gently and coincides with the adsorption line. This kind of coal reservoirs mostly develop non-parallel fractures with one end pointed out.
Figure 4-25 Isothermal Adsorption Curve Types of Low Temperature Nitrogen for Coal Reservoirs in Ordos Basin
The third type is E type, represented by Ningtiaota Coal Mine in Xinmin Mining Area, Shenbei, Shaanxi Province (Figure 4-25a and Figure 4-24d). Its adsorption line rises slowly, and its desorption line drops sharply under moderate relative pressure. It belongs to the development type of "ink bottle" cave.