А.I.Shipilov, PhD in Сhemical Sciences, R&D Director of Polyex jsc
А.V.Elsukov, Chief Chemist Analyst of Polyex jsc
А.I.Mikov, PhD in Technical Sciences Investment And Innovation Director POLYEX jsc
During the exploitation of oil and gas wells it is inevitable that maintenance and repair works are to be performed. The period between the repairs of oil producing wells equipped with ESP is about 2 years, and in the wells equipped with SRP is 1 year. Proper repair is possible only when the operation of proper killing of the well is performed.
Customer requirements to the killing fluid are rising constantly. This encourages companies involved in development and promotion of the products of this type to use all their scientific potential and experience in the development of advanced formulations and additives which make this crucial operation become non-damaging for formation and preserve the reservoir properties of the productive formation to maximal extent.
In this article the brines and additives for the killing fluids produced by POLYEX jsc are presented.
General requirements for killing fluids include requirements towards their density, corrosiveness, suspended solids content. Special requirements may include low temperature resistance, compatibility with reservoir fluids, stability over time at normal and high (reservoir) temperatures. These requirements are based on mining and geological conditions and on climate conditions in which killing fluids are applied and are subject to corresponding procedures of companies- user of subsurface resources.
POLYEX company produces chemicals, for non-damaging killing of wells with different reservoir pressure which preserve the properties of critical area at most. The company works continually on the improvement of quality of existing products, develops new materials and formulations, it also maintains author’s supervision for correct and effective application of supplied chemicals.
The company has developed a line of high density clear fluids of TZhG brand, which are basically brines, containing different specialized additives (hydrophobic modifying agents, corrosion inhibitors, scale inhibitors, demulsifiers) and which are free from polymers and solid matters. TZhG brand products are:
- TZhG-1.3 BK – calcium free system with the density of 1.28-1.32 g/cm3, with freezing point of minus 20°С, compatible with majority of reservoir fluids;
- TZhG -1.6 (Z) with the density of 1.58-1.62 g/cm3. It is a comparatively cheap system with low freezing point and low suspended solids content, freezes at temperatures lower than minus 45°С;
- TZhG -1.6 (L) with the density of 1.58-1.62 g/cm3, even cheaper system meant for milder climate conditions with lower suspended solids content; freezes at minus 15°С;
- TZhG -1.8 (Z) with the density of 1.78-1.82 g/cm3. High density fluid for abnormally high reservoir pressures, winterized form, freezes at minus 40°С.
- TZhG-2.0 (Z) with the density of 1.95-2.10 g/cm3. Low corrosive high density well killing fluid for abnormally high reservoir pressure . This fluid has been recently developed and it is ready for pilot testing.
All mentioned fluids have a low corrosiveness – not more than 0.125 mm/year.
POLYEX achieves the compliance of high density well killing fluids to the regulating documents of oil companies by adding specialized additives. By additives we mean chemicals added in small amounts, which increase significantly operating characteristics of process fluid. Among these additives there are scale inhibitors, corrosion inhibitors, hydrophobic agents, surfactant based-demulsifiers.
A very important detail in application of corrosion inhibitors in well killing fluids is that there is no versatile corrosion inhibitor. Everything depends on the particular conditions of fluid application, this is the reason why different brine systems require a different approach to the selection of inhibitors. Currently POLYEX has developed a series of corrosion inhibitors for different well killing fluids:
- NORUST® 760 А is effective in brines with the density up to 1.6 g/cm3;
- NORUST® 9805 is effective in brines with the density up to 1.6 g/cm3; provides H2S and CO2 corrosion inhibition;
- NAYTING B is effective in systems containing nitrate ions;
- SOLING V is film-forming corrosion inhibitor for high density and high temperature products;
- SOLING T – high temperature inhibitor for systems with density up to 2 g/cm3.
The experience of well killing with brines shows that multiple well killing leads to formation damage, including the forming of water screen (forming film water on the surface of the rock, reducing pore volume of the reservoir). In order to decrease this negative impact, so called hydrophobic modifying agents are added into well killing fluids produced by POLYEX. However, in our opinion, this name does not fully reflect all the functions performed by the chemicals of this type. These chemicals:
- promote breaking of water-oil emulsions due to low interfacial tension of oil-water interface area;
- repel water from hydrophilic areas;
- reduce residual water saturation and promote removal of jammed water.
Numerous data of laboratory and industrial tests point to the fact that injection of hydrophobic agents into well killing fluid in most cases provides reduction of water saturation in bottomhole pay zone and facilitates removal of water screen, hence facilitates faster resumption of stable output of the well.
In case there are fractured zones or thief zones in the reservoirs and high gas ratio is observed, our company may recommend hydrocarbon emulsions for well killing based on POLYEX’sc chemicals EKS-M (for formation temperatures up to 60°С) and EMITRIT® (for formation temperatures up to 100°С). These are emulsifiers for inverted emulsions, when a disperse medium is hydrocarbon. Well killing with inverted emulsions provides preservation of formation properties of pay zones due to low filtration and fast decrease in viscosity of emulsions at oil filtration. Hydrocarbon emulsions making technique consists in emulsifier dispersion in hydrocarbon medium with further water phase addition. Depending on the water phase density, the density of hydrocarbon emulsions may be vary from 0.9 g/cm3 to 1.25 g/cm3.
In general, the series of well killing fluids developed by POLYEX provide a wide range of densities and allow to perform technological operations of non-damaging killing of wells in the most variable climatic, mining and geological conditions. We are fully confident, that dialogue and collaborative work between POLYEX and specialists of oil producing companies in terms of well killing fluid development for specific conditions may both improve already available products and develop new compositions, allowing to perform the most effective repair operations.