A method for determining the critical physical properties of natural gas injection in tight reservoirs within overpressure zones

Abstract

A method for determining the critical physical properties of natural gas injection into tight reservoirs within overpressure zones. First, convert the mercury injection displacement pressure value of the reservoir into the gas-water displacement pressure value during the gas drive water process under geological temperature and pressure conditions, and establish the gas-water displacement pressure distribution profile during the gas drive injection process of the tight reservoir. Then, determine the natural gas accumulation time and the paleo-burial depth of the corresponding strata. Next, select sandstone samples rich in gas inclusions under a polarizing microscope to determine the paleo-pressure and residual pressure of the strata during the natural gas accumulation period. Then, determine the paleo-pressure structure of the strata. In a single overpressure system, construct a trend line of the residual pressure change gradient, that is, the effective injection power trend line. Finally, superimpose the effective injection power trend line on the converted gas-water displacement pressure distribution profile to divide the distribution intervals of effective injection and ineffective injection reservoir samples, and determine the critical physical property limit value of natural gas injection into tight reservoirs within overpressure zones. The present invention realizes the quantitative evaluation of the effectiveness of natural gas injection into deep overpressure tight reservoirs.

Description

Technical Field

[0001] The present invention relates to the technical field of geological evaluation for natural gas exploration and development in deep overpressure environments, and particularly relates to a method for determining the critical physical properties of natural gas injection into tight reservoirs within overpressure zones. Background Art

[0002] As a green energy source, natural gas is playing an increasingly prominent role in the economic society. The exploration and development of deep tight sandstone gas is the key to increasing natural gas production and reserves in China. Deep gas reservoirs are generally buried deeper than 3500 m, and the geological conditions are complex, such as tight reservoirs (air permeability K ≤ 1.0 mD), well-developed overpressure (pressure coefficient m ≥ 1.2), etc., which greatly increase the exploration and development risks. In the tight reservoirs of overpressure zones, whether natural gas can be effectively injected, that is, determining the effective injection physical property limit of natural gas (natural gas can be injected and form reservoirs only when it is higher than the critical value, otherwise it cannot), is the core content of geological evaluation of deep tight gas reservoirs. Accurately defining the critical physical property conditions for natural gas injection into tight reservoirs in overpressure environments is of great practical significance for evaluating the effective reserve scale of tight gas reservoirs, reducing the exploration and development risks of overpressure gas reservoirs, and improving economic benefits.

[0003] Currently, the commonly used research methods in this field include the simulation experiment method (Chinese Patent CN201910142938.X, titled: A Method for Determining the Critical Conditions for Natural Gas Injection into Tight Sandstone Gas Reservoirs; Jiang Fujie, 2010, Geological Thresholds and Gas Control Mechanisms during the Formation Process of Tight Sandstone Gas Reservoirs, Acta Petrolei Sinica) and the theoretical calculation method (Chinese Patent CN201410453187.0, titled: Method for Determining the Lower Limit of Physical Properties for Natural Gas Injection into Tight Sandstone; Cao Qing, 2013, Determination of the Physical Property Bounds for the Formation of Tight Sandstone Gas Reservoirs in the Eastern Ordos Basin, Acta Petrolei Sinica). The above methods have all achieved important results in geological evaluation. The disadvantages are that the geological environment is complex and harsh, which is difficult to simulate in the laboratory, and the parameters required for theoretical calculation are numerous and difficult to obtain; at the same time, the combination with the geological process during the reservoir formation period is not close enough, especially in determining the near physical properties of natural gas injection into tight reservoirs under deep overpressure conditions, and there is no targeted method. Summary of the Invention

[0004] In order to overcome the shortcomings of the above existing methods or fill the gaps, the purpose of the present invention is to provide a method for determining the critical physical properties of natural gas injection into tight reservoirs within overpressure zones, which can realize the evaluation of the effectiveness of natural gas injection into deep overpressure tight reservoirs.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] A method for determining the critical physical properties of natural gas injection into tight reservoirs within overpressure zones, comprising the following steps:

[0007] 1) Based on the high-pressure mercury injection experiment data of reservoir samples, convert the mercury injection displacement pressure value into the gas-water displacement pressure value in the gas-driving water process under geological temperature and pressure conditions, and establish the gas-water displacement pressure distribution profile during the gas-driving charging process of tight reservoirs;

[0008] 2) Synthesize geological data such as the homogenization temperature of inclusions, burial history-thermal history-hydrocarbon generation history, etc., to determine the gas accumulation time and the paleo-burial depth of the corresponding strata;

[0009] 3) Through observation under a polarized light microscope, select sandstone samples rich in gas inclusions to determine the paleo-formation pressure and remaining pressure of the strata during the gas accumulation period;

[0010] 4) Determine the paleo-formation pressure structure. In a single overpressure system, construct a trend line of the remaining pressure change gradient, that is, the effective charging power trend line;

[0011] 5) Superimpose the effective charging power trend line on the gas-water displacement pressure distribution profile converted in step 1), divide the distribution intervals of effective charging and ineffective charging samples, and then conduct statistical analysis to determine the critical physical property boundary value of natural gas charging in tight reservoirs within the overpressure zone, that is, the geological evaluation result of the physical property conditions for overpressure charging.

[0012] The reliability test of the critical physical property boundary value of natural gas charging in tight reservoirs within the overpressure zone adopts the predicted results of the gas saturation per unit reservoir in the oilfield.

[0013] The beneficial effects of the present invention are as follows:

[0014] A method for determining the critical physical properties of natural gas charging in tight reservoirs within the overpressure zone established by the present invention solves the basic problem of evaluating the gas-bearing effectiveness of tight reservoirs in the overpressure zone; this method is closely combined with geological actual conditions such as the gas accumulation periods and paleo-pressure structures in the overpressure zone, making the obtained critical physical properties boundary values of natural gas charging in tight reservoirs more practically significant and more withstand the test of geological actual situations; the obtained critical physical property boundary values of natural gas charging in tight reservoirs within the overpressure zone can be used in work such as the effectiveness evaluation of tight reservoirs in the overpressure zone, the prediction of reservoir gas-bearing properties, the evaluation of natural gas geological reserves, and the deployment of exploration and development well positions, etc., which has important application prospects and economic significance for improving the geological understanding of overpressure tight gas reservoirs, reducing the exploration and development risks of overpressure gas reservoirs, and increasing the success rate of drilling and well testing. Description of the Drawings

[0015] Figure 1 It is the mercury injection displacement pressure distribution profile of the deep reservoir in the study area of the embodiment and its conversion into the gas displacement pressure distribution profile under geological conditions.

[0016] Figure 2 It is the homogenization temperature of fluid inclusions (a-c) and Raman spectra of gas inclusions (d-e) in different intervals of the study area of the embodiment.

[0017] Figure 3 To determine the late-stage gas accumulation time and paleo-burial depth of natural gas in different intervals in the study area of the embodiment.

[0018] Figure 4 To determine the vertical distribution of formation paleo-pressure coefficient and remaining pressure during the late-stage gas accumulation of natural gas in the embodiment.

[0019] Figure 5 To divide the reservoir samples of effective injection and ineffective injection by the remaining pressure gradient trend line in the overpressure zone in the embodiment.

[0020] Figure 6 To statistically determine the physical property boundary of overpressure-driven natural gas injection in the overpressure zone in the embodiment.

[0021] Figure 7 To compare the corresponding gas saturation of sandstone reservoirs with different permeabilities in the overpressure zone in the study area of the embodiment, that is, to verify the reliability of the physical property boundary of injection. Specific implementation manners

[0022] The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0023] Embodiment: A deep overpressure gas reservoir A (containing a small amount of oil) in the study area, a method for determining the critical physical properties of natural gas injection in a tight reservoir in the overpressure zone, includes the following steps:

[0024] 1) According to the high-pressure mercury injection experiment data of reservoir samples, convert the mercury injection displacement pressure value into the gas-water displacement pressure value of the gas displacing water process under the formation temperature and pressure conditions of the study area, and establish the gas-water displacement pressure distribution profile of the gas injection process in the tight reservoir; the displacement pressure changes from a low value in the shallow part (conventional reservoir, buoyancy injection) to a high value in the deep part (tight reservoir, overpressure injection), and the turning point burial depth is about 3600 m, as Figure 1 shown;

[0025]

[0026] In the formula, P 汞 is the displacement pressure of mercury injection, MPa; δ 汞 is the mercury-water interfacial tension, 0.48 N / m; θ 汞 is the contact angle of mercury, 146°; r is the pore throat radius of the reservoir, μm; P 气 is the displacement pressure of gas displacing water injection under the geological conditions of the study area, MPa; δ 气 is the interfacial tension of gas-water under geological conditions, 0.026 N / m; θ 气 is the gas-water contact angle under geological conditions, 15°;

[0027] 2) By integrating geological data such as the homogenization temperature of inclusions, burial history - thermal history - hydrocarbon generation history, etc., determine the gas accumulation time and the paleo - burial depth of the corresponding strata.

[0028] In the study area of this embodiment, the main exploration and development layers of the deep over - pressured gas reservoir A (containing a small amount of oil) from top to bottom are the Ba3 formation, Ba5 formation, and Ba7 formation. Among them, the Ba7 formation is immediately underlain by a coal - measure hydrocarbon source rock, which has the characteristics of generating oil in the early stage and gas in the late stage, with gas generation being the main and oil generation being the secondary (Dai Jinxing, 2019). It is the main gas source rock of the over - pressured gas reservoir.

[0029] The distribution of the homogenization temperature of the associated brine inclusions in the Ba3 formation reservoir shows a bimodal state. The temperature of the front peak is between 125 - 130 °C, and the temperature of the rear peak is between 135 - 140 °C. As shown in (a) below, it reflects obvious characteristics of two - stage hydrocarbon charging in the Ba3 formation sandstone reservoir. Combining the analysis of burial history - thermal history - hydrocarbon generation history shows that the front peak of the homogenization temperature of the inclusions in the Ba3 formation corresponds to the oil charging time, about 13.0 Ma, that is, the Longjing movement period; the rear peak corresponds to the gas charging time, about 2.0 Ma, that is, the Okinawa movement period. At this time, the paleo - burial depth is about 3500 m, as shown in Figure 2 shown; Figure 3

[0030] The distribution of the homogenization temperature of the inclusions in the Ba5 formation reservoir shows a bimodal characteristic. The front peak is 135 - 140 °C, corresponding to the oil charging time of about 5.0 Ma; the rear peak is 145 - 150 °C, corresponding to the gas charging time of about 3.0 Ma, that is, the Okinawa movement period. Combining the characteristics of the peak hydrocarbon generation and evolution of the hydrocarbon source rock, the large - scale gas accumulation time in the Ba5 formation should also be in the Okinawa movement period. At this time, the hydrocarbon source rock has a large ability to generate and expel gas, and the corresponding paleo - burial depth of the reservoir is about 3800 m, as shown in (b) below, Figure 2 Figure 3 shown;

[0031] The distribution of the homogenization temperature of the inclusions in the Ba7 formation reservoir shows a unimodal state, with the main peak temperature between 140 - 145 °C, as shown in (c) below. However, there are data distributions with approximate proportions within this temperature range, reflecting the characteristics of near - source continuous hydrocarbon charging. The main peak of the homogenization temperature in the Ba7 formation can correspond to before the Longjing movement, but considering the regional geological background, the tectonic traps had not formed at this time, and hydrocarbons could not be effectively accumulated. It can also correspond to since about 6.0 Ma, with a relatively long continuous gas charging time, which conforms to the characteristics of near - source hydrocarbon accumulation. Combining the hydrocarbon generation and evolution process of the hydrocarbon source rock, the geothermal temperature increased in the later stage, and a large amount of gas was generated. The secondary peak temperature after the main peak also corresponds to the gas charging process. In summary, the main gas accumulation period of the Ba7 formation reservoir is also the Okinawa movement period, about 4.0 Ma, corresponding to a paleo - burial depth of about 4000 m, as shown in Figure 2 shown; Figure 3

[0032] ​​​Generally, the Longjing movement period in the study area is the first hydrocarbon accumulation period. The reservoir is mainly filled with a small amount of oil, supplemented by natural gas. The second hydrocarbon accumulation period is the Okinawa movement period, during which a large amount of natural gas is generated, which is the key period for the deep natural gas to be charged and accumulate. From the lower Ba7 section to the upper Ba3 section, the hydrocarbon accumulation time of natural gas is postponed in turn, which conforms to the geological actual process that the coal measure source rock under the Ba7 section generates gas, and vertically migrates and accumulates upward in turn. As Figure 3 shown;

[0033] 3) Select sandstone samples rich in gas inclusions through observation under a polarized light microscope to determine the paleopressure and residual pressure of the formation during the hydrocarbon accumulation period of natural gas;

[0034] In this embodiment, Raman tests are carried out on the gas inclusion samples developed in the above three gas-bearing sandstone intervals. The results show that the main component of the gas is CH 4 , as Figure 2 shown in (d)-(e) of; Under constant pressure conditions, the Raman shift (wave number) of CH 4 increases with the increase of temperature; Under constant temperature conditions, the Raman shift (wave number) of CH 4 decreases with the increase of pressure. Based on this, a chart of the change of Raman wave number of CH 4 with temperature-pressure is established (Chou, 2011; Wu Qiang 2019; etc.); Combining the inclusion temperature measurement data in step (2), the paleopressure (P 古 ) during the hydrocarbon accumulation of natural gas in the sandstone reservoir is obtained. Respectively, the average of the Ba3 section is 39.4 Mpa, the average of the Ba5 section samples is 66.2 Mpa, and the average of the Ba7 section is 68.2 Mpa; According to the corresponding paleo-burial depth during the hydrocarbon accumulation period, the formation hydrostatic pressure (P 静水 = ρ 水 ·g·h 古埋深 ) can be calculated, and then the formation fluid pressure coefficient (m = P 古 / P 静水 ) and the residual pressure are calculated; Calculating the paleopressure coefficient during the natural gas charging and accumulation shows that the Ba3 section is 1.15, which is an atmospheric pressure environment, and the residual pressure is 5.1 Mpa; The Ba5 section is 1.78, which is a superpressure environment, and the residual pressure is 28.9 Mpa; The Ba7 section is 1.72 and 1.76, with an average of 1.74, which is a superpressure environment, and the average residual pressure is 29.0 Mpa, as Figure 4 shown; The formation pressure coefficients and residual pressure values during the hydrocarbon accumulation of natural gas in the Ba5 section and the Ba7 section are approximately the same, which are significantly higher than those in the Ba3 section, reflecting the difference in pressure structure; The Ba5 section and the Ba7 section are in the same superpressure environment, and the Ba3 section is in an atmospheric pressure environment, as Figure 4 shown;

[0035] 4) Determine the formation paleopressure structure. In a single superpressure system, construct a trend line of the change gradient of the residual pressure, that is, the effective charging power trend line;

[0036] In this embodiment, according to step 3), the paleopressure structure of the strata in the study area is judged. The strata below the Ba 5 member are in an overpressure environment, and the strata above it are in a normal pressure environment. There are significant differences in the driving forces for hydrocarbon accumulation. The reservoirs in the Ba 5-Ba 7 members and deeper (>3800 m) are tight. Natural gas accumulates under the action of the overpressure pressure gradient, and the buoyancy effect is very weak. The reservoirs shallower than the Ba 5 member are conventional reservoirs, and natural gas accumulates under the action of buoyancy. For the overpressure system in the Ba 5-Ba 7 members, a pressure change gradient trend line is made with the remaining pressure value of its reservoir, and the starting point is set to zero to make a trend line of the effective driving force for natural gas injection, as Figure 5 shown;

[0037] 5) Superimpose the effective injection driving force trend line on the gas-water displacement pressure distribution profile after conversion in step 1), divide the distribution intervals of effective injection and ineffective injection samples, and then conduct statistical analysis to determine the critical physical property boundary value for natural gas injection into the tight reservoir in the overpressure zone, that is, the geological evaluation result of the physical property conditions for overpressure injection;

[0038] In this embodiment, the displacement pressure test data points of the experimental samples are above the trend line, indicating that the displacement pressure for natural gas injection is higher than the effective injection driving force, and the corresponding reservoir cannot accumulate. On the contrary, if it is below the trend line, it indicates that the injection driving force trend line is higher than the displacement pressure for natural gas injection, and effective accumulation can occur. The physical properties of the reservoir samples in the overpressure zone above and below the effective injection driving force trend line are respectively statistically analyzed to determine the critical physical property value for natural gas injection into the tight reservoir in the overpressure zone. The analysis shows that the permeability (K) boundary for natural gas accumulation in the tight reservoir in the overpressure zone of the study area in this embodiment is 0.2 mD, as Figure 6 shown.

[0039] Verify the reliability of the critical physical properties for reservoir injection and accumulation: For the reliability test of the critical physical property boundary value for natural gas injection into the tight reservoir in the overpressure zone, the predicted results of reservoir gas saturation of oilfield units are used to ensure the objectivity of the verification work. For the reservoirs in the overpressure zone of the study area in this embodiment, the gas saturation values of the samples with permeability higher than the critical physical property (K>0.2 mD) and lower than the critical physical property (K≤0.2 mD) are respectively statistically analyzed. Statistical analysis of a large sample data set (425) shows that the gas saturation of the reservoirs with physical properties lower than the critical value is very low, and the gas saturation of more than 70% of the samples is about 0. While the gas saturation of the reservoirs with physical properties higher than the critical value increases significantly and can be used as the exploration and development target for deep tight gas, as Figure 7 shown; It can be seen that the method of the present invention is feasible, and the determination result of the critical injection physical properties conforms to the actual geological situation.

Claims

1. A method for determining the critical physical properties of natural gas charging in tight reservoirs in overpressure zones. It is characterized in that The following steps are involved: 1) Based on the high-pressure mercury injection experimental data of reservoir samples, the mercury injection displacement pressure value is converted into the gas-water displacement pressure value of the gas-water displacement process under geological temperature and pressure conditions, and the gas-water displacement pressure distribution profile of the tight reservoir gas drive filling process is established; 2) Determine the time of natural gas accumulation and the ancient burial depth of the corresponding strata by integrating the data of inclusion homogenization temperature, burial history, thermal history and hydrocarbon generation history; 3) Select sandstone samples rich in gas inclusions through observation under a polarizing microscope to determine the paleopressure and residual pressure of the formation during the natural gas accumulation period; 4) Determine the paleopressure structure of the formation and, in a single overpressure system, construct the residual pressure change gradient trend line, i.e., the effective charging dynamic trend line; 5) superimposing the effective charging dynamic trend line with the gas-water displacement pressure distribution profile converted in step 1), dividing the distribution intervals of effective charging and ineffective charging reservoir samples, and then statistically analyzing to determine the critical physical property limit value of natural gas charging in the tight reservoir in the overpressure zone, that is, the geological evaluation result of the overpressure charging physical property condition; The data points of reservoir sample displacement pressure test are above the trend line, indicating that the displacement pressure of natural gas charging is higher than the effective charging power, and the corresponding reservoir cannot be charged into an accumulation; on the contrary, if they are below the trend line, it means that the charging power trend line is higher than the displacement pressure of natural gas charging, and effective charging forms an accumulation. The physical properties of reservoir samples above and below the effective charging power trend line in the overpressure zone are statistically analyzed to determine the critical physical property values ​​of natural gas charging in tight reservoirs in the overpressure zone.

2. The method according to claim 1, Features: The reliability test of the critical physical property limit value of natural gas charging in the tight reservoir in the overpressure zone adopts the prediction result of gas saturation of unit reservoir in the oil field to ensure the objectivity of the verification work.

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