CN112577979B - Quantitative analysis device and method for rock internal fluid saturation spatial distribution - Google Patents
Quantitative analysis device and method for rock internal fluid saturation spatial distribution Download PDFInfo
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Abstract
The invention belongs to the technical field of petroleum research. Aiming at the current situation that the quantitative analysis of the fluid saturation distribution in the current rock does not reach the pore size, the invention provides a quantitative analysis device and a method for the fluid saturation spatial distribution in the rock, wherein the rock core three-dimensional imaging D1, D2, D1 and D2 are three-dimensionally registered when no liquid is injected into the rock core under the overpressure state; d3 matched with D1 is obtained by sampling D2; determining a threshold T1 based on the overburden porosity, converting D1 into a void and matrix binary matrix; determining a threshold T2 based on the total fluid saturation, and modifying the value of a corresponding pixel in D1 according to the gray level of a gap pixel in D3; extracting a pore network based on the D1 pore pixels to obtain pores, pore throats and corresponding pixel lists; counting the pixel number and the total pixel number of the two fluids in the pore and the pore throat to obtain the saturation of the pore fluid; and counting the number, the spatial distribution and the relation with the shape factor of the pores with different fluid saturation degrees. The method can nondestructively and quantitatively obtain the spatial distribution of the fluid saturation of the internal pore scale of the rock core.
Description
Technical Field
The invention belongs to the technical field of petroleum research, relates to a testing technology of fluid saturation in rocks, and particularly relates to a quantitative analysis device and method for spatial distribution of fluid saturation in rocks.
Background
Hydrocarbon-bearing reservoir rock is a typical porous medium. In the process of forming the oil gas reservoir, the oil gas is generated in the hydrocarbon source rock and then is transported to the reservoir through the transmission and conduction system under the action of a certain driving force, and original saturated water in pores is displaced. In the oil and gas development process, the oil and gas recovery rate is improved by adopting modes of water injection, gas injection displacement and the like. Due to the influence of factors such as the complexity of a pore structure and the interaction between the fluid and the wall surface of the rock, a certain fluid cannot be completely drained. Thus, the rock pores typically contain two or more fluids, such as oil-water, water-gas, or oil-gas-water. Fluid saturation is a physical quantity describing the degree of fluid filling in the rock pores of a reservoir and is one of the key characteristic parameters for evaluating the reservoir.
When a plurality of fluids are simultaneously present in the pores, a certain flowThe volume percentage of the body is the saturation of the fluid. As the name implies, fluid saturation is the ratio of the volume of fluid to the volume of pores, in the case of water, Swater＝Vwater/Vpore. Common measurement methods include solvent distillation, pyrolysis. Solvent distillation method comprises washing oil with suitable solvent, heating and distilling to determine original mass, and water and oil mass M obtained by extractionwaterAnd MoilFurther, the volume-V occupied by oil, water and gas is obtainedoil、Vwater、Vgas(Vpore＝Voil+Vwater+Vgas) Divided by the pore volume V measured against the reference standardporeAnd obtaining the fluid saturation S. The pyrolysis method adopts different temperature intervals to evaporate water and oil respectively, and collects and reads the volume of the oil and the water through condensation. The above method measures the bulk fluid saturation of the reservoir rock.
Nuclear magnetic resonance techniques can also be used to measure fluid saturation. The nuclear magnetic resonance can measure the signals of the water rich in hydrogen nuclei 1H in the rock, and the water contents of pores with different pore diameters (corresponding to different T2 relaxation times) are obtained. Taking fig. 1 as an example, T2 relaxation time is related to the pore size of rock pores, and the curve is the content of hydrogen-containing core 1H fluid in pores with different pore sizes. When the rock pore space is saturated with water, it is measured as the ratio of pore volumes of different sizes to the total pore volume in the core. When multiple phases (e.g., gas-water) coexist, the relative content of the portion of fluid with nuclear magnetic signals (e.g., hydrogen-containing nuclei 1H) in the pores of different pore sizes is obtained. Three-dimensional images can be obtained through magnetic resonance imaging, and due to the fact that the resolution is low (the maximum is hundreds of microns), only the relative quantity of fluids with nuclear magnetic signals at different positions in space can be approximately obtained, and the pores are difficult to distinguish, and therefore information of the fluid saturation (the percentage of the fluid volume to the pore volume and the pore throat volume) in the pores and the pore throats at different positions in three-dimensional space in the rock cannot be obtained.
In addition to nuclear magnetic resonance, X-ray Computed Tomography (CT) techniques can also obtain three-dimensional images of rock without loss. The result is the X-ray attenuation coefficient or absorption coefficient for each voxel, black for low absorption regions corresponding to low density regions, and white for relatively high absorption regions corresponding to high density regions. CT has a high density resolution capability and can resolve relatively small density differences. The principle and result are shown in fig. 2, and three-dimensional digital images of rocks with resolution up to submicron can be obtained by using microfocus X-ray CT.
Fig. 3 shows a CT scanning cross-sectional image of a sandstone, the left image is an XY cross-section at a position shown by a horizontal line in the right image, and the right image is an XZ cross-section at a position shown by a vertical line in the left image. Where black is the low density region corresponding to the void (where there is no liquid in the rock) and the rest is the rock skeleton.
When the rock pore contains water or oil, the fluid part and the pore are difficult to distinguish in the gray scale image obtained by CT scanning because the absorption coefficient of the oil and the water to X-ray is low. The X-ray absorption liquid has a high absorption coefficient to X-rays after being prepared into a solution with a certain mass fraction by respectively adding iododecane or potassium iodide, sodium iodide, cesium chloride and other substances into oil or water, and the gray level of the corresponding part of the obtained CT image is obviously different from that of the gap.
And scanning by comparing the rock cores without the fluid and with the solution, comparing the results of the two times, analyzing the gray difference of corresponding pixels, determining the voxel occupied by the solution, and performing three-dimensional visualization rendering.
Chinese patent document CN102183532B discloses a method for improving the accuracy of CT measurement of fluid saturation. According to the method, CT values of fault planes of the rock core under the conditions of dryness and water containing inside and outside the rock core holder and CT values of saline water outside the holder, experimental oil and peripheral air are measured, the CT value of the saline water in the holder is calculated through a formula, the CT value of the oil in the holder is calculated by adopting the bound water saturation obtained by measuring through a metering method, and finally the oil-water saturation at different moments in the displacement process is calculated. The method tests the integral two-phase saturation of the rock core and has no spatial distribution result.
Chinese patent document CN107271460A discloses a quantitative characterization method for the spatial distribution of the internal moisture saturation change of porous material. The method comprises the steps of respectively carrying out CT tests before and after the moisture change of the same sample to obtain corresponding three-dimensional gray scale data, determining the corresponding relation of cubes with certain sizes in two groups of data by an ectopic digital volume correlation method, calculating the difference of gray scale average values of the two cubes (the change of voxel gray scale is caused by the change of water content), and defining the ratio of the gray scale average values to the gray scale of water as the change of saturation; for different voxels in the three-dimensional data volume, the saturation change of a cube centered on the voxel is calculated, and the spatial distribution of the saturation change can be obtained. Unlike the definition of "saturation" in the art to which this patent pertains. As can be seen from the calculation process, the saturation of the former is the water content in a certain sub-block, there is no concept of pore volume, not the ratio of water containing volume to pore volume, and the emphasis is on the relative change in saturation, not its absolute value. In addition, there are some problems in actual operation: 1) due to the influence of imaging quality and resolution, the gray value of water at different positions on the space may have certain variation, and is difficult to be represented by a single value; 2) due to potential factors such as instrument stability and noise during two scanning periods, even though a certain deviation may exist in the overall gray value under the condition of unchanged moisture, the method for determining the moisture change by adopting direct gray subtraction may also include the deviation, so that the false deviation is caused, and the method is not practical for directly representing the absolute value of the saturation.
Chinese patent document CN104076046A discloses a method for collecting and quantitatively characterizing micro-distribution images of residual oil in porous media. The method comprises the steps of identifying connected sub-blocks of the residual oil by determining the gray value of the residual oil after oil displacement with water, and calculating parameters such as the total number of the sub-blocks, the average volume of different residual oil sub-blocks, the contact area and the like. The communicating sub-blocks may occupy a plurality of pores and pore throats and do not involve calculation of fluid saturation.
The pore network model is an abstraction and simplification of the void space within the porous media; pore means the relatively large portion of the void space, the narrow portion of the connection between two pores being the throat. The pore network model comprises characteristics of pore size distribution, pore connectivity (coordination number, number of throats communicated with pores), and pore shape factor (shape of pore cross section) of the interior of the porous medium. Based on the pore network model, assuming that pores are spaces for storing fluid and pore throats are main channels for flowing, single-phase or multi-phase flow simulation can be carried out by giving corresponding pore throat flow relations, and the efficiency is higher than that of direct simulation. Methods including central axis, maximum axis, etc. have been developed to extract the corresponding pore network model for binary digital images. FIG. 4 shows an example of pore and pore throat relationships and a model of void space extraction pore network.
Chinese patent document CN107271460A discloses a quantitative characterization method for digital core two-phase flow simulation results based on erosion-expansion algorithm. The method comprises the steps of obtaining a digital core geometric structure file (a three-dimensional binary matrix) and a two-phase flow simulation result (two phases correspond to 2 three-dimensional matrixes, and each element represents the saturation of a corresponding fluid phase in a corresponding voxel); extracting a pore network (a network formed by mutually communicating pores and throats) from the geometric structure file by adopting an erosion-expansion algorithm; calculating the two-phase fluid saturation of each pore by summing and averaging the saturations of the pixel points corresponding to the pore throats; the average saturation of pores of the same diameter was counted. The method aims at fluid saturation data obtained by numerical simulation after the internal pore space of the rock is digitalized, and essentially performs space identification and quantitative statistics on three groups (geometrical structure, fluid 1 saturation and fluid 2 saturation) of three-dimensional matrix data with determined values. The method is characterized in that a liquid filling or displacement process of an actual rock is an image with continuous gray scale, and the problems that how to obtain digital images of different liquid injection states of an actual rock sample, how to divide rock skeletons, gaps, different phase fluids, how to determine thresholds and how to determine the basis are not disclosed in the invention.
Disclosure of Invention
The current situation that the quantitative analysis of the fluid saturation distribution in the rock does not reach the pore size, and mainly takes the overall or local average of the rock as the main situation. The technical problem to be solved or the effect to be achieved in practice by the invention is that the spatial distribution of the fluid saturation in the rock core, specifically the two-phase fluid saturation in pores and pore throats with different spatial positions and sizes, can be obtained nondestructively and quantitatively.
The technical scheme adopted by the invention is as follows:
a quantitative analysis device for the fluid saturation spatial distribution in rock comprises an X-ray three-dimensional microscopic imaging system,
the X-ray three-dimensional microscopic imaging system comprises a ray source, a clamp holder, a sample table and a receiver, wherein the ray source and the receiver are respectively and fixedly installed after adjusting relative positions, a rock sample is arranged in a cylindrical shape and is installed in the clamp holder penetrated by X-rays, the clamp holder is fixedly installed on the sample table, and the sample table rotates to control the clamp holder to rotate; during imaging, the holder rotates along with the sample table, the receiver records images at different angles, and a three-dimensional data body is established through a three-dimensional reconstruction algorithm.
The core fluid injection system is provided with a ring pressure loading cavity and an axial pressure loading cavity on the holder respectively, the ring pressure loading cavity and the axial pressure loading cavity are connected with corresponding plunger pumps through flexible pipelines respectively, and fluid is injected through the corresponding plunger pumps for pressurization; the ring pressure loading cavity is provided with an inlet and an outlet, and the heat exchange between the ring pressure fluid and the constant temperature bath is realized through the constant temperature circulating pump so as to achieve the purpose of constant temperature control. The system supports separate control of the magnitude of the axial pressure and the hoop pressure of the load and can operate at a specified temperature different from room temperature. The flexible pipeline can ensure that the clamp holder rotates at 360 ℃ along with the sample table.
Furthermore, the two ends of the holder are provided with flexible pipelines communicated with rocks, the upstream is connected with an injection pump, the injection pump is provided with a meter for measuring injected fluid, the upstream pipeline and a heating belt are wrapped in a heat insulation pipe side by side, the flexible pipeline is provided with a temperature sensor for realizing the constant temperature control of the upstream pipeline in combination with a constant temperature controller, the injection pump is used for injecting certain fluid independently or mixing the fluid according to a certain proportion, the upstream and the downstream of the pipeline are respectively provided with a pressure sensor for recording the upstream and the downstream pressures of the rocks, the upstream and the downstream of the flexible pipeline are provided with a differential pressure sensor for recording the pressure difference between the upstream and the downstream of the rocks, the outlet of the flexible pipeline is provided with a back pressure valve for controlling the outlet pressure, the rear end of the back pressure valve is sequentially provided with a three-phase separator and a meter for measuring the outlet fluid after multi-phase separation, the back pressure valve is connected with the plunger pump, and outlet pressure control is carried out through the plunger pump. The system supports flow metering of different fluids at the inlet end and the outlet end, and provides basis for threshold segmentation of fluid filling pixels of a scanned image after fluid injection. Inlet end thermostating control can perform tests at specified temperatures other than room temperature.
Further, the holder adopts PEEK, polyimide or carbon fiber material preparation to form, possesses the X ray penetrability when can satisfying confined pressure, axle load and fluid pressure intensity. Although the conventional metal holder has high strength, the conventional metal holder has strong X-ray absorption and influences the imaging quality of a core in the holder. Generally, to achieve higher imaging resolution, the holder is required to be as small in size as possible and to have as thin a wall thickness as possible; to achieve higher pressure resistance, the strength of the holder is required to be higher; the appropriate gripper material and size design can be selected based on the imaging resolution of the rock, the upper pressure limit for loading, and the strength properties of the material.
A quantitative analysis method for rock internal fluid saturation spatial distribution is characterized in that a quantitative analysis device for rock internal fluid saturation spatial distribution is used for data acquisition and then data analysis, and the method specifically comprises the following steps:
(1) firstly, placing a dried rock sample without liquid in pores in a holder, applying certain annular pressure and axial pressure, and after the rock sample is stabilized, carrying out CT scanning to obtain a 1 st three-dimensional data volume, D1, wherein the pixel gray scale is positively correlated with the X-ray absorption capacity; in the state, no liquid exists in pores, the X-ray absorption capacity is weak, the gray value is low, the X-ray absorption capacity is easy to distinguish with matrix pixels, the extraction of a pore network is carried out based on the data body, the influence of fluid can be eliminated, and a space geometric model representing the connection of the pores and pore throats of rocks is constructed;
(2) injecting a certain fluid or two fluids into the rock sample at a certain flow rate or a certain pressure, and after the fluids are stabilized, carrying out CT scanning to obtain a 2 nd three-dimensional data volume D2, wherein the pixel gray scale is positively correlated with the X-ray absorption capacity; a compound with strong X-ray absorption capacity is added into the fluid, and after entering the pores, the X-ray absorption capacity of the pore pixels is enhanced, and the gray value is increased;
the actual rock has certain elasticity, and because the rock is subjected to ring compression and axial compression, the pores may be subjected to certain changes, the positions of skeleton particles are also subjected to displacement, and in order to ensure that two data bodies are closer, the D1 data in the step (1) must be acquired under an overburden state. If the acquisition is carried out in a non-overbalanced state, difficulty and workload are increased for subsequent treatment, and the expected effect cannot be achieved.
Further, a soluble compound which is strong in X-ray absorption is added into one fluid in the step (2). Common experimental fluids such as water have poor X-ray absorption, are difficult to distinguish significantly from gases in CT scans, have low signal-to-noise ratios, and therefore require the addition of a compound that is soluble and strongly X-ray absorbing to distinguish between different fluids. (3) For three-dimensional registration of the D1 and D2 data volumes, due to the precision of the device and slight movement of a clamp or a sample during acquisition, the two data volumes may correspond to the same pixel but not the same position of the sample, an image registration algorithm is adopted for three-dimensional registration of the D1 and D2 data volumes, and a D2 value corresponding to each pixel point in the D1 data volume is obtained through a resampling algorithm, wherein the data volume is named as D3, namely, each pixel in the D1 and D3 data volumes corresponds to the same point in three-dimensional space in rocks; due to the high precision of three-dimensional microscopic imaging, any slight mechanical motion or change in the attitude of the sample may cause the spatial coordinates of each part in the sample to change between two scans. Directly carrying out difference analysis on the three-dimensional matrix obtained by two times of scanning, which may bring large errors due to dislocation and influence the final analysis precision, so that three-dimensional space registration is necessary and resampling is carried out to a uniform grid;
(4) for a D1 data volume, in order to reduce the uncertainty of threshold division, determining a division threshold T1 of a gap and rock skeleton pixel by taking helium gas porosity measurement under the same overpressure state as constraint, so that the proportion of the divided number of the pore pixels to the total number of the rock is consistent with the helium gas porosity measurement, and converting D1 data into a three-dimensional binary matrix, wherein the gap is 1, and the skeleton is 0; for skeleton pixels in the D1 data, the value of the corresponding pixel in D3 is set to 0;
(5) for the D3 data volume, determining a two-phase fluid dividing threshold value T2 by taking the fluid saturation in a sample determined by the difference between the two-phase fluid volume injected at the inlet end and the two-phase fluid volume metered at the outlet end as constraint, and for all pixels with the value of 1 in the D1 data volume, if the gray scale of the corresponding pixel in the D3 is greater than the threshold value T2, changing the pixel value in the D1 to 2; based on actually measured fluid saturation data in a rock sample, the precision of two-phase fluid threshold division is improved, and the ratio of the number of pixels with strong X-ray absorption capacity to the total pore pixel number obtained based on the threshold division is ensured to be consistent with the corresponding fluid saturation;
(6) for D1 data, taking a pixel with the value of 1 or 2 as a gap pixel, extracting a pore network by adopting a central axis-maximum sphere pore network extraction algorithm to obtain a plurality of pores and pore throats, obtaining parameters of the pores and the pore throats and a topological connection relation between the pores and the pore throats, and establishing a mapping relation between the pores and the pore throats and corresponding pixels in a D1 data body; the existing pore network extraction method does not provide the mapping relation, and the establishment of the mapping relation provides convenience for counting the number of pixels occupied by the fluid with strong X-ray absorption capacity in each pore;
(7) traversing all pores and pore throats, determining all corresponding pixels in a D1 data volume based on a mapping relation, counting the number of pixels with the value of 1 and the total number of pixels, wherein the ratio of the number of pixels to the total number of pixels is the saturation S1 of the fluid with less X-ray absorption in the pores/pore throats, and the volume occupied by the fluid is obtained by multiplying the number of pixels by the cubic power of the pixel size; 1-S1 is the saturation of the other fluid in the pore/throat S2, and the third power of the pixel size multiplied by the number of pixels is the volume occupied by the other fluid; the D1 data volume after threshold segmentation only gives information that each pixel point in the space contains fluid, and the data volume is too large and is difficult to perform correlation analysis with the characteristics of the void space; by combining the pore network extracted based on D1 data, the percentage of fluid in each pore and pore throat can be further determined, and key basic data are provided for analyzing the correlation between the fluid injection characteristics and the connectivity and geometric parameters of the pores and pore throats;
(8) on the basis of determining the fluid saturation distribution in each pore and pore throat, the method can further combine the space coordinates, the shape factors and the fluid saturation parameters of the pore and the pore throat to obtain more statistical distribution characteristics and know the characteristics of the fluid injected into the void space in more directions. The statistics S2 are respectively the number distribution of pores with intervals of 0-100% and 10%, the ratio variation of the number of pores with the S2 larger than a certain value to the total number of pores in the flow direction and the vertical flow direction is counted, and the S2 average value of the pores with different shape factors is counted.
Further, the parameters of the pore space and the pore throat in the step (6) are the diameter, the volume, the shape factor, the central three-dimensional coordinate, the diameter, the length, the volume and the shape factor of the pore space. The above parameters are the main characteristic parameters of the pore network.
Further, in the step (6), the apertures and the throats are respectively numbered by sequentially increasing numbers, each number corresponds to a two-dimensional matrix, each row has three elements, the three-dimensional coordinates of the apertures or the throats in the D1 data volume are provided, and the total number of the rows is equal to the number of the pixels corresponding to the apertures or the throats. Based on the mapping relation, all pixels corresponding to the pore/pore throat can be quickly and accurately found, and great convenience is provided for counting the fluid saturation of the pore/pore throat.
The invention has the beneficial effects that:
(1) by the simultaneous operation of the X-ray three-dimensional microscopic imaging system and the core fluid injection system, the image data of fluid injected into a sample loaded by bearing axial and annular pressure and distributed in a gap under a certain temperature condition at a certain pressure or flow can be obtained, the function is more comprehensive, the scanning device is closer to the actual underground condition of rock under the existing non-overburden pressure state scanning, and the data on a three-dimensional space can be obtained compared with the cross-section scanning.
(2) The matching degree of imaging data of different times can be improved through image registration and resampling, the influence of inevitable mechanical motion and sample posture change on the identification precision of injected fluid is reduced, the imaging precision of ten micrometers can be guaranteed, and the gap and skeleton division threshold value is restricted through helium gas measuring porosity under the same overpressure state, the two-phase fluid division threshold value is restricted through inlet and outlet fluid quantity difference, and the like, so that the gap and fluid identification precision is higher.
(3) Extracting a pore network geometric model from a scanning result of a liquid-free sample, establishing a mapping relation between pores/pore throats and corresponding pixels, and counting the volumes and the saturations of all injected fluids in the pores/pore throats by combining the results of imaging after the fluid is injected, registering, resampling and threshold dividing, so that the distribution analysis of the saturations of the fluids in the rock reaches pore size and quantification, and spatial differences can be identified. On the one hand, necessary quantitative data can be provided for understanding the influence of the pore structure on the fluid distribution; on the other hand, more comprehensive comparison and verification data are provided for numerical simulation of rock pore-scale two-phase flow, the microscopic mechanism of multi-phase flow in the rock is further understood, and technical support and foundation are further provided for establishing a more reasonable and effective macroscopic flow model.
Drawings
FIG. 1 is an exemplary graph of T2 relaxation time spectra of prior art hypotonic rocks;
FIG. 2 is a diagram of the principle and result of X-ray three-dimensional microscopic imaging in the prior art;
FIG. 3 is a sectional view of CT scan of sandstone in the prior art;
FIG. 4 is an exemplary graph of pore and throat relationship in a prior art system;
FIG. 5 is a schematic structural diagram of a quantitative analysis device for the spatial distribution of fluid saturation in rock according to the present invention;
FIG. 6 is a schematic flow chart of a quantitative analysis method of the rock internal fluid saturation spatial distribution according to the present invention;
FIG. 7 is a graph of injection flow distribution for different injection flow rates in accordance with the present invention;
FIG. 8 is a graph showing the injection amount distribution of injection fluid in different pores at different injection flow rates according to the present invention;
FIG. 9 is a graph of the number of pores at different saturations for different injection flow rates in the present invention;
FIG. 10 is a volume fraction of the present invention with more fully filled pores in the direction of injection;
FIG. 11 is a graph of pore throat shape factor versus injection fluid ratio in accordance with the present invention;
wherein, 1, an injection pump; 2. a holder; 3. a rock sample; 4. a pressure sensor; 5. a receiver; 6. a radiation source; 7. a back pressure valve; 8. a three-phase separator.
Detailed Description
The invention is further described below with reference to the accompanying drawings.
Example 1
As shown in fig. 5, a quantitative analysis device for the spatial distribution of fluid saturation in rock comprises an X-ray three-dimensional microscopic imaging system and a core fluid injection system,
the X-ray three-dimensional microscopic imaging system comprises a ray source 6, a clamp holder 2, a sample table and a receiver 5, wherein the ray source 6 and the receiver 5 are respectively and fixedly installed after adjusting relative positions, a rock sample 3 is arranged in a cylindrical shape and is installed in the clamp holder 2 through which X-rays penetrate, the clamp holder 2 is fixedly installed on the sample table, and the sample table rotates to control the clamp holder 2 to rotate; during imaging, the holder 2 rotates along with the sample table, the receiver 5 records images at different angles, and a three-dimensional data volume is established through a three-dimensional reconstruction algorithm;
in another embodiment provided by the invention, the core fluid injection system is further provided, wherein the holder 2 of the core fluid injection system is respectively provided with a ring pressure loading cavity and an axial pressure loading cavity, the ring pressure loading cavity and the axial pressure loading cavity are respectively connected with corresponding plunger pumps through flexible pipelines, and fluid is injected through the corresponding plunger pumps for pressurization; the ring pressure loading cavity is provided with an inlet and an outlet, and the heat exchange between the ring pressure fluid and the constant temperature bath is realized through the constant temperature circulating pump so as to achieve the purpose of constant temperature control.
In another embodiment provided by the invention, two ends of the holder 2 are provided with flexible pipelines communicated with rocks, the upstream is connected with the injection pump 1, a certain fluid is injected independently or mixed according to a certain proportion through the injection pump 1, the injection pump 1 is provided with a meter for metering the injected fluid, the upstream pipeline and the heating belt are wrapped in a heat-insulating pipe side by side, a temperature sensor is arranged on a circuit and is combined with a constant temperature controller to realize the constant temperature control of the upstream pipeline, the upstream and the downstream of the pipeline are respectively provided with a pressure sensor 4, the upstream and the downstream of the rocks are recorded by the pressure sensors 4, the upstream and the downstream of the pipeline are provided with differential pressure sensors, the pressure difference between the upstream and the downstream of the rocks is recorded by the sensors, the outlet of the pipeline is provided with a back pressure valve 7, the outlet pressure is controlled by the back pressure valve 7, and the rear end of the back pressure valve 7 is sequentially provided with a three-phase separator 8 and a meter, the outlet fluid is respectively metered after multiphase separation through a metering device, the back pressure valve 7 is connected with the plunger pump, and outlet pressure control is carried out through the plunger pump.
The clamp holder 2 is made of PEEK, polyimide or carbon fiber materials, and has X-ray penetrability while meeting the requirements of confining pressure, axial pressure and fluid pressure strength. The strength of the conventional metal holder 2 is high, but the absorption of X-rays is strong, so that the imaging quality of the rock core in the holder 2 is influenced.
In another embodiment provided by the present invention, as shown in fig. 5 and 6, a method for quantitatively analyzing spatial distribution of fluid saturation in rock, which includes the following steps:
(1) firstly, putting a dried rock sample 3 without liquid in pores into a holder 2, applying certain annular pressure and axial pressure, and after the rock sample is stabilized, carrying out CT scanning to obtain a 1 st three-dimensional data volume D1, wherein the pixel gray scale is in positive correlation with the X-ray absorption capacity;
(2) injecting a certain fluid or a plurality of fluids into the rock sample 3 at a certain flow rate or a certain pressure, adding a soluble compound which has strong X-ray absorption into one fluid, and after the mixture is stabilized, carrying out CT scanning to obtain a 2 nd three-dimensional data volume, D2, wherein the pixel gray scale is positively correlated with the X-ray absorption capacity;
since the rock may have some variation in compression after ring compression and axial compression, the D1 data must be collected in the overburden state in step (1) to ensure that the two data volumes are closer together. If the acquisition is carried out in a non-overbalanced state, difficulty and workload are increased for subsequent treatment, and the expected effect cannot be achieved.
(3) For three-dimensional registration of the D1 and D2 data volumes, due to the precision of the device and slight movement of a clamp 2 or a sample in the acquisition process, the two data volumes may correspond to the same pixel but not to the same position of the sample, the three-dimensional registration of the D1 and D2 data volumes is carried out by adopting an image registration algorithm, and a D2 value corresponding to each pixel point in the D1 data volume is obtained by a resampling algorithm, wherein the data volume is named as D3, namely, each pixel in the D1 and D3 data volumes corresponds to the same point in three-dimensional space in rock;
(4) for a D1 data body, determining a partition threshold T1 of a gap and rock skeleton pixel by taking helium porosity measurement under the same overpressure state as constraint, so that the proportion of the partitioned pore pixel number to the total rock pixel number is consistent with the helium porosity measurement, and converting D1 data into a three-dimensional binary matrix, wherein the gap is 1, and the skeleton is 0;
(5) for the D3 data volume, determining a two-phase fluid dividing threshold value T2 by taking the fluid saturation in a sample determined by the difference between the two-phase fluid volume injected at the inlet end and the two-phase fluid volume metered at the outlet end as constraint, and for all pixels with the value of 1 in the D1 data volume, if the gray scale of the corresponding pixel in the D3 is greater than the threshold value T2, changing the pixel value in the D1 to 2;
(6) extracting a pore network by using a pixel with the value of 1 or 2 as a pore pixel and adopting a central axis-maximum sphere pore network extraction algorithm for D1 data to obtain a plurality of pores and pore throats, and obtaining the diameters, the volumes, the shape factors, the central three-dimensional coordinates, the diameters, the lengths, the volumes, the shape factors and the topological connection relations of the pores and the pore throats; meanwhile, the pores and the pore throats are respectively numbered by sequentially increasing numbers, each number corresponds to a two-dimensional matrix, each row has three elements respectively, the three-dimensional coordinates of the pores or pore throat pixels in the D1 data volume, and the total row number is equal to the pixel number corresponding to the pores or the pore throats;
(7) traversing all pores and pore throats, determining all corresponding pixels in a D1 data volume based on a mapping relation, counting the number of pixels with the value of 1 and the total number of pixels, wherein the ratio of the number of pixels to the total number of pixels is the saturation S1 of the fluid with less X-ray absorption in the pores/pore throats, and the volume occupied by the fluid is obtained by multiplying the number of pixels by the cubic power of the pixel size; 1-S1 is the saturation of the other fluid in the pore/throat S2, and the third power of the pixel size multiplied by the number of pixels is the volume occupied by the other fluid;
(8) the statistics S2 are respectively the number distribution of pores with intervals of 0-100% and 10%, the ratio variation of the number of pores with the S2 larger than a certain value to the total number of pores in the flow direction and the vertical flow direction is counted, and the S2 average value of the pores with different shape factors is counted.
The testing of the fluid injection online imaging of the sandstone sample is carried out on a core multiphase flow online three-dimensional microscopic imaging system self-researched by the mechanical research institute of the Chinese academy of sciences. The maximum loading axial pressure and the annular pressure of the system are 35MPa, the upper limit of the fluid injection pressure is 35MPa, the upper limit of the constant flow injection flow rate is 30ml/min, the experimental temperature range is from room temperature to 100 ℃, the diameter of a clamp sample is 25mm, the length is 20-75 mm, the maximum withstand voltage of a clamp 2 is 15MPa, the imaging resolution is 14.96 mu m, the imaging size is 1920 x 1536 pixels, the imaging data is stored by 16 bits, the gray scale value is 0-65535, the output voltage of a radiation source 6 is 170kV, the current is 110 mu A, the exposure time is 0.4s, 1440 frames of scanning are carried out on each sample, and about 1 hour is consumed by single scanning. Scanning yielded three-dimensional grayscale images of samples that were dry under pressure and stabilized after saline injections (1ml/min and 10ml/min) at different rates. By the method, images of different times are registered, threshold segmentation is respectively carried out, pixels occupied by saline are determined, and three-dimensional rendering display is carried out, so that the result is shown in fig. 7, specifically, fig. 7a is an injection fluid distribution diagram under a small injection flow rate; fig. 7b is a graph of injection flow distribution at a large injection flow rate.
With the method of the present invention, the injection volume fraction of fluid in each pore and pore throat of the extracted pore network is obtained, see fig. 8, where each sphere represents a pore and the sphere size represents the absolute amount of injected fluid.
Based on this quantitative analysis method, the pore distribution characteristics of the fluid injection of the tested sample were further analyzed. The fluid saturation of individual pores was found to be distributed mainly in the two intervals 10-40% and 90-100%. The injection flow rate is reduced so that more pore fluid is injected more fully (saturation > 90% and above, as shown in fig. 9). When the high-flow rate injection is carried out, the pore injection near the inlet end is relatively sufficient, and the injection is stable after the rapid attenuation towards the outlet direction; the low flow rate injection advances farther (as shown in fig. 10).
The effect of the shape factor on the fluid injection ratio was further analyzed. The pore space of the sample is mainly triangular section, and the pore throats of the triangular section and the square section are equivalent in number. The larger the pore throat form factor, the higher the injection fluid fraction of the connected pores. Reducing the injection flow rate allows for more complete fluid injection for smaller form factor apertures (as shown in fig. 11).
The above description is not meant to be limiting, it being noted that: it will be apparent to those skilled in the art that various changes, modifications, additions and substitutions can be made without departing from the true scope of the invention, and these improvements and modifications should also be construed as within the scope of the invention.
Claims (7)
1. A quantitative analysis method for rock internal fluid saturation spatial distribution adopts a quantitative analysis device for rock internal fluid saturation spatial distribution to carry out data acquisition and then data analysis, and is characterized by comprising the following steps:
(1) firstly, placing a dried rock sample without liquid in pores in a holder, applying certain annular pressure and axial pressure, and after the rock sample is stabilized, carrying out CT scanning to obtain a 1 st three-dimensional data volume, D1, wherein the pixel gray scale is positively correlated with the X-ray absorption capacity;
(2) injecting a certain fluid or two fluids into the rock sample at a certain flow rate or a certain pressure, and after the fluids are stabilized, carrying out CT scanning to obtain a 2 nd three-dimensional data volume D2, wherein the pixel gray scale is positively correlated with the X-ray absorption capacity;
(3) carrying out three-dimensional registration on the D1 and D2 data volumes, carrying out three-dimensional registration on the D1 and D2 data volumes by adopting an image registration algorithm, and obtaining a D2 value corresponding to each pixel point in the D1 data volume by adopting a resampling algorithm, wherein the data volume is named as D3, namely, each pixel in the D1 and D3 data volumes corresponds to the same point in three-dimensional space in the rock;
(4) for a D1 data body, determining a partition threshold T1 of pores and rock skeleton pixels by taking helium porosity measurement under the same overpressure state as constraint, so that the proportion of the number of the partitioned pores to the total number of the rock pixels is consistent with the helium porosity measurement, and converting D1 data into a three-dimensional binary matrix, wherein the pores are 1, and the skeleton is 0; for skeleton pixels in the D1 data, the value of the corresponding pixel in D3 is set to 0;
(5) for the D3 data volume, determining a two-phase fluid dividing threshold value T2 by taking the fluid saturation in a sample determined by the difference between the two-phase fluid volume injected at the inlet end and the two-phase fluid volume metered at the outlet end as constraint, and for all pixels with the value of 1 in the D1 data volume, if the gray scale of the corresponding pixel in the D3 is greater than the threshold value T2, changing the pixel value in the D1 to 2;
(6) taking a pixel with the value of 1 or 2 as a pore pixel for D1 data, extracting a pore network by adopting a central axis-maximum sphere pore network extraction algorithm to obtain a plurality of pores and pore throats, obtaining parameters of the pores and the pore throats and a topological connection relation between the pores and the pore throats, and establishing a mapping relation between the pores and the pore throats and corresponding pixels in a D1 data body;
(7) traversing all pores and pore throats, determining corresponding pixels in a D1 data volume based on a mapping relation, counting the number of pixels with the value of 1 and the total number of pixels, wherein the ratio of the number of pixels to the total number of pixels is the saturation S1 of the fluid with less X-ray absorption in the pores/pore throats, and the volume occupied by the fluid is obtained by multiplying the number of pixels by the cubic power of the pixel size; 1-S1 is the saturation of the other fluid in the pore/throat S2, and the third power of the pixel size multiplied by the number of pixels is the volume occupied by the other fluid;
(8) the statistics S2 are respectively the number distribution of the pores with the intervals of 0-100% and 10%, the ratio variation of the number of the pores with the S2 larger than a certain value to the total number of the pores in the flow direction and the vertical flow direction is counted, and the S2 average value of the pores with different shape factors is counted;
the quantitative analysis device for the fluid saturation spatial distribution in the rock is characterized by comprising an X-ray three-dimensional microscopic imaging system,
the X-ray three-dimensional microscopic imaging system comprises a ray source, a clamp, a sample table and a receiver, wherein the ray source and the receiver are correspondingly arranged; the rock sample is arranged in a cylindrical shape and is arranged in a holder which is penetrated by X rays, the holder is fixedly arranged on the sample table, and the rotation of the holder is controlled by the rotation of the sample table; during imaging, the holder rotates along with the sample table, the receiver records images at different angles, and a three-dimensional data body is established through a three-dimensional reconstruction algorithm.
2. The quantitative analysis method for the spatial distribution of fluid saturation in rock according to claim 1, characterized in that a core fluid injection system is further provided, the core fluid injection system is provided with a ring pressure loading cavity and an axial pressure loading cavity on the holder, the ring pressure loading cavity and the axial pressure loading cavity are respectively connected with corresponding plunger pumps through flexible pipelines, and fluid is injected through the corresponding plunger pumps for pressurization; the ring pressure loading cavity is provided with an inlet and an outlet, and the heat exchange between the ring pressure fluid and the constant temperature bath is realized through the constant temperature circulating pump to achieve constant temperature control.
3. The quantitative analysis method for the spatial distribution of fluid saturation in rock according to claim 1 or 2, characterized in that, flexible pipelines communicated with rock are arranged at two ends of the holder, an injection pump is connected at the upstream, a meter is arranged on the injection pump to measure the injected fluid, the upstream pipeline and the heating belt are wrapped in the heat preservation pipe side by side, a temperature sensor is arranged on the upstream pipeline, the upstream pipeline and the constant temperature controller are combined to realize the constant temperature control of the upstream pipeline, pressure sensors are respectively arranged at the upstream and the downstream of the flexible pipeline, the upstream and the downstream of the rock are recorded by the pressure sensors, differential pressure sensors are arranged at the upstream and the downstream of the flexible pipeline, the differential pressure sensor is used to record the pressure difference between the upstream and the downstream of the rock, a three-phase separator and a meter are sequentially arranged at the outlet of the flexible pipeline by a back pressure valve, the meter is used to respectively measure the fluid after multi-phase separation, the back pressure valve is connected with the plunger pump, and the outlet pressure is controlled by the plunger pump.
4. The method for quantitatively analyzing the spatial distribution of the fluid saturation in the rock as claimed in claim 1 or 2, wherein the holder is made of PEEK, polyimide or carbon fiber, and has X-ray penetrability while bearing the strength of confining pressure, axial pressure and fluid pressure.
5. The method for quantitatively analyzing the spatial distribution of fluid saturation in rock according to claim 1, wherein D1 data must be collected under the overpressure condition in the step (1); adding a soluble compound with strong X-ray absorption into one fluid in the step (2).
6. The method for quantitative analysis of the spatial distribution of fluid saturation in rock according to claim 1, wherein the parameters of pore space and pore throat in step (6) are pore diameter, pore volume, shape factor, central three-dimensional coordinates, pore throat diameter, pore length, pore volume, and shape factor.
7. The method for quantitative analysis of spatial distribution of fluid saturation in rock according to claim 1, wherein in said step (6), the pores and pore throats are numbered sequentially and progressively, each number corresponds to a two-dimensional matrix, each row has three elements, the three-dimensional coordinates of the pore or pore throat pixels in the D1 data volume are determined, and the total number of rows is equal to the number of pixels corresponding to the pore or pore throat.
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