While physically simulating oil-water displacement, quantitative three-dimensional oil-water saturation data of the core are obtained by CT scanning, so as to directly analyze the fluid migration state in the core. The research content involves the test method and process of CT scanning, the reconstruction of oil-water flow state and remaining oil distribution state by CT images, and how to deal with CT digital images. Through two examples, the history of saturation distribution in each stage of physical simulation of oil-water flooding is successfully reproduced, and the mechanism of EOR methods such as reverse water flooding and polymer flooding is clearly observed.
CT； Physical simulation; Prosodic model; Saturation mechanism; Polymer flooding; recovery ratio
I. Introduction
It is a widely accepted technology at home and abroad to study the seepage characteristics in porous media by computer tomography (CT). Its advantages are: while displacing the core, using CT scanning to obtain quantitative three-dimensional oil-water saturation data in the displaced core, so as to intuitively analyze the fluid migration state in the core. However, conventional tests can only give the average value of the internal saturation of the whole core.
Figure 1 volume CT scanning core diagram
CT has been used in seepage research for more than 20 years, and a complete experimental system has been formed abroad. In recent years, CT research has been combined with new theories and methods of reservoir engineering, which has promoted the technical progress of seepage mechanics. 65438-0997 Institute of Geological Sciences of Shengli Oilfield Co., Ltd. introduced a set of micro-focus industrial CT system with volume chromatography function from the United States, and invested a lot in seepage physical simulation, data processing and software development, forming an effective research method. The research content involves the test method and process of CT scanning, the reconstruction of oil-water flow state and remaining oil distribution state by CT images, and how to process the obtained slice data. The test process involved is similar to the conventional test, that is, pumping vacuum saturated water first, then injecting water to drive oil to establish an irreducible water state, and then injecting water to drive residual oil. The difference is that CT scanning is continuous during the experiment. In addition, using CT scanning displacement test, it is necessary to add a substance that can cause ray attenuation to a certain item of oil and water, such as adding NaI to the water phase.
Second, CT machine and scanning method
The experiment adopts ACTIS industrial CT/DR system, which has advanced volume CT function, and is used for scanning homogeneous columnar core displacement experiment to realize the three-dimensional spatial oil-water saturation distribution of core. The system configuration is shown in figure 1. The X-ray source emits a cone-shaped ray beam and passes through the core placed on the turntable. Through core rotation, the projection data is obtained, and 100 slices are reconstructed at one time to complete the scanning of a three-dimensional data volume.
The system has the function of digital radiography (DR) for scanning two-dimensional models. The system configuration is shown in Figure 2. The fan-shaped ray beam passes through the model and completes the two-dimensional DR image through the horizontal displacement of the model. This function is more effective for scanning the displacement of multi-layer compound rhythmic reservoir model.
Third, the basic principle of calculating saturation distribution by CT
The test core is divided into several grids in I, J and K directions (Figure 3), and the X-ray linear attenuation coefficient on each grid is measured by CT scanning. In practice, in order to convert the attenuation coefficient into CT number conveniently, the CT number of each grid is expressed by CTijk. When the core is saturated with oil and water, the number of CT in each unit is as follows:
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Where: —— CT number of each grid during oil-water displacement;
φijk- porosity of each grid,%;
—— Oil saturation of each grid,%;
—— Water saturation of each grid,%;
—— CT number of each grid rock skeleton;
CTO-CT value of oil;
CTW CT value of water.
Fig. 2 schematic diagram of dr scanning two-dimensional displacement model
Fig. 3 Schematic diagram of core grid division
Scan the core of completely saturated water or completely saturated oil, and the CT number is as follows:
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Where:-CT number of each grid when water is completely saturated;
-CT number of each grid when the oil is completely saturated.
The oil-water saturation of each grid is obtained from (1), (2) and (3):
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The distribution of CT number when oil and water are completely saturated can only be obtained by CT scanning, and the CT number at different times can be obtained by scanning during displacement, so as to calculate the saturation distribution at different times.
Fourthly, CT digital image processing.
The dynamic change of oil and water during displacement cannot be reflected by CT images only, and the saturation distribution can only be calculated by ray hardening, image matrix transformation, ray drift processing and volume CT position correction.
1. ray hardening treatment
Because X-ray is a continuous polychromatic spectrum, it will cause the artifact of ray hardening. This kind of image is characterized by large edge gray scale and low center gray scale, so the saturation distribution image obtained in this way also has the characteristics of high edge saturation value and low middle value. At present, the feasible method is to create an anti-hardening image by using the average number of CT slices to eliminate ray hardening.
2. Image matrix transformation
For particle P in porous media, the volume at the center of mass is △V, the pore volume is △Vp, and the porosity is:
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Draw a curve between φ and△ V. When△ V shrinks to a certain volume near△ V 0, φ begins to oscillate violently. When △V is on the pore, φ =1; φ= 0 when △V is on the skeleton particle. The same is true for saturation calculation. If the CT pixel matrix is simply used as the grid, the calculated saturation distribution value will oscillate violently. This is the reason for matrix transformation.
3.x-ray drift correction
Because the scanning displacement test lasts for ten to dozens of hours, the radiation attenuation value of the same material varies greatly due to the aging of the radiation detector and the random change of external conditions during the test. The attenuation data of calibration materials must be used to correct ct image data at different times.
4. Volume CT position correction
On the one hand, volume CT uses a cone-shaped X-ray beam, and the intensity of the edge and the middle of the ray is different. On the other hand, except the central slice, other slices of volume CT are actually the superposition of attenuation values of multiple ellipsoids. These two situations lead to a slight difference in the average CT number of each slice in the K direction of the core. The test needs to be calibrated with a uniform columnar material.
Application example of verb (abbreviation of verb)
Internal displacement characteristics of homogeneous core scanned by 1. Computerized x-ray tomography
The main purpose of the test is to study the test and calculation method of oil-water saturation distribution in CT scanning displacement test, the saturation distribution characteristics of homogeneous cores with different permeability, and determine the displacement laws of water flooding and polymer flooding through CT scanning.
The experiment selected homogeneous artificial core, which was completed at normal temperature and pressure, and the injection rate was 0. 1 ml/min. The core permeability is 684× 10-3μ m2 and the porosity is 27.8; The diameter of the core is 2.5cm, and the length is 5.7cm. The viscosity of crude oil is 38 MPa sec.
Fig. 4 Relationship curve between oil recovery and injection multiple
CT scanning can be used to calculate the distribution of oil-water saturation, the distribution of residual oil in water flooding and the enrichment of residual oil in polymer slug, and the recovery curve can be obtained at the same time. Fig. 4 is data obtained after CT image processing. It can be seen that the effect of enhancing oil recovery is obvious after injecting polymer slug. After injecting polymer slug, the oil recovery is about 76%, which is about 16% higher than that of water flooding.
The mechanism of enhanced oil recovery can be clearly observed in axial slices after CT image processing (Figure 5). Fig. 5 is the imaging processing result of Experiment 1. Black represents high oil saturation and white represents high water saturation, reflecting the numerical distribution of oil-water saturation under different injection multiples. It can be seen that with the continuous injection of water, the image turns from black to white, and the oil content in the core decreases continuously. In the stage of water flooding, the water channeling channel is obviously formed. In the channeling channel, the oil saturation is obviously lower than that in other areas, and the microwave heterogeneity is very obvious. Injecting 0.2PV polymer slug can effectively enrich the remaining oil after water flooding, forming piston displacement. It is generally believed that polymer flooding can improve the macro sweep coefficient, and this experiment has successfully observed that it can also effectively improve the sweep uniformity at the micro level.
Saturation distribution under different injection multiples.
The darker the color, the higher the oil saturation. The water saturation decreases from top to bottom, and the injection multiple is 0, 0.36, 1.3 1, 2.58 and 2.88. 1 is the state of bound water, 2 ~ 4 is the saturation distribution during water flooding, and 5 is the state after injecting 0.2PV polymer slug and 0.4PV water.
2. The displacement law of prosodic model is studied by digital radiography.
Experimental purpose: to observe the displacement characteristics of rhythmic model at different injection rates, the sudden movement law of water channeling channel along permeable layer, and the distribution characteristics of oil-water saturation in different displacement stages.
The model is a three-layer anti-prosodic model with dimensions: length× height× width = 20cm× 5cm×1.5cm; The height of the three-layer structure from top to bottom is 1.5cm, 2.5cm and 1cm, respectively, and the permeability is 4μm2, 2μm2 and 1μm2, respectively. The experiment was carried out at room temperature (25℃), and the average injection linear velocity was 4mm/min, 2mm/min and 65438 0 mm/min, respectively. The displacement oil is mixed engine oil, and its viscosity is115 MPa s at 25℃.
Like CT scanning, DR scanning can be used to calculate the porosity distribution of the model in two-dimensional plane, the oil-water saturation distribution during displacement, the remaining oil distribution in water flooding, the injection-production curve and so on. Compared with CT image data processing, DR image processing is relatively simple, and water-driven microchannels can also be observed by using DR image subtraction technology.
Fig. 6 imaging results of prosodic model DR
As can be seen from Figure 6, both images show that after the injection speed is 1mm/min and 0.07PV aqueous solution is injected, the fluid flows along the high permeability layer. Fig. 6A reflects the saturation distribution state; Fig. 6B shows a flow pattern obtained by image subtraction. The scouring mode of fluid is different from experience, and the microscopic fingering is abnormal.
Main references
S.l. Wellington, H.J. vinegar. X-ray computed tomography. SPE 16983:885~898。
[2] Agrag, Alcove Tusk. Construction of detailed distribution of remaining oil saturation during water injection development by CT scanning and neural network technology. SPE 35737:695~7 10。