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Introduction X-ray tomography of core is a relatively new direction to investigate reservoir properties of oil deposits today. The method is based on study of X-ray specter characteristics of rocks. Theoretical basis of the method applied to the study of reservoir properties (porosity and permeability) is covered in detail in [1]. Analysis of the world experience shows that the method of X-ray tomography begins to be used not only in study of petrophysical characteristics of rocks [2-4], but also in evaluation of enhanced oil recovery methods [5, 6]. This paper shows the results of possible X-ray tomography applications while studying petrophy-sical properties of carbonate reservoir cores on example of several oil fields in Perm region. The microfocus system of X-ray control with computer tomography Nikon Metrology XT H 225 was used. To study the effect of sample size on the tomography results 25 pairs of same core samples were used. Each pair is a standard petrophysical sample 30 mm in diameter and smaller (5 and 10 mm). The aim of the research is to obtain the characteristics of X-ray specter heterogeneities, voids, fluid distribution in hollow core space. The methods of detailed assessment of rock structures are most relevant during study of carbonate reservoirs with complex geology [7]. Heterogeneity of carbonate deposits and impact of facial conditions on its reservoir properties are proved in [8]. Accordingly, underestimation of heterogeneity of carbonate reservoirs reduces accuracy of final oil recovery assessment that is shown on the example of number of fields development [9, 10]. During the study following objectives were highlighted: - lithological description and study of structure of reservoir voids; - to build 3D-models of voids and calculate porosity coefficient (Kp); - to select radiographic solutions and model ireducible water saturation in samples; - to build 3D-models of fluid distribution, calculate initial oil saturation (Ki.o.) and irreducible water saturation (Ki.w.). Lithological description came down to determination of mineral composition, structural and textural features, cement nature, cracks development. All of these data are used in the interpretation of core tomography results. The results of X-ray tomography studies are as follows: 2D-slices of longitudinal and transverse cross-sections of samples; 3D-models of void distribution, seals, initial oil saturation; charts with distribution of porosity and irreducible water saturation along sample height; histograms of pore diameters in sample [11]. Computer modeling of objects and subsequent calculations were performed in the software package Avizo (Visualization Sciences Group, France). Study of void structure of carbonate deposits Study of void nature and distribution of seals in the samples allowed to type core depending on its void structure. The results are shown in Table 1. Table 1 shows that X-ray tomography allows you to select types of voids of different genesis. The porous type of structure is caused mainly by secondary inner voids. The method can determine primary (intergranular) and secondary (interpar-ticle, intraparticle) voids. Secondary voids include cracks and cavities that were formed during leaching of fossils with carbonate composition. Allocation of crack zones is one of the important areas of practical application of core tomography. According to [12] for a number of carbonate deposits there is a discrepancy between oil recovery and geological reserves, evaluated by standard methods. Integration of X-ray tomography analysis with well test for carbonate reservoirs in [13] allowed determining reservoir zones with pore-crack type along the interval with absence of granular reservoir. In [14] there are examples of fixation convergence (or absence) of cracks on X-ray tomograms and results of experiments during varying hydraulic compression of core samples. Table 1 In the analysis of porosity evaluation results for samples of different diameters it is necessary to compare exactly that part of a larger sample, which smaller is made from. Conducted this way comparison showed a high convergence of Kp values (r = 0.97). During the comparison of average values over the entire volume of samples of 30 mm the correlation reduces significantly (r = 0.77) due to the influence of its heterogeneity. Figure 1 shows an example of interpretation of tomography and distribution of pores according to identified void size for samples of 5 and 30 mm diameter. The sample of 5 mm was made from the bottom of the 30 mm sample. Comparison of the interpretation results shows their high convergence. The distribution of porosity along 30 mm sample height indicates that Kp value in its lower part is the closest to interpretation results of 5 mm sample. In general, analysis of the results of core studies by X-ray tomography shows that for samples of smaller size the elements of voids and X-ray density heterogeneities are visualized in much detail. It is determined that for 30 mm diameter the minimal size of visible specter is 0.08 mm, for 10 mm diameter - 0.04 mm and for 5 mm diameter - 0.02 mm. Discrepancy of visible pore size during the study of samples of different diameters leads to underestimated results for bigger samples. For instance, for dense difference in porosity of 5 % results of Kp evaluation in absolute values are underestimated by 1 %, for high dense differences of 10 % - 1.5 %. Thus, relative error of porosity determination for bigger samples decreases due to decreased share of pores with small diameter. Study of fluid saturation of carbonate deposits In order to determine nature of distribution and ratio of irreducible water and residual oil X-ray tomography was used on dry sample, sample with 100 % of water saturation and sample with ireducible water saturation. Irreducible water saturation was modeled by semi-permeable membrane under overpressure of 1.2 MPa. Radiographic solutions NaI (200 g/l), CsNO3 (200 g/l), LiBr (1500 g/l), LaCl3·3H2O (100 g/l) were used as reservoir fluid models. Non-radiographic four-normal solution of NaCl usually used as reservoir fluid in petrochemical study is a reference sample in this research. Determination of the volume occupied by irreducible water and residual oil, as well as nature of fluid distribution in voids of sample is carried out by binarization of 3D-model using Tresholding function of AvizoFire 7.0. In addition, Ki.w. was determined by the difference between void space of dry sample and sample with irreducible water in case when irreducible water was not radiographic enough. Results of fluid saturation study by X-ray tomography show that the most contrast in X-ray are water solutions of sodium iodide and lanthanum chloride. Figure 2 shows 2D-slices and 3D-models of core samples saturated by NaI (200 g/l) and LaCl3·3H2O (100 g/l). It is observed from the figures that in longitudinal and transverse 2D-slices saturating solutions appear as areas with higher X-ray density (filled white), equal density of the mineral skeleton or higher. It allowed creating 3D-model of initial oil saturation distribution (filled orange) and calculating behavior of Ki.w. change along sample height. The results revealed that large pores, cracks and cavities are substantially free of irreducible water that always occupies fine pores (see Fig. 2). Due to X-ray density of NaI closed to the density of rock mineral matrix, allows to estimate the amount occupied by oil (see Fig. 2a). Thus, to assess Ki.w. with use of NaI it is needed to apply results of scanning of dry sample and sample with irreducible water. The solution of AlCl3·3H2O, having X-ray density higher than mineral skeleton (see Fig. 2b), allow to allocate zones occupied by irreducible water without dry sample scanning. The values of Ki.w. obtained from tomography were underestimated in comparison with Ki.w. determined by semi-permeable membrane. The method allows visualizing voids higher than 0.021 mm for 5 mm samples and 0.075 mm for 30 mm samples. According to X-ray tomography big pores, crack and cavities do not have water. It is contained in the fine pores of diameter less than resolution of images obtained by tomography. Pores with inner fluid flow (as indicated by standard petrophysical studies) are in the range of 0.01-0.06 mm. Conclusion Based on the analysis of core of carbonate deposits by X-ray tomography following conclusions were obtained: 1. The method allows getting an idea of space distribution (visualization, 3D-models) of quantitative void reservoir properties of rocks (coefficients of porosity, irreducible water and residual oil saturation, crack opening, voids diameters) and determining reservoir type based on morphostructural features. It is easy to distinguish cavities, cracks, pores of large and medium size and determine its void. 2. The resolution of images obtained by X-ray scanning has minimal threshold depending on sample size. For samples of 30 mm in diameter, minimumal size of fixed object is 0.08 mm, for samples of 10 mm in diameter - 0.04 mm, for samples of 5 mm in diameter - 0.02 mm. 3. Determination of closed porosity is complex, since the estimation is made on the core with 100 % of water saturation. In case samples have large voids, then under the action of gravity saturating solution leaves voids in the first minutes of scanning. Therefore, voids that have no solution are erroneous referred to isolated. Moreover, if the solution is not sufficiently radiographic then it cannot be registered in large voids, even if they are filled. On the other hand it is registered as 100 % water saturation in the voids of smaller diameter. That leads to overestimation of open porosity. 4. Interpretation of X-ray heterogeneities without lithological description of the core is possible only while solving problems related to the establishment of their spatial distribution. 5. The test with 4 radiographic solutions showed that the best results in modeling of irreducible water were obtained for compositions with concentration of 200 g/l of NaI and 100 g/l of LaCl3·3H2O at. 6. During determination of irreducible water saturation it was found that large voids (cavities, cracks) have no irreducible water. 7. During evaluation of irreducible water saturation in the matrix it was determined that pore size has great influence on final results. It was noted that for reservoirs with predominance of small-diameter pores Ki.w. value was underestimated (overestimation of Ki.o.). It should be noted that the X-ray tomography of core have to be carried out in complex with standard petrophysical and petrographic studies, since the method has its limitations due to physics of scanning process and minimal sensitivity threshold depending on the size of scanned samples. In order to develop the method it is relevant to improve mathematical apparatus of software for tomography data processing by change from X-ray images to 3D digital model [15].

Об авторах

Артём Александрович Ефимов

Автор, ответственный за переписку.
Email: lpfi@pstu.ru
614990, Россия, г. Пермь, Комсомольский проспект, 29

кандидат геолого-минералогических наук, доцент кафедры геологии нефти и газа

Ян Владимирович Савицкий

Email: yansavitsky@yandex.ru
614990, Россия, г. Пермь, Комсомольский проспект, 29

ассистент кафедры геологии нефти и газа

Сергей Владиславович Галкин

Email: gnfd@pstu.ru
614990, Россия, г. Пермь, Комсомольский проспект, 29

доктор геолого-минералогических наук, профессор кафедры геологии нефти и газа

Серж Шапиро

Email: shapiro@geophysik.fu-berlin.de
12249, Германия, г. Берлин, ул. Мальтезерштрассе, 74-100, стр. Д

доктор технических наук, профессор, руководитель департамента геофизики

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