Analysis of the degree of influence of geological and technological factors on the initial flow rates of wells commissioned after their downtime

. The results of statistical processing of data from 97 wells put into re-operation, after their long downtime due to flooding, are presented. In which, depending on the geological and physical conditions and the state of development of oil deposits, different results are obtained. With an average initial oil flow rate of 1.12 tons/day in reservoir-vaulted deposits with low-viscosity oil in the Fergana oil and gas region - 0.5 tons/day, in reservoir-arch deposits with high-viscosity oil in the Surkhandarya oil and gas region - 0.97 tons/day, in sub-gas deposits of the Bukhara-Khiva region - 2.5 tons/day. By assessing the informativeness of geological, physical and technological factors using the Kulbak method, the degree of their influence on the initial oil flow rate of wells after their re-commissioning was established. Taking into account the degree of influence of the factors considered, it is recommended to put into re-operation wells located in the arched part of the reservoir, having the highest effective oil-saturated thicknesses, in areas with a low oil recovery factor and a lower degree of reduction in initial reservoir pressure, as well as watered in a short period of operation.


Introduction
At present, in the development of fields to achieve high rates of extraction and recovery factor of oil, various types of flooding of oil deposits are widely used. Many of these deposits have entered a late stage of development and are increasing the number of abandoned and mothballed wells due to their flooding. Many scientists have proved that despite the application of the flooding method, from 40 to 60% of geological oil reserves remain in productive formations, and increasing the degree of recovery by recommissioning abandoned and mothballed wells is important for the oil and gas industry, all oil-producing countries [1][2].
According to the existing ideas of scientists, the residual oil reserves in the depleted reservoirs remain in the form of: oil remaining in weakly permeable interlayers and areas not covered by water; oil remaining in stagnant zones of homogeneous formations; oil remaining in lenses and impermeable screens not opened by wells; capillary retention and film oil [5][6].
Theoretical and experimental studies have shown that the process of reformation of residual oil in the reservoir is influenced by the forces associated with gravitationalcapillary segregation, capillary impregnation, natural filtration flow of water, processes of elastic redistribution of pressure, diffusion, thermal conductivity, etc. [7,9].
An analysis of the results of the commissioning of wells published by the works of geological and field materials on the results of putting wells into re-operation, after their long downtime, shows that its efficiency varies within large limits and depends on many geological, physical and technological factors. In this regard, it is relevant to establish the geological, physical and technological factors that determine the effectiveness of the reexploitation of depleted oil deposits and mothballed wells.

Materials and methods
To date, in Uzbekistan, at 73 fields containing oil deposits, 54 development facilities are in the late (in fact, at the final) stage of operation. . Of these, 38 are located in the Fergana Depression, 5 in the Surkhandarya mega-syncline and 11 in Western Uzbekistan According to the results of studies by U.S. Nazarov, A.Kh. Agzamov, N.N. Makhmudov, N.Kh. Ermatov [3,7] at 38 objects of the Fergana Depression, the expected average ORF is 40.2%. At the same time, at 24 facilities developed using flooding, the average ORF is 41.8%, and at 14 facilities developed using natural regimes -27.6%.
At 5 fields of the Surkhandarya mega syncline, the achieved average oil recovery factor is 24.1%.
At 11 facilities in Western Uzbekistan, an average ORF of 23.9% will be achieved. In general, for 54 facilities in Uzbekistan that are in the late stage of development, the average oil recovery factor will be 32.4%, i.e. 67.6% of the initial oil reserves will remain in the reservoir.
In the fields of Uzbekistan with oil deposits, there is a large fund of abandoned and mothballed wells due to flooding of their products. In this regard, in recent years, work has been carried out to return these wells to re-operation. For the period 2013-2017 at 97 wells of 36 fields, work was carried out to return their liquidation and conservation fund. These fields and wells are located in various oil and gas regions of Uzbekistan: Fergana oil and gas region -(FOGR) 40 wells at 13 fields; Surkhandarya Oil and Gas Region (SOGR) -39 wells at 10 fields and Bukhara-Khiva region (BKhR) -18 wells at 13 fields ( Figure 1).
The oil deposits of the FOGR fields are mainly of the reservoir-vault type, they are represented with both terrigenous and carbonate rocks, the average depth varies from 350 m to 4300 m, mainly with a small effective oil-saturated thickness of productive formations from 3 m to 18 m, the reservoir temperature is from 31 to 136 0C, the initial reservoir pressure is from 3 to 45 MPa.
The coefficients of porosity and oil saturation are 11-20% and 52-75%, permeability 0.013-0.250 mkm 2 . The density and viscosity of reservoir oil vary from 829 to 894 kg/m 3 and 1.05 to 9.5 MPa s, respectively.
The life of oil deposits ranges from 13 to 118 years. Despite the large limit of change in the current oil recovery factor from 2.42 to 47.45%, the depletion of recoverable reserves is very high from 60.12 to 38.36%.

Results and Discussion
The oil deposits of the fields of the Surkhandarya oil and gas region (SOGR) are of the reservoir-arch type. All known productive horizons are represented by carbonate-type rocks, the depth of the horizons is small from 300 m to 1460 m, the effective oil-saturated thickness of the horizons varies from 2.8 m (Uchkyzyl field) to 34.5 m (Mirshady field), reservoir pressure from 30 to 56 0 C, initial reservoir pressure from 3 to 23.3 MPa. Category I wells -returned from liquidation on reservoir-type deposits with low-viscosity oils; Category II wells -returned from liquidation on reservoir-type deposits with high-viscosity oils; Category III wells -returned from liquidation at sub-gas oil wells deposits (from oil rims).
The filtration and capacitance properties of the reservoirs are quite high, the porosity coefficient varies from 12.5 to 20%, the oil saturation coefficient is from 68 to 80%, the permeability is from 0.120 to 0.451 mkm 2 . Oil deposits are classified as heavy (density in reservoir conditions 849-980 kg/m 3 ) and high-viscosity (oil viscosity in reservoir conditions 10-129 mPas).
Oil deposits have been exploited for quite a long time from 11 years (Korsagli field) to 87 years (Uchkyzyl and Haudag fields). The current oil recovery rate is from 13.32 to 36.07%, and the depletion of recoverable reserves is from 58.14 to 95.04%.
The oil deposits of the Bukhara-Khiva region (BKhR) are of the sub-gas type. The coefficient of porosity of reservoirs varies from 7.2 to 18.9%, the coefficient of oil saturation from 61.1 to 80%, permeability from 0.085 to 0.532 mkm 2 . Oils of all fields are classified as low-viscosity. The density of reservoir oil varies from 867 kg/m 3 to 930 kg/m 3 , viscosity from 0.7 to 1.8 mPa In contrast to the FOGR and SOGR deposits, the BKhR oil deposits are operated for a relatively short time from 8 to 64 years. The oil recovery factor and the depletion of recoverable reserves are also relatively small: from 0.04 to 8.58% and from 0.18 to 39.83%, respectively.
At 97 wells, after their return to re-operation, an average initial oil flow rate of 1.12 tons per day was obtained. At the same time, the initial average flow rates of the wells of the SOGR deposits are comparable to the average values of 0.97 tons per day, the FOGR is almost 2 times lower -0.50 tons / day, and the BKhR is more than 2.5 times (Figure 2).
The results of the return of 97 wells to re-production once again confirm that the effectiveness of this event and their oil production depend on many geological and technological factors.
Based on the results of the generalization of theoretical, experimental and field studies, the following were selected as the main factors determining the effectiveness of the return of wells to reproduction:  Characterizing productive horizons: effective oil-saturated thickness, average depth, reservoir temperature.
 Characterizing the filtration and capacitance properties of reservoirs: porosity coefficient, oil saturation coefficient, permeability.
 Characterizing properties of fluids: viscosity of reservoir oil, difference in densities of water and oil.
 Characterizing technological parameters: the life of the deposits, the degree of reduction of the initial reservoir pressure, the oil recovery factor, the degree of depletion of recoverable reserves. The limits of changes in these factors are given in Table 1. It shows that the limits of change in the effective oil-saturated thickness of the reservoir are from 2.8 m to 34.5 m, the average depth of occurrence of productive formations from 300 m to 4300 m, the reservoir temperature from 30 0 C to 136 0 C, the degree of reduction of the initial reservoir pressure from 0.01 to 86.6%, the average porosity coefficient from 7.2% to 20%, the oil saturation coefficient from 52% to 81%, average permeability from 0.013 mkm 2 to 0.532 mkm 2 , difference in water and oil densities from 96 kg/m 3 to 231 kg/m 3 , viscosity of reservoir oil from 0.7 mPas to 129 mPas, well operation time from 8 years to 118 years, current oil recovery factor from 0.04% to 47.45%, depletion of recoverable reserves from 0.18% to 99.24%. Fig. 2. Histogram of the initial average daily flow rate of wells (tons/day) returned from liquidation: Category I wells -returned from liquidation at reservoir-type deposits with low viscosity oils; Category II wells -returned from liquidation at reservoir-type deposits with high-viscosity oils; Category III wells -returned from liquidation at sub-gas oil deposits (from oil rims). To evaluate the importance and level of data content of geological and field factors, the most commonly applied ways are non-parametric criteria, which are used in the tasks of comparing two groups of observations with a distribution law. In these methods, two relative pairs of groups of progress objects are accounted. The first group contains objects with the studied parameters less than its average value (A), the second with more than its average value (B).
To assess the degree of information content of each factor, we used the Kullback measure [16]. I accordance to this method, the information content of the i-th range of the jth factor is determined by the formula: (1) Where DC(x j i ) is the diagnostic coefficient of the j-th factor; P((x j i )/A) -probability of getting into the group A of the i-th range of the j-th factor; P((x j i )/B) -the same for group B.
The current oil recovery factor in % is calculated as the ratio of cumulative oil production (Q) (as of 01/01/2021) to the initial geological reserve (ORO): (2) The degree of reservoir pressure reduction (in %) is calculated by the formula: Where R n -R t -initial and current reservoir pressure.
The degree of depletion of recoverable reserves is calculated by the formula: Where NIRR is the initial recoverable oil reserves.

Results
The outcomes of calculating the data content of the accounted geological and technological causes are described in Table. 2 in descending order of the degree of their impact on the first oil production rate of wells put into re-operation.

Discussion
The above generalization of the results of re-exploitation of deposits and wells showed that the efficiency of the process depends on a larger number of geological and technological factors. For this reason, when studying the reformation of oil deposits in the process of their conservation, methods of analysis and comparison of field data, experimental studies, as well as mathematical modeling were used. Each of these methods has its advantages and disadvantages. Studies summarizing the results of analysis and comparison of field data are mainly limited to considering oil and liquid flow rates, as well as water cut of the extracted well products. In these works, the influence of the process on the effectiveness of the implemented development system and ORF is practically not considered. The findings and recommendations are applicable only to the geological and physical conditions of the analyzed objects.
The reliability and validity of the results of the study by the methods of geological and hydrodynamic modeling in many respects depends on the sufficiency and quality of information about the geological structure of deposits, the properties of reservoir fluids, the filtration and capacitance properties of reservoirs, etc. Since many of the analyzed fields were put into development in the 30-50s of the last century, there is naturally a lack of initial geological and field information on them. This complicates and affects the reliability of the results obtained.
At present, many scientific centers around the world have experimental equipment that allows them to conduct high-quality research on the study of the process of reformation of depleted oil deposits. However, due to the activity of studying this process and the large material costs, the number of experimental studies is not much.
Under these conditions, the most reasonable approach to solving the tasks is to use the approaches developed in the oil and gas industry to analyze and compare the indicators of the exploitation of deposits and wells, as well as methods for identifying the most significant factors from a large number of mutually related geological and technological indicators.

Conclusion
Based on the results of calculating the information content of geological, physical and technological factors, the following conclusions can be drawn:  -The efficiency of returning wells to reproduction after their long downtime mainly depends on the value of the initial effective oil-saturated thickness of the reservoir. This is confirmed by almost all known fishing data. In which it is noted that the best results (relatively high oil flow rates and a decrease in the water cut of the extracted products) were achieved in wells located in the arched parts of the deposits with the highest effective oil-saturated thicknesses of productive formations.
 -The degree of influence of technological factors on the performance of wells put into re-operation is very high. Especially the time of operation of wells before their shutdown and the degree of recovery of geological and recoverable oil reserves.
 -The degree of influence of other factors on the performance of wells put into reoperation is much less. Although they play an important role in the process of reformation of depleted deposits during the period of their conservation.
 -The selection of wells for re-operation after their long downtime (return from liquidation and conservation) should first of all be carried out in wells that have been watered for a relatively short time, located in the arched part of the reservoir in areas with a relatively low degree of depletion of oil reserves, as well as a decrease in the initial reservoir pressure.