Enhancement of Horizontal Well Oil Recovery by Means of Chemical Stimulation

Complex of self production technologies solves the problem of preservation, restoration and enhancement of natural collection characteristics of bottom hole formation zone and, moreover, enhancement of active draining field and level of hydrocarbon selection by oil wells of different construction in non-uniform porous fractured carbonate reservoirs. The scientific and methodological basis for complex of well stimulation technologies in carbonate reservoirs is the following principle – phased, consistent, rational inclusion in the development and exploitation of the entire producing formation thickness, and only thereafter – successive, phased realization of physical and chemical influence on the formation-reservoirs depth and extent. The article deals with the solution of up-to-date problems of restoration and enhancement of productivity of oil wells in Tatarstan carbonate reservoirs under conditions of import substitution of advanced technologies of the formation oil recovery enhancement. Stimulation issues on the entire chain of oil extraction technological process are regarded from the formation drilling-in until repair-isolation works. Reduction in oil recovery cost is taken into account by means of the effective application of complex technical solutions, operations matching in time and power inputs on production enhancement in wells from water-bearing horizon. Enhancement of technical and economic efficiency of one's own technologies of acid treatment of vertical and horizontal wells leads to enhancement of current and final coefficient of hydrocarbon resources extraction. Solution of the above stated tasks was performed on the basis of the suggested by the authors principally new, scientifically based, mastered in production and inculcated in industrial scales complex of our own technical and technological solutions providing achievement of considerable enhancement of the efficiency of hydrocarbon raw materials extraction in Tatarstan.


Introduction
Generalized analysis of foreign companies activity directed at the development of stimulation shows that academic research is devoted to two main tendencies (Matrix Stimulation, 2005): on chemistry of processing materials and on technological methods of their delivery and injection into formation (Matrix Stimulation, 2005-Brian & Metcalf, 2009).
Concerning the first method, the development of acid self deflecting and self regenerating in the formation acid compositions, acids with regulated reaction kinetics with slowing down as well as acceleration of the selective reactions on mineralogical components, hydrocarbon compositions with the enhanced capacity pollutants, highly active emulsifying agents, penetrating agents and dispersants and also application of organic acids and their mixtures with inorganic.Abroad oil service companies, as a rule, apply chemically pure hydrochloric acids with initial concentration 28-38% of chlorine hydride with addition of from 3-4 to 10 and more special ingredients of different classes (Matrix Stimulation, 2005).Naturally, prices on such acid compositions (Clay Acid, RNF, Regular Mud Acid, Sandstont Acid, Half Strenght Mud and others) are very high.Analysis of technical and patent foreign literature showed that development achievements in the field of chemical stimulation of horizontal wells (HW) work is held in the direction of the complicated system of inflatable packer with relief (bypass) valves use (Coulter, 1993).HW cavity is divided into small parts in advance.Acid is cyclically directed into these pipe fields between packers, gradually, and this way the entire HW or its calculated intervals are being treated.
Halliburton, Schlumberger, Chell, Vestern, Dovell, Nowsco, Chevron Phillips, the leading foreign companies dealing with the development of acid compositions on the basis of hydrochloric acid and compositions with hydrofluoric acid, carry out research on foam-acid compositions thickened (polymers, surface active compositions) acids, acid containing emulsions, generation of acids in the formation itself.For deep influence on carbonates in the USA polymers of acryl row are used, gels on the basis of alkaline metal silicate, boric acid salts, biopolymer reagents.Baker Huges, Schlumberger, Halliburton, McDermott, BJ Services Company created, patented and successfully continue to use for treatment their own acid solutions on the basis of pure syntetic hydrochloric acid with addition of the definite set of complex additives, organic acids which provide the enhancement of the wells outputs, preservation of the formation's collector properties, heat-resistance of compositions and protection of oil producing equipment from corrosion.However, import of such products in Russia with beginning of sanctions in the gas and oil extracting field gradually decreased.
St. James Oil Company improved acid stimulation of wells.To achieve longer efficiency of hydrochloric acid treatment, phosphonic acid with hydrochloric acids are injected.
Exxon Mobil -stimulation of powerful carbonate formations, injection strategy, calculating and modelling of acid treatments, HF projecting.
Chevron -methods of operational formation qualities disturbances evaluation, chemistry for hydrochloric acid treatments and HF.The basic characteristic feature is high well productivity owing to hydrocarbon inflow through large surface area, there exist restrictions for technology -oil saturated formation thickness should be not less than 15 m, rocks dissolubility -more than 80-90%, the absence of water-saturated rocks, porous structure.
Jogmec-Tokio conducts research in technologies of acid HF.
Eni Exploration -complex research of objects and technologies of inflow stimulation.
Apache Corporation -the use of slowed down reaction of acetic acid under conditions of low intensity of liquid injection regulating at the same time water influx of well production.The essence of technology is based on organic acid of high concentration injection in rock with large volume of fresh water.Resuming the above said, it is possible to make certain conclusion that new technological approaches being developed at present in JSC Tatneft in the field of oil inflows stimulation and wells BZT in complexly built porous fractured carbonate reservoirs have a perspective to become basic geological and technical events and solve the given tasks on intensification of hydrocarbon recovery.

Discussion
Analysis of modern state of the filtration capacity properties (FCP) of the downhole formation zone (DFZ) during wells opening, mastering, exploitation and repair and also generalization, systematization of BZT technologies and wells stimulation allowed to substantiate perspective tendencies of their development and work out methodology of their systemic, complex application on the entire technological chain of well exploitation in carbonate reservoirs.Analysis indicates the following priority of the most important tasks (Musabirov, 2007).
1. Quality opening (initial and secondary) of producing formation.The problem must be solved by means of industrial operation of irrigating solutions and perforation liquids, penetration of which or their filtrates in DFZ does not have negative influence on FCP (Kazemi, 1969).
2. Effect achieved from quality formation opening must stay during the operational period.It becomes possible by means of special technological liquids application during repairing works with minimal influence on reservoir properties of DFZ (Renard & Dupuy, 1990).
Complex approach, management and regulation of quantity and fluid inflow to HW quality (Joshi, 1988;Peaceman, 1983): -quality initial formation opening with optimal-effective geometry of borepath with preservation of FCP reservoir; -constructing of downhole with maximum filtration area; -engineering projecting of rational installing of cut off packers regulating fluid inflow from fractured zones of HW; -obligatory removal of illuvial components of drilling fluid from HW side; -stimulation of fluid inflow with HW oil saturated intervals and its whole length, when it is necessary carrying out of reliable WW with fractured as a rule water-bearing intervals cutting off.
-optimization of energetic parameters of inflow process (optimal balance of formation and hole pressures); thus, regulation of water free operational period and water content in production; -exclusion of negative effect of FCP formations of water technological liquids during repairing works.

Conclusion
On the bais of BZT complex of new and improved technical and technological solutions was industrially introduced in oil fields providing systemic successive implementation of import substituting block technologies: • Selective (controlled) chemical formation BZT on the basis of so called "smart" "self deflecting" AC of the last generation and polymer compositions with chemical cross-link and destruction (regulated behavior rheology): • Methods of complex jet acid, shock wave, dynamic formations BZT, physical and chemical and hydro-impulse influence on DFZ of uneven (heteragenous) in structure and penetrability of reservoirs; • Regulation of oil and water selection (technologies of water inflows isolation in VW and HW, complex stimulation technologies with simultaneous restriction of water inflow).
Within realization technical and technological solutions provided the following indices of technological and economic efficiency and social effect in regions of inductrial works carrying out (Ibragimov, Musabirov, & Yartiev, 2014).
• introduction in 1163 wells; • -995 th.t; additional oil recovery -995000 t; • total effect from oil recovery stimulation technologies was 1.5 billions roubles; • additional payments in budget and non-budgetary funds in RF are estimated in 9.3 billions roubles; • multiplicative effect taking into account adjacent fields was estimated in 19.1 billions roubles; • reduction in staff in main production is absent since company's oil recovery remains on the level achieved and even increases.

References
Babadagli, T., Al-Bemani, A., & Boukadi, F. (1999) Saudi Aramco -mechanism of acid reactions, acid treatment of dense matrix blocks, mechanism of rock etching by acids of different modifications.Acid jet technologies StimTunnel, РVDSA, BJ Services Company -are technologies of arborescent well building (with several open side tracks) in carbonate reservoirs allowing to enhance the filtration surface area with further oil recovery.In fact, this technology of rock erosion by acid in open track and well creation containing basic track with many subsidiary tracks on different levels, each of which contsisting of smaller fractal channels formed from penetration of acids and dissolution of rock.Example of arborescent well is presented in Figure 1 (Espina, Guerrero, Colmenares, Dias, Rae, & Lullo, 2011).

Table 2 .
Wells of 4488H and 4859H operation dynamicsThe essence of technology lies in the following.In the zone of producing formation in oil-well tubing column double packer arrangement with use of inflatable packer descends.Arrangement is placed with fixed interval of HW open bore starting from toe.Further acid treatment with use of special acid compositions allowing deep penetration in the oil-bearing collector zone is produced.Then double arrangement with inflatable packers is moved to the next interval of open bore closer to heel with region's treatment with acid compositions.Such an operation is repeated several times depending on the open length of HW bore.In table 3 dynamics of the main wells operation before and after BZT on application of foam acid BZT is presented.Priority of the developed objects -the reduced formation pressure, Tournaisian sediments.Average daily output increase for the operational period in three successful wells was +3,1 t/a day (on the level of technologies PAKS, KSMD).In well 9304ha NGDU Nurlatneft and 16024ha NGDU Yamashneft, 9304ha NGDU Nurlatneft, 15289ha NGDU Leninogorskneft -negative result, there occured production watering.In wells 35396ha NGDU Leninogorskneft, 17507ha NGDU Yamashneft -insignificant result.
. Seawater as IOR fluid in fractured chalk.Paper SPE 93000 presented at SPE International Conference on Oilfield Chemistry, Houston, Texas, USA, 2-4 February.