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Sorokin A.V., Sorokin V.D. Omega-K, Tyumen



Knowledge of fractional analysis and physico-chemical properties of oil is used in many areas of petroleum science to tackle a dazzling array of practical issues. Again, in many cases, the properties of oil are used as a model. And this always brings up the question of whether the chosen model reflects the real values of the properties of oil.

Models of fractional analysis and physico-chemical properties of in-place oil are used for solving the problems of oil origin, the specificity of processes that occur in oil before it migrates into traps where it currently resides, the degree of change it undergoes during storage, and the study of the impact on its composition and properties of processes that occur during storage of selected oil samples during laboratory investigations, etc.

Among the hands-on tasks that involve the use of values of in-place oil properties, mention should be made in the first place of reserves assessment, calculation of oil recovery factor during lab tests, etc.

Hence, the parameters of the model describing the fractional analysis and physico-chemical properties of in-place oil indirectly affect the assessment of the investment appeal of an oil production scheme at a specific field.

Models of fractional analysis and physico-chemical properties of mobile oil are used in hydrodynamic simulation of oil displacement process, selection of relevant displacement technology, they are also incorporated into technical requirements for equipment for oil production, treatment and transport, and considered when choosing process solutions for oil field processing of petroleum, etc.

Models of fractional analysis and physico-chemical properties of commercial (degassed) oil are utilized in addressing the issues of storage and delivery of oil via trunk pipelines, selection of technologies and equipment for its processing, and affect the process for optimizing the range of obtained products.

From the above-mentioned models, models of fractional analysis and physico-chemical properties of degassed oil are currently the best-fit models that reliably reflect the actual object which is due to physical accessibility of oil in any volume, and the well-developed methods of taking and testing of samples.


Models of the physico-chemical properties of mobile oil are far less adequate to the real object of investigation due to a number of constraints imposed on methods deployed for drawing samples of this oil at various stages of oil field development[1].

There are currently no adequate models of in-place oil by reason of lack of procedures and devices for taking in-place oil samples. To a large extent this is due to the fact that in regulatory documents and technical literature there is no clear-cut distinction yet between the terms : "in-place oil" and "mobile oil".

The meaning of the terms "in-place oil" and "mobile oil" do not coincide because both in the composition and values of the physico-chemical properties of oil-in-place and mobile oil there is a substantial difference as shown in paper[2].

Mobile oil is only one of the components of in-place oil and therefore the values of their properties do not coincide. This example demonstrates the exceptional importance of working out relevant terms and their definitions which then set the scene for further research in many areas of petroleum science and practice.

In-place oil is a natural system and for this reason its fractional analysis and physico-chemical properties cannot be governed by the technology of oil recovery, methods of its study, etc.

Therefore, the process of generating a model of the physico-chemical properties and fractional analysis of in-place oil for a specific production facility should be maximally independent of the technogenic impact on oil in-place.

The notions of oil in-place and mobile oil are not treated as separate entities in applicable regulatory documents. The values of properties of the latter tend to vary during the period of oil field development and are also a function of the technology deployed and parameters of the recovery process.

And thus, in actual practice, while taking and testing mobile oil samples the obtained results are identified with the properties of in-place oil without any appropriate substantiation. And since the physico-chemical properties of mobile oil are known to change their values [3] in the process of field development, then this also proves the fact that there is a disparity in the values of physico-chemical properties of oil in-place and mobile oil.

The information structure of in-place oil is presented in the study[2]. The need for developing such structure is dictated by reasons based on different methods of study and personalized estimate of the fraction of each in-place oil component.

According to the proposed structure, in-place oil is divided into mobile and immobile components. The mobile component is divided, in turn, into recovered mobile and non-recovered mobile oil.

The immobile oil consists of the following components: oil adsorbed by the reservoir surface, oil residing in structured layers and oil located beyond the deposit drainage zone. The composition and physico-chemical properties of mobile recovered component of in-place oil are studied by taking and investigating bottom-hole or recombined oil samples from the products of producing oil well. The composition and physico-chemical properties of non-recovered mobile component of in-place oil have not been studied experimentally, but can be obtained computationally by extrapolating the values of recovered mobile oil properties.

The composition and physico-chemical properties of oil located in adsorption layers on reservoir surface, in structured layers in the near-surface zone of the reservoir are studied by means of laboratory investigation methods. Physical models of the corresponding in-place oil component are also used in the experiments. The composition and properties of oil located outside the drainage zone correspond to those of in-place oil as an immutable object during the life of a field.

In practice, however, when each in-place oil component is investigated by its own technique, acquiring an adequate model of its composition and physico-chemical properties is feasible only through synthesis of information about fractions and the properties of all components.

This method of generating a model of composition and physico-chemical properties of in-place oil is cited in reference [2, 4]. Using volumetric data for in-place oil with parameters of mobile oil when calculating hydrocarbon reserves may bias the results of estimation of geological resources.

With this approach the results of calculating the hydrocarbon reserves indicate a lesser reliability and hence oil reserves are usually understated (for the groups of beds B and Yu of deposits in Western Siberia by 10-20%) while the reserves of oil gas are somewhat overstated.

A model of physico-chemical properties of mobile oil is utilized in geological and hydrodynamic simulation of the process of oil recovery. Today it is an established fact that this model is static. It has been established by numerous investigations including those based on the results of dedicated field experiments conducted in different regions of Russia that the composition and values of physico-chemical properties of mobile oil tend to change in the process of field development and are also contingent on operating parameters of the well.

Information on the results of these investigations can be gleaned from works [2, 3, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14] and elsewhere. From this it follows that parameters of the model describing the composition and properties of mobile oil are non-stationary, are clearly dependent on selected technology of oil production, the degree of impact of oil displacement methods, etc which should be considered during model generation. The sum total of the methods to account for changes in the physico-chemical properties of mobile oil that occur during the period of field development which makes allowance for the impact of main technogenic effects is presented in a list of works cited [2, 3, 12, 13, 14].

A lower quality of geological and hydrodynamic models of the process of oil displacement, especially at the fields under the late stage of development is the upshot of using models of the composition and properties of mobile oil that ignore the dynamics of their change during field development.

In a number of cases, history matching of the geological and hydrodynamic model of the oil displacement process fails to bring the desired result without considering the phenomenon of variability of values of the physico-chemical properties of mobile oil.

According to the conclusions drawn by researchers in the study [15] in order to achieve the required accuracy (10%) of field development indices while computing the hydrodynamic model of the process of oil displacement, the parameters featuring the values of the physico-chemical properties of oil should be defined with the following accuracy: oil density 1%, viscosity 4%, oil formation volume factor 3%. The physico-chemical properties of mobile oil during the process of development at the oil fields in Western Siberia vary in their values over a far greater extent (examples are provided in work [3]). The question of the value of in-place oil volume factor remains undecided due to lack of formation oil model. The first attempts at generating a mathematical model of oil in-place are outlined in a list of works cited [2, 3].

The question of separation of in-place oil in the drainage zone between its mobile and immobile components is a separate issue since their fractions are calculated using the oil recovery factor (ORF) whose calculation is based on an experimentally obtained coefficient of oil displacement by water. To determine the coefficient of oil displacement it is essential that a physical model of in-place oil should be used rather than its mobile component model as is the current practice.

When using the currently effective OST 39-195-86 [16] whose objective is regulation of jobs to determine the coefficient of oil displacement by water, obtaining objective results in defining this coefficient may be impossible for a number of reasons. The clause 1.5 of this OST prescribes using dry crude oil or isoviscous model of formation oil to conduct the studies and use is also allowed of recombined samples of oil in-place.

The first part of this requirement is impossible to implement if only because no one has yet succeeded in taking a sample of in-place oil at the present stage of scientific development. The current potential of available methods and sampling technology allow one to only take a sample of mobile component of in-place oil and even then with varying degree of validity.

The isoviscous model of in-place oil is not devoid of the following drawbacks. Since parameters of oil in-place are unknown, the isoviscous oil parameters are selected to fit those of recovered mobile oil model.

A sample of degassed oil is taken as a basis for the model and organic solvents are added to it to reduce its values of viscosity until they are equal to those of mobile oil. As a result, a disparity in the fraction ratio develops in the isoviscous physical model with regard to oil in-place model: the gas component is totally absent, the portion of heavy fractions is insufficient which are contained in increased amounts in the structured layers of in-place oil. The fractional oil content of structured layers depends on both reservoir properties and the properties of oil and, in the opinion of the study author[17] , may be quite substantial in some specific cases.

Despite the "match" between the viscous characteristics of the two oil models, the surface tension coefficient of the isoviscous model of in-place oil determined by capillary method is 1.5 -2 times higher than that of mobile oil model determined under similar thermobaric conditions. Therefore, the coefficients of displacement of these fluids (the isoviscous model and mobile oil model) by water will have different values.

It is also important to point out here that when using in the experiments the in-place oil model which, as a rule, features much higher values of density and viscosity under reservoir conditions and a lesser relative content of light components in its composition than does mobile oil, the difference in the values of surface tension coefficient between the in-place oil model and that of isoviscous oil should decrease.

Thus, if we consider to define experimentally the coefficient of oil displacement by water there remains an option of selecting an oil model through recombination with mandatory condition that parameters of the recombined oil model should correspond to those of the in-place oil model.

To achieve the highest possible degree of approximation, the selection of parameters of this physical model should be done by combining the individual fractions of mobile oil. It is necessary to develop in advance a mathematical model of the composition and physico-chemical properties of in-place oil according to procedures outlined in works [2, 4].

Usually when drawing up process documentation based on the coefficient of oil displacement by water the value of ORF is set such that it becomes virtually unattainable in the majority of cases with the use of only one technology of oil displacement by water. Therefore, in development practice to achieve the preset value of ORF other technologies are also employed such as hydrofrac, chemical methods of enhanced oil recovery, etc. This provides yet another proof that the value of the coefficient of oil displacement by water obtained in a laboratory experiment is overstated.

It is our opinion that the error in question emerges as a result of using in the experiment an oil model inadequate by its properties to oil in-place. Analysis of the foregoing compels the following conclusions:

• To further develop the process of investigations of the fractional analysis and physico-chemical properties of oil in-place and mobile oil it is essential to establish and standardize a system of terms and to provide their definitions;
• Because it was found impossible to take samples of in-place oil its composition and values of physico-chemical properties have not been defined experimentally and in consequence it is necessary to use computational methods to simulate the properties of in-place oil;
• The composition and values of the physico-chemical properties of mobile oil are prone to change during field operation;
• Parameters of the physical model used to define the coefficient of oil displacement by water must correspond to parameters of in-place oil model with the presence in it of components and fractions that actually exist in the oil in-place.

Labels: Fractional analysis models, oil gas russia, Omega K, reservoir analysis, Tyumen

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