Using microbiological technology to increase oil recovery for terrigenous oil deposits

. In this paper, the authors consider one of the widely used methods to increase the efficiency of flooding within the terrigenous reservoirs of the Volga-Ural oil and gas province - microbiological effects on the matrix of rocks to align the profile of pick-up and production. The process of the influence of metabolic products produced by microorganisms on the change in oil recovery coefficients over time has been comprehensively studied, as a result of which a significant increase in oil mobility has been established. The results of the measures and their main technical and economic indicators before the impact and after the operation are presented, on the basis of which it is recommended to use this technology to increase the efficiency of oil displacement in fields at the final stage of development as one of the alternative options for implementing high-quality energy resource management.


Introduction
In most fields of the Volga-Ural and West Siberian oil and gas provinces with terrigenous reservoirs, productive strata are sharply heterogeneous in thickness and in the predominant fraction of constituent grains, which causes a difference in their hydrodynamic and capacitive characteristics volumes along the section and strike [1][2][3][4][5][6].
The geochemical appearance of oils is characterized by significant variability under the influence of various factors due to the intensive development of fields with the injection of large volumes of water.Oil conversion products are slow-moving heavy oxidized oils and almost immobile bitumen.Residual oil in waterflooded formations jas similar properties, which complicates its extraction.
It is promising to use vital bacteria for injection into the formation.Experiments showed that the bacteria clog areas with high permeability faster, i.e. a much larger amount of injected fluid enters there.

Methods and materials
For the geological and field conditions of the field under consideration, based on the criteria for the application of enhanced oil recovery methods (EOR) and the analysis and generalization of Russian and foreign experience for oil deposits of the Kizilov horizon, biotechnology based on dry activated sludge (ASI) should be used; for deposits of the Pashiysky, Bobrikovsky and Tula horizons, the gel-forming technology based on a biopolymer (BP-92) should be used.
The technology is based on the use of a bioreagent consisting of dry activated sludge (microbial biomass) and a biostimulator -hydrolytic sugar.The addition of a biostimulant to activated sludge makes it possible to activate microorganisms that are part of the activated sludge and formation microflora, which affects the residual oil, increasing its mobility.
The technology is based on the following mechanisms: selective plugging of highly permeable interlayers with microbial biomass and the resulting biopolymer, which increases the sweep efficiency of the reservoir by flooding and microbiological processes occurring in two stages [7][8][9].
Stage 1 -aerobic and intermediate (facultative) forms of bacteria in the bottomhole formation zone decompose organic matter using the available reserves of dissolved oxygen.With incomplete oxidation of organic matter, acids, alcohols and other compounds are formed.
Stage 2 -anaerobic: in the presence of methane-forming bacteria, the fermentation process ends with the conversion of organic acids into gaseous end products -methane and carbon dioxide, increasing the residual oil displacement ratio.

Results and Discussion
The technology of microbiological impact based on DAS (dry activated sludge) developed at BashNIPIneft has been introduced since 1996; 146 well treatments have been carried out on the carbonate reservoirs of the Novouzybashevsky, Volkovsky and Znamensky fields, additional 153.9 thousand tons of oil have been produced (Table 1).The results of the use of activated sludge in the injection wells of the Bobrikov horizon, developed by NGDU Tuimazaneft, are shown in Table 2 [10].The average efficiency of one treatment is 1.0-1.5 thousand tons of additional oil.Production wells begin to react 1-3 months after the injection.The duration of the effect is 24-36 months, depending on the geological structure of the reservoir and the value of its residual reserves.The frequency of repeated treatments is 1 year.The injection of a complex bioreagent does not require field development and reequipment of wells.Formation biotreatment can be carried out through one focal injection well, a block-comb or a sewage pumping station acting on a group of wells.The amount of reagents is depends on the permeability, injectivity, and formation thickness.Bioreagents are delivered to the mouth of the injection well in tank trucks.Activated sludge and biostimulant are mixed immediately before the injection.The bioinfluence is as follows: activated sludge 0.2-1.0t (from the injectivity of wells) and sugar (1.5-3.0 t) are loaded into the tank, using TsA-320M or AN 700 pumpt, the contents of the tank are mixed, then it is pumped into the well.Fresh water buffer injection is required for more intensive microbial processes in the reservoir.When treating, the pressure at the wellhead is recorded, then the well is connected to the conduit.
The industrial implementation of the technology will begin in 2004 and end in 2047.For 43 years, it is planned to treat 57 injection wells, additional oil production will amount to 61.1 thousand tons.The consumption of reagents will be as follows: dry activated sludge -57 tons, hydrolytic sugar -171 tons.An increase in the oil recovery factor will be 0.9% of the balance reserves or 7.7% of the remaining recoverable reserves [11].
In the 1990s, researchers discussed the potential of biopolymer flooding in the development of oil fields.Within the state scientific and technical program "Progressive technologies for the integrated development of fuel and energy resources of the subsoil of Russia (SSTP "Nedra of Russia") of the Ministry of Science of the Russian Federation, a fundamentally new technology of highly efficient biopolymers was developed.The production of biopolymers is aimed at replacing PAA with other thickening agents that are resistant to extreme conditions (temperature, degradation, etc.).The most suitable are exopolysaccharides -polymers produced on the basis of microbial biosynthesis.Strains producing exopolysaccharides (biopolymers) were isolated and produced.Unlike the wellknown polyacrylamide, biopolymers are highly resistant to mechanical, thermal and chemical degradation, not affected by hydrogen sulfide and environmentally safe (its biodegradation period is 12-15 months).The disadvantage is its incompatibility with mineralized waters.
Biopolymer BP-92 (TU 9199-001-1793259398) is produced by the Siberian Biotechnical Enterprise in liquid form (unlike foreign ones) with a polysaccharide content of up to 1%.BP-92 has no raw material restrictions, and its cost is $250-300/t, which is 10 times less than the cost of polyacrylamide.
It was found that when a biopolymer is injected, the filtration rate in a low-permeability interlayer accelerates and slows down in a high-permeability one that is flooded, the oil recovery factor increases by 13 points, and the WOR decreases.
Pilot works and technology testing were conducted in Western Siberia (Tallinskoye, Tarasovskoye, Pokamasovskoye, Severo-Pokurskoye, Yuzhno-Aganskoye, Vatinskoye fields, etc.) Field tests confirmed the results of experimental studies.The injection of biopolymer (BP-92) into injection wells changed the injectivity profile, increased oil production rates in the surrounding production wells, and decreased the water cut.With biopolymer flooding, the specific technological efficiency will be at least 100 tons per 1 ton of reagent.The technological effects of the technology in Siberia are not given due to the fact that the West Siberian fields differ significantly from the fields of Bashkortostan (depth of occurrence, types of reservoir, physical and chemical properties of oil and water, degrees of depletion of oil reserves, etc.).
In 1987-1993 in Bashkortostan field trials of the similar biopolymer Simusan were carried out in the Yusupovskaya and Novo-Khazinskaya areas of the Arlanskoye field (TTNK facility), both in separate foci and in the field.With focal treatments per ton of biopolymer, a specific technological effect of 50-80 t/t was obtained.The water cut decreased by 2-3% regardless of the current water cut (or at the current water cut of 80%).
When calculating predictive indicators of the technological efficiency from BP-92, the following initial data were taken:  Reagent consumption (BP-92) for 1 well-treatment -3 tons. Consumption of CMC reagent for 1 well-treatment -0.03 t.  Consumption of KCr(SO4)•12H2O reagent for 1 treatment well -0.015 t.
It is planned to treat 12 injection wells on the Pashiysky horizon.Additional oil production will amount to 7.6 thousand tons, which will increase the oil recovery factor by 2.1%.
For the Bobrikov horizon, after 12 treatments of injection wells, additional oil production will amount to 11.8 thousand tons, which will increase the oil recovery factor by 2.8%.
It is planned to treat 9 injection wells along the Tula horizon.Additional oil production will amount to 7.2 thousand tons, which will increase the oil recovery factor by 2.3%.

Conclusion
 Oil-displacing metabolic products (biogases, biosurfactants) generated by microorganisms increase the mobility of residual oil and improve the displacement efficiency. The technology is based on the injection of biomass of dry activated sludge organisms into injection wells in oil fields of the middle and late stages of development, where waterflooding efficiency is low.

Table 1 .
Results of DAS use in Carbonate Reservoirs.

Table 2 .
The results of activated sludge use in injection wells of the Bobrikov horizon.