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Objectives The sustained long-term development of oil industry in Russia depends largely on how effectively oil reserves in the domanik sediments and other hard-to-recover reserves are brought on stream. Study and development of the domanik sediments–tight, low-permeability carbonate rocks–requires heavy investments and significant intellectual assets. The main challenge of nowadays’ is to find effective stimulation methods to enhance production from the domanik sediments. Development of technologies that can give insight into the reservoir dynamics to make decisions on the optimum completion strategies is one of the priorities of PJSC TATNEFT. Currently, common stimulation methods include treatments of the bottomhole zone and hydraulic fracturing. So, the focus is on the technologies that help to understand the process of hydraulic fractures propagation into the reservoir in the process of multistage hydraulic fracturing, to estimate the extent of the stimulated reservoir volume, and to assess zones of likely proppant penetration into fractures.

Main fields in the Republic of Tatarstan have entered the closing stage of development characterized by high extent of reserves depletion (86 % of OOIP) and increase of the share of unconventionsl hard-to-recover reserves (from 33 % to 80 %). A considerable portion of PJSC TATNEFT’s unconventional reserves (36 %) is found, primarily, in carbonate and carbonate-silicious (shale) low porosity/permeability or fractured reservoirs, which are very challenging targets. In this light, study of low-permeability and porous reservoirs has recently acquired a prime importance. Comprehensive studies are needed to understand stress and strain properties of reservoir rocks in challenging geomechanical in-situ conditions. To understand fracturing pattern of carbonate reservoirs in Tatarstan, the Company has been employing different survey methods, including aerial and space imagery, side-view seismic location (SVSL), offset vertical seismic profiling, tracer surveys, core analysis, including oriented and quick capture cores, 3D CDP seismic survey. Particular attention has been given to analyses of drilling troubles and vertical and horizontal wells’ performance. The results obtained make it possible to select optimal orientations of horizontal wellbores with a view to maximize future production. This paper presents the Bavlinskoye oil field case study. Recommendations on downhole surveys are presented, as well as guidelines to select an optimal orientation of a horizontal wellbore in fractured reservoirs.

The objective of the paper is to assess incremental oil production due to low-salinity waterflooding (LSW) in the Pervomaiskoye oil field operated by PJSC TATNEFT vs. conventional waterflooding by high-salinity water. In this field, low-salinity water has been injected for a relatively lengthy period enabling to assess the effects of low-salinity flooding. The procedure was as follows. Laboratory coreflood experiments included preparation of cores, their saturation with formation water and oil, core flooding using high-salinity and low-salinity water. In both cases, oil production was recorded and fines release was analyzed. Core test results were adjusted using a special numerical fines migration model, and relative permeabilities were calculated. Then a 3D model was built and history matched using the function that allows change of relative permeability to water. After this, the model was recalculated without this function and incremental oil production due to LSW was estimated. 3D modeling showed that cumulative additional oil production due to LSW in Pervomaiskoye oil field is 4.2 million m3, incremental oil recovery is 3.5 %. The effect was prominent in wells with wellstream water cut from 20 % to 90 %. Targeted application of LSW is recommended – in injection wells as an EOR method when wellstream water cut ranges from 20 % to 90 %, and in production wells as a water control technology.

Pilot projects on heavy oil production in the Republic of Tatarstan proved the efficiency of Steam Assisted Gravity Drainage (SAGD) and Cyclic Steam Stimulation (CSS) technologies for horizontal wells. This paper discusses ways of improving CSS technology and search for alternative methods of extra-viscous and heavy oil production from thin net pay zones. Various configurations of vertical wells and a single horizontal well have been considered. It has been shown that using vertical wells in combination with a horizontal well can provide a two-fold increase in ultimate oil recovery compared to CSS method. The best configuration has been identified to provide maximum recovery of heavy oil from thin pay zones. Pilot test program has been developed to test the suggested well configuration in the Ashalchy pilot area.

The objective of the work is to provide for competitive ability of the Company in the light of changing external market situation. Novel scientific and technological solutions that ensure innovation-based growth of the Company have been developed. Innovative upstream and downstream technologies allow increasing geological prospecting effectiveness, increment of hydrocarbon reserves and enhanced production. Among the innovative technologies developed and widely used by the Company are the following: –Integration of CDP 2D, 3D seismic survey and low-cost geological prospecting technologies (geological-geochemical survey methods, neuroseismic methods, etc.) to optimize geological prospecting. Cost effectiveness analysis and improvement of well survey methods (wireline formation testing, side wall coring, coring, variable density logging, gamma-ray spectrometric logging, pulse neutron generator, microimager- MCI, MPAL, offset vertical seismic profiling).–Methods to investigate the sedimentary mantle cross-section in naturally-fractured and tight rocks to evaluate the hydrocarbon potential and oil reserves in the long term.–G&G model and geochemical model of the sedimentary mantle with the objective to work out a strategy of geological prospecting of non-conventional reserves in domanik sediments, heavy oil, and natural bitumen. Oil reserves increment due to development of novel geological prospecting methods and technologies applied in new exploration targets and small-sized structures, re-evaluation of reserves of fields under development, commercial development of nonconventional carbonate low-permeability reservoirs, domanikites and domanikoids, heavy oil accumulations, increase of oil recovery factor in fields under development–all these are the most important conditions of sustainable industrial and social development of the Republic of Tatarstan. Introduction on a commercial scale of scientifically-based methods and technologies allowed increasing exploratory drilling success ratio to 75 %, exploratory wells–to 81 % in the Republic of Tatarstan, and from 20 % to 56 % outside Tatarstan, in PJSC TATNEFT license areas. The paper demonstrates that the task of effective development of non-conventional oil reserves is being successfully solved. New reservoir engineering technologies have been worked out in the process of oil fields' development. A significant factor contributing to the effectiveness of works aimed at reserves increment, development of non-conventional reserves, and oil recovery increase is the governmental support by way of tax incentives regarding production of depleted fields with cumulative production of OOIP more than 80 %, development of new small-sized fields, production of heavy oil reserves and reserves of domanikite and domanikoid sediments. The novel geological prospecting and reservoir engineering technologies developed and tested by PJSC TATNEFT hold great potential for petroleum industry. The technologies and equipment that have been developed in the process of non-conventional hydrocarbon reserves production can be used for development of similar reserves in other Russian regions.

The paper presents the results of laboratory nuclear magnetic resonance (NMR) studies of oil-saturated core samples of the Sheshminskian horizon of heavy oil fields in the Republic of Tatarstan. The analysis of the results showed that T2 spectra of oil recovered via centrifugation from the pre-heated core and oil remaining in the core are substantially different from each other. Magnetic relaxation rate of residual oil is much faster, and thus its viscosity is significantly higher than that of the recovered oil. The difference can be up to ten times. The spectrum of relaxation times T2 in the original oil-saturated core integrates information about all the fluid contained in the rock. The results and findings underscore the ability of NMR logging tools to characterize the entire oil fluid in the rock in situ.

The technology of multistage fracturing of horizontal wells for the development of low-permeability reservoirs has gained wide acceptance. The performance of multistage fracturing treatments may vary significantly: one treatment may yield large initial oil production rates but rapid increase of water cut and low ultimate oil recovery, while the other treatment may result in low oil production rates but moderate water cut increase and higher oil recovery. The purpose of this study is to establish the technologies aimed at achieving the utmost production performance of low-permeability reservoirs; namely, maximum initial oil production rates and oil recoveries. This paper considers three patented technologies which have been designed based on the results of reservoir simulation modeling and actual multistage fracturing performance data in real wells. These technologies have comprised the following research results: the most efficient wellbore orientation and well spacing, optimal angles (30° to 60°) between maximum in-situ stress vector and horizontal wellbore direction as well as between multiple hydraulic fractures, displacement front and natural fractures; appropriate number of fracturing stages N (N=1+L/100, where L is horizontal length) and induced fracture spacing (10 to 100 m), well operation conditions depending on reservoir pressure reduction in the course of reservoir development (up to 3% a year) etc. Numerical reservoir simulation studies have shown incremental oil recoveries of 2–5% as compared to the existing technologies of multistage fracturing in horizontal wells. Actual field data from Well 2917G penetrating the Domanic deposits of the Bavlinskoye field suggest sufficiently high oil production rates and low water cut when hydraulic fractures propagate at 45- degree angle with respect to horizontal wellbore. In recent years, boosting production performance of low-permeability (up to 2 mD) oil reservoirs has become one of top-priority tasks in Russia owing to mineral extraction tax incentives. The proposed technologies enable enhanced oil production from such low-permeability oil-saturated reservoirs and increased production rates through improvement of sweep efficiency.

Low-salinity waterflooding (LSW) and its effect on oil recovery factor have been studied around the world during the last 30 years. Low-salinity/fresh water was used in the initial stage of waterflooding because of deficiency of produced water. The Pervomaiskoye oil field in Republic of Tatarstan is one of the examples. Waterflooding started in 1966. Water was taken from the nearest water source, the Kama River. After more than 40 years, about two PVI has been injected, and 30.1% of this injected water was low-salinity. This paper presents the results of laboratory study of double core flooding using high-salinity (formation) and low-salinity (river) water. Two plugs were taken with the clay content of about 0.8% and 0.1% and the permeabilities of 432 and 291 mD, respectively. Dead oil was used with the viscosity of 37.5 mPa.s. TDS values were about 252,738 ppm for high-salinity water and 848 ppm for low-salinity water. Dry and vacuumed plugs were saturated, first, with high-salinity water, then, with oil, to simulate connate water saturation; then, oil was displaced with the same water. After this, the cores wre restored to the connate water by a secondary oil flood, and finally, LSW was simulated. In both cases, oil recovery was recorded and fines release was analyzed. Processing of the low-salinity corefloods data by a numerical model shows a two-fold decrease of water relative permeability for the first plug and 1.5 times reduce of residual oil saturation for the second plug. Fines release and formation damage was observed in the first plug and wettability alteration in the second plug. Oil relative permeability didn't change. Also, the unusual behavior of water relative permeability in response to low-salinity waterflood was observed, namely, it decreased at water saturation values more than 0.4. To investigate the low-salinity EOR effect seven pilots on the Pervomaiskoye field were analyzed, with high-salinity waterflooding following LSW. Incremental oil recovery for these pilots, that has been achieved by now, is approximately 5-9%. Calculations of low-salinity EOR effect in the field show that LSW through 96 injectors ensures to date the incremental recovery factor of 3-4%. The reservoir 3D model of five-spot pattern for the Pervomaiskoye field for fines-assisted waterflooding gave incremental oil recovery by 5-11%. The laboratory relative permeabilities were used in the model. Incremental oil recovery depends on the number of the flooded layers.

This paper presents the results of laboratory and nuclear magnetic resonances researches of seventy-seven crude oil samples from the Devonian, Carboniferous and Permian formations of Tatarstan oil fields with a wide range of viscosity varying from several cps to 60,000 cps. The results can be used to estimate the viscosity of the oil, including in situ viscosity. It has been found that the NMR relaxation characteristics of the tested samples presented in the form of T2 time distribution spectra confirm that crude oil is multicomponent. Numerical characteristics resulting from the analysis of the spectrum of spin-spin relaxation time T2 have been determined, which show good correlation with the viscosity of the hydrocarbon fluid. The paper presents the obtained experimental correlations between the rheological properties and NMR characteristics for oil samples studied.

This paper presents the results of research programs performed by OAO TATNEFT aimed at additions to reserves and increased production from non-conventional reservoirs found in Tatarstan. It is worth mentioning that TATNEFT has been studying non-conventional reservoirs since 1950s. To date, most of Tatarstan's oil fields have entered their closing stage of development with 86% cumulative production of OOIP and increase of hard-to-recover reserves share from 33% to 80%. A considerable part (36%) of TATNEFT hard-to-recover reserves falls on carbonate low-permeability sediments with low-thickness fractured reservoirs. Aiming at study of shale, or tight, oil in the territory of Tatarstan, TATNEFT has concentrated its efforts on the search for oil accumulations and study of low-permeability reservoirs and source rocks in the Tournaisian-Sargaevian formations. Prospecting of nonconventional hydrocarbon reserves and nonconventional reservoir study is a challenging, still, a very promising line of activity associated that can add value and yield good returns, considering that the Tournaisian-Sargaevian formations are widespread in Tatarstan, the domanikites are source rocks with high organic matter content, and proved oil potential of these deposits. A certain tight oil potential is also associated with the Samara, Ulyanovsk, Orenburg, Nenets Autonomous District, and Kalmykia leases. It is worth mentioning that evaluation of shale plays is not based on conventional methods involving seismic survey, logging etc., but rather on analysis of chemical-mineralogical and mechanical rock properties. Analysis of available well data has been performed; underway are large-scale core recovery projects, including oriented core and quick capture. Considering geologic environment of the domanikites and the domanikoids in the Volga-Ural oil-and-gas province, petrophysical properties of these rocks, fluid composition and properties, G&G information is analyzed to design hydraulic fracturing treatment in wells targeting the domanikites and the domanikoids. Candidates for hydrofrac jobs have been selected. Dedicated drilling and reservoir management technologies are developed.

Initial stage of development of microbial enhanced oil recovery technology for carbonate petroleum reservoirs is presented. OAO TATNEFT has been successfully employing microbial technology in terrigenous formations for a long time. Results of carbonate reservoir characterization and microbial community analysis show prospectivity of microbial technology development for carbonate reservoirs as well. It has been found that microorganisms isolated from formation water can produce metabolites with high oil displacing potential when growing on carbohydrates. The main features of microbial method under development are formulated.

To-date, more than 450 heavy oil fields and deposits have been identified in the Republic of Tatarstan with oil-in-places ranging between 1.4 and 7.0 billion tons, according to different estimates. Thermal recovery has proved itself as a reliable method and seemed a logical solution however it did not work properly in vertical wells. Building on the experience of development of the Yaregskoye field (the Komi Republic, Russia) and oilsands in the Canadian province of Alberta, the focus was shifted on working out of recovery methods involving horizontal wells. Engineering solutions accounting for concrete geologic environment and the steam assisted gravity drainage method used for production of heavy oil aim at high-quality casing, leak control in the interstring space, sand control, etc. In shallow heavy oil reservoirs with variable oil saturation of utmost importance are production control practices to ensure profitable oil production, to maintain optimal steam-oil ratio and safe operating pressure to ensure cap rock integrity.

Cement has been the main form of annular zone isolation for oil well construction since the beginning of the industry. It is a science in its own right comprised of technical experts continually improving materials and processes. Often the failure of cement to achieve zonal isolation can be traced to the lack of best practices. When the cement does not provide required annular isolation, the result can be one of the following: Excessive water productionLost production from cross flowGas migration revealed as sustained casing pressure / sustained casing flowContamination of shallow water zones Excessive water production diminishes the value of an asset and results in extra capex/opex for remedial operations. It can also result in delayed oil production. By concentrating on the zonal isolation process during the completion phase, many zonal isolation issues can be mitigated. A detailed evaluation of the cement program can highlight areas of improvement in both materials and operations. This is especially critical for areas where there are multiple stacked productive zones (oil on water) and varying reservoir pressures. An approach to improve annular integrity and the overall cementing process is the use of swellable packers. This practice has become more commonly accepted over the past few years. This paper will discuss the best practices adopted to mitigate water production up to the time of implementing swellable packers followed by the additional improvement achieved by including swellable packers with the best practices.

Carbonate reservoirs which contains significant amount of the remaining oil reserves in the Tatarstan Republic and in Russia are normally always naturally fractured. The presence of natural fractures creates additional heterogeneity of the reservoirs structure and makes an essential impact on the development strategy for such reservoirs. An integration of modern logging methods and special studies of oriented cores can give better characterization of such complex reservoirs. To estimate naturally fractured reservoirs' characteristics in carbonate deposits of the Tatarstan Republic the network of key wells were drilled with oriented cores recovered. To date ten such wells have been drilled. Core studies for qualitative and quantitative fracture system analysis have been made. Based on these studies main types, morphology and orientation of occurred fractures were identified. The depth, dipping angle, azimuth and distribution function has been determined for each fracture type from oriented core studies. Analysis of full-size core allowed estimating secondary porosity in intervals with caverns present. In the same intervals in addition to standard logging set special open hole logging was done including micro-resistivity and acoustic imagers, ultrasonic logging. Core measurements and logging data was analyzed in specialized software FracaFlow. Based on this analysis it was shown that core data and log data are in good agreement to each other and can be used in combination for better reservoir description in carbonates. The core orientation technique works best for wells with high inclination while micro-imagers can only be used in low angle wells (less than 40°). Low aperture fractures can be successfully identified on core in thick formations however the core orientation method fails in thin interbedded formations. The enhancement for core orientation technique was suggested which uses formation dipping angle data. Data received will help to evaluate carbonate reservoirs in more details. It will be taken in account while drilling new wells and for design of oilfields' development.

Formation evaluation provides the input data for reserves’ estimation and oil recovery strategy. Usually core analysis and log-interpretation calibrated with core data are the primarily sources of information. If there are some unsolved problems during core analysis stage, both log-interpretation and following estimations and conclusions could be wrong. We faced with such situation while evaluating properties of weakly consolidated sandstones and un-consolidated sands in Permian formations of Tatarstan Republic in Russia a few years ago. The problems were primarily associated with destruction of the rocks structure during the core recovery. Weight hydrocarbon saturation and grain size distribution were the only properties determined for these formations by conventional core analysis. It was impossible to determine porosity, permeability and volume hydrocarbon saturation which are critical for modeling and reservoir engineering purposes. Improved non-conventional method of core analysis described in this paper has been applied to solve this problem. The method suggests different approach depending on rocks’ consolidation. Since year 2005 more than 900 meters of core from twenty-three parametric wells were studied using the method proposed in this paper. As a result, necessary correlations for log interpretation have been adjusted. Application of the method allowed reasonable evaluation of the heavy-oil Ashalchinskoye field. The estimation for porosity has been increased from 30% to 34%, and permeability estimates were increased from 300 mD to 1-3 D. The new estimates of the reservoir properties allowed to suggest SAGD method for oil recovery, which otherwise would be uneconomical. The SAGD method is currently being implemented in this oilfield. Thus, the described unconventional core analysis method has proved its efficiency and can be recommended for further usage.

Approaches and methods for oil and gas exploration have improved significantly with maturing of major oilfields. At the initial stage of geologic exploration in Tatarstan prospects were identified by single coverage reflection profiling and structural drilling. The accuracy of these methods sufficed to identify large anticlinal closures. Efficiency of geologic drilling to evaluate the potential of deep intervals was rather high at that time, 88%. In 1980-ies Tatarstan witnessed a wide-scale development and application of 2D seismic methods to search for small oil habitats, structural and lithologically-screened structural pools. The discovery ratio of prospects identified with 2D seismics in Tatarstan in 1980-ies reached 72–95%, in the Volga-Ural oil and gas province 12–89%, in Russia–20%. At the initial and medium stages of exploration of zones with proved oil and gas potential primary objectives are structures with areal extent not less than 5 sq. km and closure at least 15 m. At mature stage of exploration (Republic of Tatarstan) primary targets are features with areal extent 0.5–1.0 sq. km and 5–10 m closure. Complicated geologic and economic environment attributed to exploration of deep carbonate pools, complex trapping mechanism, small-size and small-closure anomalies necessitated improvement of geologic prospecting methods. Nowadays Tatneft Company successfully uses the technology of forecasting of local hydrocarbon accumulations based on a package of 2D and 3D seismic prospecting and non-conventional exploration methods, the major advantage of which is low costs. So, lately in its exploration activity both on Tatarstan territory and in its license areas, Tatneft Company has been widely using advanced methods of geologic prospecting including investigation of seismic prospects with the help of neural network (NEUROSEISM system), geophysical and gas geochemical exploration methods, electromagmetic (EM) sounding, low-frequency seismic sounding, geochemical survey (Gore Sorber), and a number of others.

Simultaneous injection of heated exhaust gas (mainly CO2 and N2) and steam is one of EOR methods used in Russia for heavy oil recovery. In this case the energy released by gas-steam generator is used more efficiently compared to pure steam injection. Another benefit of using this EOR method is an additional reduction of the oil viscosity caused by exhaust gases solution in oil. This technology is particularly efficient when used for the small reservoirs being developed by a single well. Practical implementation of this technology is not always successful. One of the main reasons of failure is that parameters of the injected gas-steam mixture are far from being optimal. This study presents results of numerical simulation of the gas-steam injection process, so called huff-and-puff injection. The aim of the modeling was to estimate an influence of reservoir geometry and heterogeneity as well as huff-and-puff injection parameters on oil recovery factor. The reservoir's and oil properties were typical for Volga-Ural oil province of Russia. The radial geometry was used in numerical simulation with model radius equal to 400 meters. The main results of this study can be summarized as follows: heavy oil reservoir development based on primary depletion is not profitable; EOR methods are needed;efficiency of the huff-and-puff gas-steam injection increases with oil column thickness increasethere is optimal time for well being shut-in after gas-steam injection, this time is dependant on volume of injected energy;the reservoir heterogeneity plays significant role in thermal oil recovery process.

In the paper published by us [1] we set forth the methods and results of multivariate formation classification using the data base of geophysical parameters. Four types of formations have been identified with a 87–94% accuracy presenting non-reservoirs, water saturated and oil saturated reservoirs and one class which we called intermediary or mixed which judging by the results of standard conclusions on well-logging data, includes in various proportions the formations of all the above-mentioned classes. The present paper illustrates the results of classification with identification of other 6 or 8 groups of formations with the purpose of their detailization according to lithology and fluid saturation. The used data base included 6 geophysical parameters characterizing 14,546 formation intersections in 1,707 wells. The classification method lies in building of a ‘Tree of Classes’ according to the Ward method and appropriate classifying equations of discriminant analysis, and in determining the position of classes on the ordination plot in compliance with canonical equations. The classification results in the delivery of: –Borders of classes identified implicitly by means of discriminant analysis and described by classifying functions;–Tables with the results of classification quality on the training subsets, including the percentage of correct attribution of the interval to the class;–Statistical description of classes. Even though interpretation results were not used when making up classifying equations, the comparative analysis of classification results with standard interpretation data points to possibility and practicability of including the generated classifying and canonical equations into the active software for interpretation of well-logging data. This will increase accuracy and unambiguity of issued conclusions on the lithology of formations and their fluid saturation, which is very important for building geological and hydro-dynamic models of oil fields and estimation of reserves and designing of development.

Main fields of the Republic of Tatarstan in Russia entered late stage of development. In recent years more than 60 methods on enhanced oil recovery and stimulation of producing wells have undergone comprehensive tests in a wide range of geologic conditions. Based on geologic information and experience of fields development there have been created geologic and hydrodynamic models for analysis and forecasting of technological indices of oil production, as well as data bases on application of methods for enhanced oil recovery and stimulation of wells. Based on artificial intelligence, procedure is developed for selection of enhanced oil recovery method using model of residual reserves for different structures of development. Models apply neural network, capable to synthesize recommendations based on training by data base for realization of more than 60 methods on enhanced oil recovery and wells stimulation at similar structures of Tatarstan fields. These technologies are characterized by a set of criteria, including data on reservoir, formation fluids, operating conditions of wells, information on peculiarities of the technologies application. Models are trained on more than 2200 examples, reflecting correct conditions of realization of EOR and stimulation technologies, using considered technologies. During testing more than 70% of data sets are confidently recognized, which correspond to criteria of application of enhanced oil recovery methods, realized through injection wells, and more than 90% of methods on wells stimulation. The developed procedure allows to forecast and rank the application of different methods for enhanced oil recovery and stimulation of wells at specific producing and injection wells in real-time operation mode. This procedure is used in conditions of fields development in the Republic of Tatarstan. The conducted work allows to combine two modern approaches - modeling of field development and application of artificial intelligence for selection of most effective technologies on enhanced oil recovery and stimulation of wells. Data base has been developed for methods of enhanced oil recovery and stimulation of wells for conditions of Tatarstan fields.