A method for inverting the hydrate saturation in sediments using the thermal conductivity

Abstract

The present invention discloses a method for inversely calculating the hydrate saturation in sediments by using the thermal conductivity, which includes fabricating a sediment sample containing hydrates, determining the heating time and the temperature rise of the sediment during the heating process through a temperature-measuring and heating probe, multiple groups of temperature-pressure sensor probes, and a constant-current and constant-voltage output device, then calculating to obtain the hydrate state in the sediment, and finally calculating the in-situ hydrate saturation in the sediment by using the hydrate saturation inversion formula. The present invention takes into account the influence of the latent heat of hydrate decomposition phase change when calculating the hydrate thermal conductivity, introduces the hydrate saturation in the sediment into the thermal conductivity calculation, and inversely obtains the in-situ hydrate saturation distribution in the sediment by using the principle that the thermal conductivity obtained by the mixture theory is equivalent to the thermal conductivity measured by the hot wire method. This method realizes the inverse calculation of the saturation distribution in hydrate-containing sediments with phase change based on the thermal principle, providing convenience for the calculation of the hydrate saturation in sediments.

Description

Technical Field

[0001] The present invention belongs to the technical field of hydrate saturation inversion methods, and relates to a method for inverting hydrate saturation by using thermal conductivity. Background Art

[0002] The identification of gas hydrate saturation in sediments is crucial in the fields of natural gas hydrate exploitation and carbon dioxide geological sequestration by the hydrate method. Especially during the exploration of natural gas hydrates, determining the hydrate saturation in the formation is an important indicator for selecting sweet spots for hydrate development, which determines the hydrate burial volume at the sweet spots. For carbon dioxide geological sequestration by the hydrate method, monitoring the change in hydrate saturation in the reservoir after sequestration is the key to measuring the sequestration effect. Therefore, it is of great significance to obtain the hydrate saturation distribution in sediments.

[0003] Currently, research on hydrate saturation inversion mainly focuses on seismic wave impedance inversion, resistivity inversion, based on dielectric constant / conductivity dual parameters, etc. These methods estimate hydrate saturation by monitoring the acoustic and electrical properties of hydrate sediments and combining with geophysical models. However, these methods are relatively complex and costly. There are also some that directly measure the chloride ion concentration of hydrate-bearing sediments by in-situ coring, and use the characteristic of diluting the chloride ion concentration in water after hydrate decomposition to invert the hydrate saturation. However, in-situ coring faces many difficulties, and the damage to sediments leading to a decrease in measurement accuracy cannot be ignored.

[0004] Patent No. 2020105940803 discloses a non-in-situ pressure and fidelity preservation measurement device and method for hydrate thermal conductivity, Patent No. 2014106588055 discloses an in-situ test device and method for gas hydrate thermal conductivity, and 2014100152714 discloses a method and device for measuring the thermal conductivity of natural gas hydrates in porous media. The above use the method of maintaining pressure stability to avoid hydrate decomposition when calculating the thermal conductivity of hydrate sediments, which is not applicable to the calculation of thermal conductivity during the hydrate decomposition process and cannot invert hydrate saturation. Therefore, there are few methods for estimating hydrate saturation using thermics currently. And when measuring the thermal conductivity during the decomposition process of hydrate sediments, it is necessary to consider the fact that the heat provided by the external heat source causes the hydrate to decompose and absorb latent heat, which greatly affects the test results of the thermal conductivity of hydrate sediments and also provides a technical entry point for inverting hydrate saturation. Summary of the Invention

[0005] The object of the present invention is to provide a method for inverting hydrate saturation by using thermal conductivity to obtain the hydrate saturation distribution in sediments.

[0006] To achieve the above object, the technical solution of the present invention is as follows:

[0007] A method for inversely calculating the hydrate saturation in sediments using the thermal conductivity includes the following steps:

[0008] (1) The dry sediment is evenly divided and loaded into the reaction kettle n times, and each time it is compacted m times until the cylindrical reaction kettle is filled. The pore volume and porosity of the sediment are obtained by the weighing method at this time;

[0009] (2) Dry sediment with the same weight as in step 1 is mixed with water. The volume of water used is obtained by multiplying the preset initial water saturation by the pore volume obtained in step 1. Then, the mixture is filled into the cylindrical reaction kettle in the filling manner of step (1) to obtain a specimen. Multiple groups of temperature-pressure sensor probes are inserted into the specimen. Each pair of temperature-pressure sensor probes is a group and is used to monitor the temperature and pressure changes at the location. Multiple groups of temperature-pressure sensor probes are evenly distributed in the radial and axial directions of the specimen; The temperature measurement-heating probe is inserted along the axis of the top surface of the specimen and penetrates the entire specimen;

[0010] (3) The reaction kettle is cooled to the set temperature and left standing for no less than 12 h, and then a gas that can form hydrates (such as methane and carbon dioxide, etc.) is injected into the reaction kettle until the gas hydrate phase equilibrium pressure is exceeded;

[0011] (4) The multiple groups of temperature-pressure sensor probes are connected to a digital display device. When the pressure in the reaction kettle drops and the temperature rises, it is considered that hydrates are formed. When the pressure drops no more than 0.01 kPa within 12 hours, it is considered that the formation of hydrates is completed, that is, a sediment specimen containing hydrates is obtained;

[0012] (5) A constant current I and voltage U are output to the temperature measurement-heating probe to increase its temperature. The temperature T of the temperature measurement-heating probe and the changes of pressure P and temperature T measured by the temperature-pressure sensor probes with time are recorded in real time; After the temperature of the specimen reaches stability, the current and voltage output device is turned off;

[0013] 1) Calculate the hydrate state at each measurement point of the specimen using equations (1) and (2)

[0014] Δp = p - e a (1)

[0015] a = -1.94138504464560×10 5 +3.31018213397926×10 3 *(T + T d ) -

[0016] 2.25540264493806×10 1 *(T + T d ) 2 +7.67559117787059×10-2 *

[0017] (T + T d ) 3 -1.30465829788791×10 -4 *(T + T d ) 4 +8.86065316687571×

[0018] 10 -8 *(T + T d ) 5 )(2)

[0019] Wherein, p and T are the pressures and temperatures of each measuring point measured by each temperature - pressure sensor probe in the sample; T d is a parameter related to the salinity in the sediment. When the seawater salinity is 3.5%, T d is taken as 0.98K;

[0020] The state of hydrate in the sediment is judged by Δp. When Δp > 0 at the measuring point, the hydrate here is still in a stable state and has not decomposed; when Δp = 0 at the measuring point, the hydrate here is in the process of decomposition; when Δp < 0 at the measuring point, the hydrate here no longer exists and has been completely decomposed;

[0021] 2) Combine the calculated hydrate state and obtain the hydrate saturation distribution according to the following method;

[0022] a) Before the start of heating and temperature increase, Δp > 0 at the measuring point; after the start of heating, Δp gradually decreases. Capture the moment t1 corresponding to Δp = 0 at each measuring point and the temperature of the temperature - measuring and heating probe at moment t1; continue heating until Δp < 0, record the moment t2 and the temperature of the temperature - measuring and heating probe at moment t2. At this time, the temperature and pressure of the sediment containing hydrate are lower than the hydrate phase equilibrium, and the hydrate is completely decomposed. Calculate the thermal conductivity of the sediment at each measuring point using formula (3), that is, the hot - wire method;

[0023]

[0024] Wherein, l is the length of the temperature - measuring and heating probe, I is the current intensity, A; U is the output voltage, V; π is the pi; T1 and T2 are the temperatures of the temperature - measuring and heating probe corresponding to moments t1 and t2 respectively, K; L is the heat absorbed by the decomposition of the hydrate, kJ / kg; ρ h is the density of the gas hydrate, which is a known constant value, kg / m 3 ;

[0025] b) Calculate the equivalent thermal conductivity of the sediment at moment t2 using formula (4);

[0026]

[0027] In the formula, λ s is the thermal conductivity of sediment particles, which is determined by the composition of sediment particles and is a known quantity, W / m / k; λ w and λ g are the thermal conductivities of water and gas respectively, and are also known quantities, W / m / k;

[0028] According to the hydration reaction equation of hydrate: N H H2O + CH4 = CH4·N H H2O (5) In the formula, N H is the hydration number and takes a fixed value;

[0029] At time t2, the hydrate completely decomposes into gas and water. Assuming that the original hydrate saturation in the sediment is s h0 , then the saturations of the gas and water generated after its complete decomposition are calculated by the following formulas (6) and (7) respectively

[0030]

[0031]

[0032] In the formula, ρ w is the density of water, kg / m 3 ; the gas density is calculated by , kg / m 3 ; M w is the molar mass fraction of water, g / mol; M h is the molar mass fraction of hydrate, g / mol; M g is the molar mass fraction of methane gas, g / mol;

[0033] Therefore, the thermal conductivity of the system when the hydrate decomposition in the sediment is completed at time t2 calculated by formula (4) is as follows:

[0034]

[0035] In the formula, porosity initial water saturation s w0 are all obtained when preparing the hydrate sample;

[0036] c) Combine formulas (3) and (8) to calculate the hydrate saturation at all measurement points, that is, obtain the distribution of all hydrate saturations in the sediment;

[0037]

[0038] Advantages of the present invention: The present invention ingeniously utilizes the characteristic that the influence of the latent heat of hydrate decomposition phase change is considered in the calculation of the thermal conductivity of hydrates, introduces the physical quantity of hydrate saturation in the sediment into the thermal conductivity calculation formula, and based on the principle that the thermal conductivity obtained by the mixture theory is equivalent to the thermal conductivity measured by the hot wire method, inversely obtains the in-situ hydrate saturation distribution in the sediment. This method realizes the inverse calculation of the saturation distribution in hydrate-containing sediments with phase change based on thermal principles, providing convenience for the calculation of hydrate saturation in sediments. Detailed implementation manners

[0039] A method for inversely calculating hydrate saturation using thermal conductivity of the present invention will be described.

[0040] Principle of a method for inversely calculating hydrate saturation using thermal conductivity of the present invention: By capturing the temperature and pressure changes in the sediment to determine the state of the hydrate, thermal conductivity calculation methods are established respectively based on the hot wire method and the mixture theory. The hydrate saturation is introduced, and the heat absorbed by the hydrate decomposition is converted into the temperature increment that the sediment should rise. Combining the hydrate phase identification and thermal conductivity calculation methods, the thermal conductivity expression at the moment when the in-situ hydrate is completely decomposed is obtained, and based on the principle that the thermal conductivities obtained by the two methods are equal, the in-situ hydrate saturation in the sediment is inversely obtained.

[0041] The following describes an example of the detailed implementation manner of the present invention in detail in combination with the technical solution.

[0042] (1) Divide 100 ml of dry sediment into the reaction kettle in 10 equal portions and compact it 5 times each time until the reaction kettle with a radius of 3 cm and a height of 3.53 cm (effective volume of 100 ml) is filled. Use the weighing method to obtain the pore volume of the sediment as 50 ml and the porosity as 0.5 at this time.

[0043] (2) Preset the initial water saturation to 40%. Take 100 ml of dry sediment and 20 ml of water and mix them. Then fill the mixture into the cylindrical reaction kettle according to step (1) to obtain a sample. Multiple groups of temperature-pressure sensor probes are inserted into the sample. Each pair of pressure sensor probes is a group and is used to monitor the temperature and pressure changes at the location. Multiple groups of temperature-pressure sensor probes are evenly distributed in 5 groups in the radial and axial directions of the sample; the length l of the temperature measurement-heating probe is 3.53 cm and is inserted along the axis of the top surface of the sample, and the probe penetrates the entire sample.

[0044] (3) Cool the reaction kettle to the set temperature and let it stand for no less than 12 h. Then fill the kettle with methane until the pressure reaches above the hydrate phase equilibrium pressure to generate hydrates.

[0045] (4) Connect multiple sets of temperature-pressure sensor probes to the digital display device. When the pressure in the reactor decreases and the temperature rises, it is considered that hydrate formation occurs. When the pressure drops no more than 0.01 kPa within 12 hours, it is considered that the hydrate formation ends. That is, a sediment sample containing hydrate is obtained.

[0046] (5) Output a constant current of 10 A and a voltage of 5 V to the temperature-measuring and heating probe to increase its temperature. Record the temperature T of the temperature-measuring and heating probe and the temperature-pressure P measured by the temperature-pressure sensor probe and the change of T with time in real time. After the sample temperature reaches stability, turn off the current and voltage output device.

[0047] 1) Calculate the hydrate state at each measurement point of the sample using the following formulas (1) and (2)

[0048] Δp = p - e a (1)

[0049] a = -1.94138504464560×10 5 +3.31018213397926×10 3 *(T + T d ) -

[0050] 2.25540264493806×10 1 *(T + T d ) 2 +7.67559117787059×10 -2 *

[0051] (T + T d ) 3 -1.30465829788791×10 -4 *(T + T d ) 4 +8.86065316687571×

[0052] 10 -8 *(T + T d ) 5 ) (2) In formulas (1) and (2), p and T are the pressures and temperatures at each measurement point measured by each temperature-pressure sensor probe in the sample; T d is a parameter related to the salinity in the sediment. When the seawater salinity is 3.5%, T d is taken as 0.98 K.

[0053] The state of hydrate in the sediment is judged by Δp. When Δp > 0 at the measuring point, the hydrate here is still in a stable state and has not decomposed; when Δp = 0 at the measuring point, the hydrate here is in the process of decomposition; when Δp < 0 at the measuring point, the hydrate here no longer exists and has been completely decomposed.

[0054] 2) Combining with the calculated hydrate state, the hydrate saturation distribution can be obtained by the following method.

[0055] a) Before the heating-up starts, Δp > 0 at the measuring point. After the heating starts, Δp gradually decreases. Capture the moment t1 when Δp = 0 at each measuring point and the temperature of the temperature-measuring and heating probe at this time. Continue heating until Δp < 0, record this moment t2 and the temperature of the temperature-measuring and heating probe at this time. At this time, the temperature and pressure of the sediment containing hydrate are lower than the hydrate phase equilibrium, and the hydrate is completely decomposed. Use formula (3) to calculate the thermal conductivity of the sediment in the area between each measuring point and the heating probe (hot wire method).

[0056]

[0057] In the formula, l is the length of the heating probe, I is the current intensity, A; U is the output voltage, V; π is the pi; T1 and T2 are the temperatures of the temperature-measuring and heating probe corresponding to the times t1 and t2 respectively, K. L is the heat absorbed by the decomposition of hydrate, 334.6 kJ / kg; ρ h is the density of gas hydrate, which is a known fixed value, 800 kg / m 3 ;

[0058] b) Use formula (4) to calculate the equivalent thermal conductivity of the sediment at time t2.

[0059]

[0060] In the formula, λ s is the thermal conductivity of sediment particles, which is determined by the composition of sediment particles, 2 W / m / k; λ w and λ g are the thermal conductivities of water and gas respectively, which are 0.58 W / m / k and 0.03 W / m / k respectively.

[0061] According to the hydration reaction equation of hydrate

[0062] N H H2O + CH4 = CH4·N H H2O (5)

[0063] In the formula, N H is the hydration number, generally taken as 5.6.

[0064] At time t2, the hydrate is completely decomposed into gas and water. Assuming that the original hydrate saturation in the sediment is s h0 , then the saturation of gas and water produced after complete decomposition is calculated by the following equations (6) and (7) respectively:

[0065]

[0066]

[0067] In the formula, ρ w is the density of water, equal to 1000kg / m 3 is the density of the hydrate, and the gas density can be obtained from Calculated. kg / m 3 ;M w is the molar mass fraction of water, 18.016 g / mol; M h is the molar mass fraction of the hydrate, 124.138 g / mol; M g is the molar mass fraction of methane gas, 16.042 g / mol; M_w is the molar mass fraction of water, 18.016 g / mol; M_h is the molar mass fraction of hydrate, 124.138 g / mol; M_g is the molar mass fraction of methane gas, 16.042 g / mol. The coefficients of the system when the decomposition of hydrate in the sediment at time t2 are calculated by formula (4) are as follows:

[0068]

[0069] In the formula, porosity Initial water saturation s w0 Both can be obtained when preparing hydrate samples.

[0070] c) By combining equations (3) and (8), the hydrate saturation at all measuring points can be calculated, and the distribution of all hydrate saturations in the sediment can be obtained.

[0071]

[0072] If 5min and 10min are the times corresponding to Δp=0 and Δp<0 at a certain measuring point (t1=5min and t2=10min), the corresponding heating probe temperatures are 5° and 10° (T1=5° and T2=10°), respectively. At t2, the temperature of the measuring point is 6° and the pressure is 5.25MPa, which is exactly at the critical temperature and pressure of Δp<0. Then the hydrate saturation of the sample at the measuring point can be calculated by formula (9), which is 42.53%.

[0073] The above description is only for the purpose of explaining in combination with the present calculation process. For those skilled in the art, what changes and modifications can be made to the present invention. Any modifications, equivalent replacements, improvements, etc., made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for inverting the hydrate saturation in sediments using the thermal conductivity, characterized in that It includes the following steps: (1) The dry sediment is evenly divided into n portions and loaded into the reaction kettle, and compacted m times each time until the cylindrical reaction kettle is filled. The pore volume and porosity of the sediment are obtained by the weighing method at this time. (2) Take the same weight of dry sediment as in step 1 and mix it with water. The volume of water used is obtained by multiplying the preset initial water saturation by the pore volume obtained in step 1. Then, the mixture is filled into the cylindrical reaction kettle in the filling manner of step (1) to obtain a specimen. Multiple groups of temperature-pressure sensor probes are inserted into the specimen. Each pair of temperature-pressure sensor probes is a group and is used to monitor the temperature and pressure changes at the location. The multiple groups of temperature-pressure sensor probes are evenly distributed in the radial and axial directions of the specimen. The temperature measurement-heating probe is inserted along the axis of the top surface of the specimen and penetrates the entire specimen. (3) Cool the reaction kettle to the set temperature and let it stand for no less than 12 h, and then fill the reaction kettle with the gas that can form hydrate to a pressure above the gas hydrate phase equilibrium pressure. (4) Connect the multiple groups of temperature-pressure sensor probes to the digital display device. When the pressure in the reaction kettle drops and the temperature rises, it is considered that hydrate formation occurs. When the pressure drops no more than 0.01 kPa within 12 hours, it is considered that the hydrate formation ends, that is, a sediment specimen containing hydrate is obtained. (5) Output a constant current I and voltage U to the temperature measurement-heating probe to increase its temperature. Record the changes of the temperature T of the temperature measurement-heating probe and the pressure P and temperature T measured by the temperature-pressure sensor probes in real time with time. After the temperature of the specimen reaches stability, turn off the current and voltage output device. 1) Calculate the hydrate state at each measuring point of the specimen using equations (1) and (2). Δp = p - e a (1) a = -1.94138504464560×10 5 + 3.31018213397926×10 3 *(T + T d ) - 2.25540264493806×10 1 *(T + T d ) 2 + 7.67559117787059×10 -2 *(T + T d ) 3 - 1.30465829788791×10 -4 *(T + T d ) 4 + 8.86065316687571×10 -8 *(T + T d ) 5 ) (2) where p and T are the pressure and temperature at each measurement point measured by each temperature-pressure sensor probe in the sample; T d is a parameter related to the salinity in the sediment, and when the seawater salinity is 3.5%, T d is taken as 0.98 K; Judge the state of the hydrate in the sediment through Δp. When Δp>0 at the measuring point, the hydrate here is still in a stable state and has not decomposed. When Δp = 0 at the measuring point, the hydrate here is in the decomposition process. When Δp<0 at the measuring point, the hydrate here no longer exists and has been completely decomposed. 2) Combine the calculated hydrate state and obtain the hydrate saturation distribution according to the following method. a) Before the heating starts, Δp>0 at the measuring point. After the heating starts, Δp gradually decreases. Capture the corresponding time t1 when Δp = 0 at each measuring point and the temperature of the temperature measurement-heating probe at time t1. Continue heating until Δp<0, record the time t2 and the temperature of the temperature measurement-heating probe at time t2. At this time, the temperature and pressure of the sediment containing hydrate are lower than the gas hydrate phase equilibrium, and the hydrate is completely decomposed. Calculate the thermal conductivity of the sediment at each measuring point using formula (3), that is, the hot wire method. In the formula, l is the length of the temperature measurement-heating probe, and I is the current intensity, A. U is the output voltage, V; π is the pi; T1 and T2 are the temperatures of the temperature-measuring and heating probes corresponding to the times t1 and t2 respectively, K; L is the heat absorbed by the hydrate decomposition, kJ / kg; ρ h is the density of the gas hydrate, which is a known fixed value, kg / m 3 ; b) Calculate the equivalent thermal conductivity of the sediment at time t2 using equation (4). where λ s is the thermal conductivity of sediment particles, which is determined by the composition of sediment particles and is a known quantity, W / m / k; λ w and λ g are the thermal conductivities of water and gas, respectively, and are also known quantities, W / m / k; According to the hydration reaction equation of hydrate: N H H2O + CH4 = CH4·N H H2O (5) where N H is the hydration number, which is a fixed value; At time t2, the hydrate completely decomposes into gas and water. Assuming that the original hydrate saturation in the sediment is s h0 , then the saturations of the gas and water generated after its complete decomposition are calculated by the following equations (6) and (7) respectively where ρ w is the density of water, kg / m 3 ; the gas density is obtained by calculation, kg / m 3 ; M w is the molar mass fraction of water, g / mol; M h is the molar mass fraction of hydrate, g / mol; M g is the molar mass fraction of methane gas, g / mol; Therefore, the thermal conductivity of the system when the hydrate in the sediment decomposes completely at time t2 calculated by equation (4) is as follows: In the formula, the porosity the initial water saturation s w0 are all obtained when preparing the hydrate sample; c) Combine equations (3) and (8) to calculate the hydrate saturation at all measuring points, that is, obtain the hydrate saturation distribution in all sediments.

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