CN111562206A - Method for measuring pore size distribution characteristics of oil-bearing rock of unconventional oil and gas reservoir - Google Patents
Method for measuring pore size distribution characteristics of oil-bearing rock of unconventional oil and gas reservoir Download PDFInfo
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Abstract
The invention discloses a method for measuring the pore size distribution characteristics of oil-bearing rocks of an unconventional oil and gas reservoir, which comprises the following steps: determination of K for octamethylcyclotetrasiloxane using silica standard pore size samplesGTA value; determining the time interval of octamethylcyclotetrasiloxane as probe liquid in a temperature plan tested by a low-field nuclear magnetic resonance freezing and thawing method by using a rock sample (such as compact sandstone) of an unconventional oil and gas reservoir as a sample to be tested; preprocessing an oil-bearing rock sample of an unconventional oil and gas reservoir; performing oil washing treatment on an unconventional reservoir oil-bearing rock sample by using a Soxhlet extraction method; octamethylcyclotetrasiloxane is used as probe liquid, and the low-field nuclear magnetic resonance freezing and thawing method is used for testing the unconventional oil and gas reservoir petroliferous rock sample to obtain the pore size distribution characteristics of the unconventional oil and gas reservoir petroliferous rock sample. The invention improves the accuracy of numerical characterization of the unconventional oil and gas reservoir rock pore size distribution and solves the problem that the unconventional oil and gas reservoir rock pore size distribution characteristics are difficult to accurately characterize.
Description
Technical Field
The invention relates to the field of geology, in particular to a method for measuring the pore size distribution characteristics of oil-containing rocks in unconventional oil and gas reservoirs.
Background
In recent years, with the continuous deep development of oil and gas exploration and development, the key point of oil and gas resource research at home and abroad is gradually changed from a shallow layer to a deep layer, and from a conventional oil and gas reservoir to an unconventional oil and gas reservoir. Unconventional oil gas such as dense gas, shale gas, coal bed gas, dense oil and the like shows huge potential under the existing economic and technical conditions, the conventional oil gas resources in China are relatively poor, but the unconventional oil gas resources are abundant, and particularly the dense oil gas has huge potential for development and utilization. Therefore, unconventional oil and gas has important significance for our country. The rapid development of unconventional oil and gas exploration and development breaks through the conventional reservoir physical property lower limit in the traditional sense, and discovers that the nano holes of unconventional oil and gas reservoir rocks can store rich oil and gas resources. Therefore, the research on the nano-pore structure of unconventional oil and gas reservoir rocks is very important for the exploration and development of compact oil and gas.
The pores of unconventional oil and gas reservoir rocks are mainly nano-pores, and the wettability of part of the reservoir rocks is oil-wet. The precise characterization of pore size distribution characteristics of unconventional reservoir rocks for nanometer oil wettability remains very difficult. The currently used test methods mainly comprise four methods, including mercury intrusion method, nitrogen adsorption method and nuclear magnetic resonance T2Spectrum and low-field nuclear magnetic resonance freeze-thaw method. Among them, mercury porosimetry and nitrogen adsorption are most widely used. However, mercury porosimetry and nitrogen adsorption have many limitations in the characterization of tight sandstone. And low field nuclear magnetic resonance T2Staff gauge with difficulty in accurate measurementA nanoporous.
The application of the low-field nuclear magnetic resonance freezing and thawing method in the pores of unconventional oil and gas reservoir rocks is not researched much, and particularly the application aims at the rocks containing oil in the unconventional oil and gas reservoir rocks. Research shows that the low-field nuclear magnetic resonance freezing and thawing method has considerable advantages in the aspect of quantitative characterization of the pore size distribution of unconventional oil and gas reservoir rocks. In the low-field nmr freeze-thaw theory, the probe fluid has a significant impact on the experimental results due to the large difference between the sample and probe fluid properties. Water and cyclohexane are the most commonly used probe liquids. When water is used as the probe liquid, the analysis result is relatively accurate for hydrophilic sandstone, but the analysis result is relatively large in error for unconventional reservoir oil-bearing rock, and water is difficult to penetrate into oil-wet pores due to the existence of the surface tension of the pores. Furthermore, water interacts strongly with the pore surfaces, which may disrupt the pore structure of the rock. Furthermore, due to K of waterGTAnd the measurement range of the low-field nuclear magnetic resonance freezing and thawing method when water is used as probe liquid is 2-500nm according to the accuracy of a temperature control system and the molecular size. However, unconventional reservoir rock, especially tight sandstone, is porous and widely distributed. Thus, water as a probe liquid presents great difficulties in testing unconventional hydrocarbon reservoir rock. When cyclohexane is used as the probe liquid, it is found that it is easier to saturate the pores whose wettability is oil-wet, depending on the nature of cyclohexane, but the nuclear magnetic resonance signal intensities of the liquid and solid components of cyclohexane are difficult to distinguish. Although it is possible to ensure that the tested nmr signal intensity is completely from the liquid phase by setting a larger echo time, for example, 10 ms or even more, the whole low-field nmr freeze-thaw analysis experiment time is too long and even exceeds the machine limit. Due to the non-hydrophilic property of cyclohexane, accurate determination of hydrophilic rocks is not facilitated. Based on this current state of the art, a new probe liquid, octamethylcyclotetrasiloxane, was used by some scholars. Considering the properties of octamethylcyclotetrasiloxane, it is suitable for use as a probe liquid in low-field nuclear magnetic resonance freeze-thaw method. K of probe liquid can be obtained from Gibbs Thomas equationGTAssay for influencing nuclear magnetic resonance freeze-thaw analysis methodAmount ranges. Thus, the K of octamethylcyclotetrasiloxane was determinedGTIs extremely important. But when different scholars use octamethylcyclotetrasiloxane as probe liquid and apply low-field nuclear magnetic resonance freeze-thaw method to measure pore size distribution characteristics of petroselite in unconventional oil and gas reservoirs, K for octamethylcyclotetrasiloxaneGTThe values are chosen to be different values, and the exact K is not used in consideration of the properties of unconventional oil and gas reservoir rocksGTValue, resulting in errors in the test method. Furthermore, one parameter important in the design of a thermometer program during low-field nmr freeze-thaw analysis is the size of the time interval between the two temperature points. The size of this time interval will determine whether thermal equilibrium of the probe liquid in the test sample pores can be guaranteed, and is critical to the test results.
Disclosure of Invention
The invention aims to provide a method for measuring the pore size distribution characteristics of the petroliferous rock of the unconventional oil and gas reservoir, so as to solve the problem that the pore size distribution characteristics of the unconventional oil and gas petroliferous rock are difficult to accurately represent.
In order to achieve the purpose, the invention adopts the technical scheme that:
a method for measuring the pore size distribution characteristics of petroliferous rocks of unconventional oil and gas reservoirs comprises the following steps:
step S1, determining K of octamethylcyclotetrasiloxane using silica standard pore size sampleGTA value;
step S2, using a rock sample of an unconventional oil and gas reservoir as a sample to be tested, and determining the time interval of octamethylcyclotetrasiloxane as probe liquid in a temperature plan tested by a low-field nuclear magnetic resonance freeze-thaw method;
step S3, preprocessing an oil-bearing rock sample of an unconventional oil and gas reservoir;
step S4, performing oil washing treatment on the oil-bearing rock sample of the unconventional reservoir by using a Soxhlet extraction method;
and step S5, using octamethylcyclotetrasiloxane as probe liquid, and testing the unconventional oil and gas reservoir oil-containing rock sample by using a low-field nuclear magnetic resonance freeze-thaw method to obtain the pore size distribution characteristics of the unconventional oil and gas reservoir oil-containing rock sample.
In the step S1, three silica standard pore size samples with different pore sizes are selected to saturate octamethylcyclotetrasiloxane, the phase transition process of octamethylcyclotetrasiloxane in the three silica standard pore size samples is analyzed and tested by using a low-field nuclear magnetic resonance freeze-thaw method, and K of octamethylcyclotetrasiloxane is calculated by using the relationship between the melting point change value of octamethylcyclotetrasiloxane and the silica standard pore sizeGTThe value is obtained.
In step S2, recording a thermal equilibrium process of octamethylcyclotetrasiloxane in the pores of the sample to be tested within two temperature intervals of 0.5K for the saturated octamethylcyclotetrasiloxane of the sample to be tested, and obtaining a time interval for two temperature points of octamethylcyclotetrasiloxane during the low-field nuclear magnetic resonance freeze-thaw analysis, wherein the time interval is sufficient to ensure that octamethylcyclotetrasiloxane reaches thermal equilibrium in the pores of the sample at each temperature point.
In the step S3, mechanically crushing unconventional oil and gas reservoir rock samples; for tight sandstone, a sample of 20-35 mesh is selected, and for shale and coal, a sample of 35-50 mesh is selected. In order to avoid mixing of metal powder during the sample crushing, an agate mortar was used during the crushing.
In the step S4, the solvent is a mixture of dichloromethane and methanol at a volume ratio of 9:1 until the residual oil or organic matter in the unconventional reservoir petroliferous rock sample is removed, and finally, the unconventional reservoir petroliferous rock sample is dried in a 373.15K vacuum oven for 24 hours.
In the step S5, for an unconventional oil and gas reservoir petroliferous rock sample saturated octamethylcyclotetrasiloxane, the specific steps are as follows: placing an unconventional oil-gas reservoir oil-containing rock sample in a vacuum box, and vacuumizing for 12 hours; adding octamethylcyclotetrasiloxane into the chromatographic bottle containing the vacuumized unconventional oil and gas reservoir rock sample, and balancing for 6 hours; and centrifuging the chromatographic bottle for 2h to fully saturate the octamethylcyclotetrasiloxane in the unconventional oil and gas reservoir rock sample.
Has the advantages that: the invention determines octamethylcyclotetrasulfame by using a silicon dioxide standard aperture materialSiloxane as probe liquid KGTThe time interval in the octamethylcyclotetrasiloxane temperature plan is determined by using the compact sandstone as a sample to be detected, the pore size distribution characteristic of the unconventional oil-gas petroliferous rock is accurately represented by using a low-field nuclear magnetic resonance freeze-thaw method, the problem that the pore size distribution characteristic of the unconventional oil-gas petroliferous rock is difficult to accurately represent is solved, and the adaptability and the technical advantages of the probe liquid in the pore size representation of the unconventional oil-gas petroliferous rock are discussed. Compared with the prior art, the method has the following advantages:
1. the measurement method of the invention firstly determines the K of octamethylcyclotetrasiloxaneGTThen, the method is applied to the oil-containing sample of the unconventional oil and gas reservoir and has the following advantages: octamethylcyclotetrasiloxane has a larger K after being tested by silicon dioxide standard aperture YangpinGTThe value is that when Octamethylcyclotetrasiloxane (OMCTS) is used as probe liquid to carry out low-field nuclear magnetic resonance freeze-thaw test, the measurable aperture range is 4-1388 nm, and compared with the analog value used in the prior art, the value is more accurate.
2. The measuring method adopts octamethylcyclotetrasiloxane as nuclear magnetic resonance probe liquid, and the interval time of temperature points is determined through testing. The probe liquid in the pore can fully reach the thermal equilibrium. More accurate than the estimates used by other scholars.
Drawings
FIG. 1 is a schematic flow chart of a measurement method of the present invention;
FIG. 2 is a graph showing the relationship between the variation of melting point of octamethylcyclotetrasiloxane in porous silica media and the standard pore size of silica;
FIG. 3 is a thermal equilibrium process of octamethylcyclotetrasiloxane in tight sandstone pores;
fig. 4 is a pore size distribution curve of unconventional oil-bearing reservoir rock obtained by taking oil-bearing tight sandstone as an example in the example.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In the invention, the unconventional oil and gas reservoir oil-bearing rock sample is tested by using a low-field nuclear magnetic resonance freezing and thawing method, the low-field nuclear magnetic resonance freezing and thawing method is based on a Gibbs-Thomson equation, and the equation describes the relationship between the melting point reduction value of the solid in the hole and the hole diameter.
Wherein,
is the melting point of a bulk solid (which can be considered as infinitely large crystals); t ism(D) Is the melting point of a crystal with a diameter D; sigmaslIs the surface energy of a solid-liquid interface △ HfIs the enthalpy of fusion of the bulk solid; rhosIs the density of the solid, D is the pore diameter, △ TmIs the amount of change in melting point of the material.
The right side of the equation equal sign except the aperture D all other items are probe liquid physical property parameters, and for the same substance, the same substance can be regarded as a constant in the temperature range of the test, and the simple form of the Gibbs-Thomson equation is as follows:
KGTis the Gibbs-Thomson constant, and is related to the thermodynamic properties of the probe liquid.
Examples
Before actual testing, 3 representative oil-bearing samples, one shale sample, one oil-tight sandstone sample, and one coal sample, were selected from unconventional oil and gas reservoirs. The wettability of all three samples was oil-containing. And determining the sample which can represent the petroliferous physical property of the unconventional oil and gas reservoir most from the unconventional oil and gas reservoir as the optimal sample, thereby avoiding the test result from the unconventional oil and gas reservoir rock heterogeneity from reflecting the real reservoir physical property.
Referring to fig. 1, the method for measuring the pore size distribution characteristics of the petroliferous rock of the unconventional oil and gas reservoir comprises the following steps:
step S1, determining K of octamethylcyclotetrasiloxane using silica standard pore size sampleGTA value; the method specifically comprises the following steps: selecting three silicon dioxide standard pore size samples with different pore sizes, saturating the three silicon dioxide standard pore size samples with octamethylcyclotetrasiloxane, analyzing and testing the phase change process of the octamethylcyclotetrasiloxane in the three silicon dioxide standard pore size samples by a low-field nuclear magnetic resonance freeze-thaw method, and calculating the K of the octamethylcyclotetrasiloxane according to the relation between the melting point change value of the octamethylcyclotetrasiloxane and the silicon dioxide standard pore sizeGTA value;
in this example, three standard pore size samples of silica with uniform pore size were selected, namely 24nm, 38.1nm and 50 nm. The three samples were dried under a vacuum of 372.15K for 24 hours to remove residual moisture from the pores of the porous media of the samples. Then, the octamethylcyclotetrasiloxane was saturated for three samples, and the saturation process was performed by vacuum self-priming for 24 hours. Then, the phase change process of the octamethylcyclotetrasiloxane in the porous medium is observed by using low-field nuclear magnetic resonance. K of octamethylcyclotetrasiloxaneGTValues are calculated from the relationship between the melting point variation of octamethylcyclotetrasiloxane and the silica standard pore size, and the KGT value for octamethylcyclotetrasiloxane suitable for very conventional reservoir oil rock testing is 138.8, see fig. 2.
Step S2, using a rock sample of an unconventional oil and gas reservoir as a sample to be tested, and determining the time interval of octamethylcyclotetrasiloxane as probe liquid in a temperature plan tested by a low-field nuclear magnetic resonance freeze-thaw method; the method specifically comprises the following steps: recording the thermal equilibrium process of the octamethylcyclotetrasiloxane in the pores of the sample to be detected within two temperature intervals of 0.5K for the saturated octamethylcyclotetrasiloxane of the sample to be detected, and obtaining the time interval of two temperature points of the octamethylcyclotetrasiloxane in the low-field nuclear magnetic resonance freeze-thaw analysis process, wherein the time interval is enough to ensure that the octamethylcyclotetrasiloxane reaches thermal equilibrium in the pores of the sample at each temperature point;
in the embodiment, the tight sandstone is selected as a sample to be tested, the tight sandstone is firstly crushed to 20-35 meshes, dried for 24 hours in a vacuum environment of 372.15K, then the octamethylcyclotetrasiloxane is saturated, and finally the tight sandstone sample is placed in a low-field nuclear magnetic resonance instrument for testing. The temperature of the sample was first lowered to 270.15K by using a temperature control device and left for 3 hours. After this time, the temperature was gradually increased and stabilized at 287.65K for 1 hour. The sample compartment temperature was then adjusted to 288.15K and the entire change was recorded using a low field nmr, with data being recorded every two minutes for ten acquisitions, i.e. 20 minutes, see figure 3. The sample compartment temperature was then adjusted to 288.65K and the entire change was recorded using a low field NMR spectrometer with data being recorded every two minutes for ten total acquisitions, i.e., 20 minutes, see FIG. 3. It can be seen that octamethylcyclotetrasiloxane equilibrates after 14 minutes in the temperature interval between 287.15 and 288.15K and 16 minutes in the temperature interval between 288.15 and 288.65K. Thus, a time interval of 20 minutes for the two temperature points of octamethylcyclotetrasiloxane during low-field nmr freeze-thaw analysis is sufficient to ensure that octamethylcyclotetrasiloxane reaches thermal equilibrium in the sample pores at each temperature point.
Step S3, preprocessing an oil-bearing rock sample of an unconventional oil and gas reservoir; the method specifically comprises the following steps: mechanically crushing the unconventional oil and gas reservoir rock sample; selecting a sample with 20-35 meshes for the compact sandstone, selecting a sample with 35-50 meshes for the shale and the coal, drying the selected sample, weighing by using an analytical balance, recording the mass M, and then filling the sample into a chromatographic bottle;
in this embodiment, when the oil-bearing rock sample of the unconventional oil and gas reservoir to be tested is mechanically crushed, an agate mortar is preferably used, and a metal grinding body such as a copper pot and a stainless steel pot cannot be used, so that the influence of metal on the test result of the sample to be tested is avoided. And crushing the sample, and then sorting by using a sieve, wherein the compact sandstone sample is a 20-35-mesh sample, and the shale and the coal are 35-50-mesh samples to be detected. Generally, the size of the sample particles is selected according to the mineral composition of the sample to be tested, the tight sandstone cementation degree and other properties, and 20-35 mesh samples are used in the embodiment of the invention. If the sample is too large, the probe liquid is not saturated in the later period, so that the test result is influenced; too small sample particles can accumulate to create a large number of micro-nano-particle pores and can also affect the test results.
Step S4, performing oil washing treatment on the oil-bearing rock sample of the unconventional reservoir by using a Soxhlet extraction method; the method specifically comprises the following steps: the unconventional reservoir petroliferous rock sample is subjected to oil washing by using a Soxhlet extraction method, a solvent is a mixture of dichloromethane and methanol according to a volume ratio of 9:1 until residual oil or organic matters in the unconventional reservoir petroliferous rock sample are removed, and finally, the unconventional reservoir petroliferous rock sample is dried in a 373.15K vacuum oven for 24 hours.
Residual oil or organic matter in petroliferous samples of unconventional oil and gas reservoirs must be removed because the nuclear magnetic resonance signal intensity of the probe fluid in the pores of the sample from which the residual oil has not been removed will be affected by the nuclear magnetic resonance signal intensity of the residual oil or organic matter in the sample. The washing oil treatment is carried out by washing with an organic solvent which is a mixture of dichloromethane and methanol in a volume ratio of 9: 1. In the process of washing the oil, a Soxhlet extraction method is adopted for processing. And then drying the sample. For the drying process, it is preferable that the selected sample is placed in a vacuum chamber, set at 373.15K, and dried for 24 hours. If the environment is not vacuum environment, the phenomena that part of minerals are oxidized and the like can occur, so that the original sample is changed. The dried sample is weighed using an analytical balance, about 1-1.5g of sample is selected, and the mass M of the test sample is recorded. In this example, a weighed sample of the oil-bearing rock in the unconventional reservoir is filled into a chromatographic bottle to prepare for the next test, and preferably, the coil inner diameter of the nuclear magnetic resonance instrument is 10mm, so that a 2.5mL glass chromatographic bottle is used for containing the sample, but it is understood that chromatographic bottles with different sizes or materials can be used according to different instruments.
Step S5, using octamethylcyclotetrasiloxane as probe liquid, and testing the unconventional oil and gas reservoir oil-containing rock sample by using a low-field nuclear magnetic resonance freeze-thaw method to obtain the pore size distribution characteristics of the unconventional oil and gas reservoir oil-containing rock sample; the method specifically comprises the following steps: saturated octamethylcyclotetrasiloxane is used for an oil-containing rock sample of an unconventional oil and gas reservoir, then the central frequency of a permanent magnet is determined by using a sample with strong nuclear magnetic signals, the frequency of radio frequency signals is corrected, and the radio frequency pulse width is determined; testing alcohol by using a nuclear magnetic resonance freezing and thawing method, and determining a nuclear magnetic resonance signal intensity temperature correction coefficient lambda; determining the relation between the nuclear magnetic resonance signal intensity and the content of octamethylcyclotetrasiloxane; determining nuclear magnetic resonance experiment parameters of unconventional oil and gas reservoir rocks, setting a temperature plan, and measuring the unconventional oil and gas reservoir rock original shape by using a nuclear magnetic resonance freeze-thaw method.
In this example, the step of saturating the petroliferous rock with octamethylcyclotetrasiloxane in an unconventional oil and gas reservoir is as follows: processing by using a vacuum saturation device, placing an unconventional reservoir oil-containing rock sample in a vacuum box, and vacuumizing for 12 hours; adding octamethylcyclotetrasiloxane into the chromatographic bottle containing the vacuumized unconventional oil and gas reservoir rock sample, and balancing for 6 hours; centrifuge the chromatography vial for 2 h. The method has the advantages that the method is simple and convenient to process the oil-bearing rock of the unconventional oil and gas reservoir by using the vacuum saturation device, is convenient to operate, and preferably comprises the following specific steps: placing an unconventional reservoir oil-bearing rock sample in a vacuum box, and vacuumizing for 12 hours; and adding octamethylcyclotetrasiloxane liquid into a 2.5mL chromatographic bottle containing the vacuumized unconventional oil and gas reservoir rock sample, and balancing for 6 hours to fully saturate the unconventional reservoir oil-containing rock sample with octamethylcyclotetrasiloxane. As a preferred example, the centrifugation step is to centrifuge the 2.5mL chromatography vial using a centrifuge at 5000r/min for 2h to ensure adequate centrifugation. It will be appreciated that the time for the evacuation and equilibration and centrifugation may be routinely adjusted as required by the actual experiment. Then, a low-field nuclear magnetic resonance freeze thawing method is used for testing, firstly, the instrument is subjected to conventional parameter correction, wherein the conventional parameter correction comprises center frequency, 90-degree pulse width, 180-degree pulse width and the like, in the embodiment, the 90-degree pulse width is 3.4 mu s, the 180-degree pulse width is 7.2 mu s, then, CPMG sequence parameters for analyzing the sample to be tested by using the low-field nuclear magnetic resonance freeze thawing method are set according to experimental requirements, wherein the CPMG sequence parameters comprise the waiting time of 2500ms, the echo time of 1ms, the number of echoes of 2000 and the scanning times of 32, a temperature plan is set, and finally, the analysis is started. Taking the oil-containing tight sandstone as an example, the test result is that the pore-size distribution curve of the oil-containing tight sandstone is shown in figure 4. It can be seen from fig. 4 that the pores of unconventional oil and gas reservoir rock are mainly nano-scale pores, and the nano-scale pores of the tight sandstone are rich in a large amount of crude oil. For research on compact sandstone oil and gas, the pore size distribution characteristic is very important.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
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| CN115876825A (en) * | 2021-09-28 | 2023-03-31 | 中国石油化工股份有限公司 | Method for determining self-absorption rule in nano-pores by utilizing nuclear magnetic resonance instrument and application |
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| CN119846008A (en) * | 2025-01-08 | 2025-04-18 | 南京大学 | Method for measuring porosity of micro-dense rock crushed sample based on amphiphilic saturated liquid nuclear magnetic resonance method |
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Inventor after: Yao Suping Inventor after: Xiao Hanmin Inventor after: Pei Wenming Inventor after: Hu Wenxuan Inventor after: Cao Jian Inventor after: Qin Yang Inventor after: Liu Xuewei Inventor before: Yao Suping Inventor before: Pei Wenming Inventor before: Hu Wenxuan Inventor before: Cao Jian Inventor before: Qin Yang Inventor before: Xiao Hanmin Inventor before: Liu Xuewei |
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Effective date of registration: 20201104 Address after: Nanjing University (Xianlin Campus), 163 Xianlin Avenue, Qixia District, Nanjing City, Jiangsu Province Applicant after: NANJING University Applicant after: Exploration and Development Research Institute of China National Petroleum Corp. Address before: Nanjing University (Xianlin Campus), 163 Xianlin Avenue, Qixia District, Nanjing City, Jiangsu Province Applicant before: NANJING University |
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