Modern EOR Review
|1. EOR Goals|
|2. EOR Classification|
|3. Main EOR Methods|
|3.1. Thermal EOR|
|3.2. Gas EOR|
|3.3. Chemical EOR|
|3.4. Hydrodynamic EOR|
|3.5. Oil Production Intensification methods|
|3.6. Petros EOR|
|4. EOR Results|
|5. EOR in the World|
The effectiveness of oil recovery from oil-bearing formations using modern industrial methods is considered unsatisfactory in all oil producing countries, while the consumption of petroleum products is growing worldwide every year.
Average ultimate oil recovery in different countries and regions is ranging from 25 to 40% and makes, for example, in Latin America and Southeast Asia 24-27%, in Iran 16-17%, in the USA, Canada and Saudi Arabia 33-37% , in Russia up to 40%.
It means, that the residual or non-recoverable oil runs up to 55-75% of the initial geological oil resources (Pic.1).
Therefore application of Enhances Oil Recovery (EOR) technologies, which can considerably increase recovery in already developed oil reservoirs obviously becomes the first priority.
|1. EOR Goals|
Interest towards Enhanced Oil Rcovery methods is increasing every year all over the world and researches aimed at finding scientific approach to choosing the most effective EOR are developing rapidly.
In order to improve the economic efficiency of oil field development and to reduce direct capital investments the entire period of oil field development is usually divided into three main stages.
At the first stage of oil production (primary production) the natural energy of an oil field is used as much as possible. This energy is mostly the elastic energy, the energy of the dissolved gas, the energy of the gas cap and the potential energy of gravitational forces. (Pic. 2).
At the second stage methods to maintain reservoir pressure by injecting water or gas are implemented. These methods were called methods of secondary production. (Pic. 3).
At the third stage enhanced oil recovery (EOR) methods are used to improve the production efficiency. This stage is generally associated with so called tertiary production. (Pic. 4).
The actual distribution of residual oil requires that EOR methods first of all effectively influence oil in flooded and gassy areas as well as by-passed zones not covered by the current system of oil production.
It is absolutely obvious that with such a wide variety of the residual oil saturation conditions as well as under large differences in the physical properties of oil, water, gas and permeability of oil-saturated zones there cannot one universal method for enhanced oil recovery.
|2. EOR Classification|
All the known EOR methods are generally classified as follows:
1. Thermal EOR
Х Steam treatment ;
2. Gas EOR
Х Air injection;
3. Chemical EOR
Х Surfactant flooding (including foam);
4. Hydrodynamic EOR
Х Integrated displacement technologies;
5. Combined EOR
In most cases combined EOR methods are implemented. These are different combinations of hydrodynamic and thermal, hydrodynamic and physicochemical, thermal and physicochemical and other methods.
|3. EOR Classification|
|3.1. Thermal EOR|
Thermal EOR methods which stimulate oil inflow rate and increase the oil well productivity are based on artificial temperature increase in the well hole and the bottom zone area. These methods are used mainly for the production of highly paraffin oil (Pic.5). The warming leads to oil liquefaction, melting down of paraffin, resinous substances accomulated on the pipes surface and in the bottom hole area.
Steam treatment. Steam oil drive is an EOR method mostly used to displace high-viscosity oil. In this process steam is injected from the surface down to the low temperature and high viscosity oil formations through special steam injection wells.
The steam with a high heat capacity provides the oil formation with a significant amount of heat energy which heats the reservoir oil and reduces its relative permeability and viscosity. As a result the following three zones differing in temperature and saturation appear in the oil bearing formation:
1) Steam area around the injection well with the temperature varying from the temperature of steam to the temperature of condensation (400-200 ∞C), which provides extraction of oil light fractions (oil distillationl) and displacement of oil in the formation, i.e., joint filtration of steam and light oil fractions.
2) Hot condensate zone, in which temperature varies from the temperature of the condensation beginning (200 ∞C) to the reservoir temperature and hot condensate (water) displaces oil under non- isothermal conditions..
3) Zone with the initial formation temperature not covered by thermal effect. In this zone oil is displaced by reservoir water.
After steam heating the following processes take place: oil is distillated, reservoir fluids viscosity is reducing and all the formation agents are expanding their volumes, permeability, wettability of formation and mobility of water and oil are also changing.
In situ combustion. The EOR method of oil extraction is based on the ability of reservoir hydrocarbons (oil) to join the air oxidation reaction with oxygen, accompanied with a release of large amounts of heat. It differs from burning on the surface. Generation of heat directly in the reservoir is the main advantage of this method. (Pic. 5).
In situ combustion starts near the bottom-hole of an injection well usually by means of air heating and further injections.
The sources of the heat are commonly special bottom-hole electric heaters, gas burners and oxidation reactions.
After burning fire source at the well bottom-hole is set the further in situ combustion is supported by continuous air injection into the formation and diversion of the combustion products (N2, CO2, etc.) from the fire front.
Oil remaining in the formation after the displacement front is utilized as a fuel for further combustion . As a result the heaviest fractions of crude oil are burned out.
In case of conventional (dry) in-situ combustion the process is carried out by injecting only air into the oil reservoir. Since the air heat capacity is lower than that of the reservoir rock the rock heating front is moving behind the air combustion front. As a result the bulk of the heat generated in the formation (up to 80% or more) remains behind the air combustion front and is hardly used for the displacement but largely dissipated in the surrounding reservoir rock.
This heat has some positive impact on the subsequent displacement of oil by water in the reservoir zones not covered by the in-situ combustion process. It`s, however, clear, that the use of the bulk of the heat in the area ahead of the combustion front, i.e. approximation of the generated heat to the front of oil displacement, significantly increases the efficiency of the process.
Moving of the heat forward to the front is possible if an agent ( such as water) with a higher than air heat capacity is added to the injected air. This EOR method of wet combustion has been recently successfully applied in some Russian oil fields and abroad.
During the wet in-situ combustion water injected into the formation together with air evaporates after contacting the heated rock . The vapor transfers heat to the reservoir zone ahead of the combustion front where large heated areas saturated with steam and condensed hot water are created.
Cyclic steam treatment. Cyclic steam treatment is a periodic direct steam injection into the oil formation through production wells. After the injection period the well is shut in for some time and then is put back on production of heated (low viscosity) oil and condensed steam. The purpose of this technology is to heat the formation and oil in the bottom-hole zone of the producing wells, to reduce oil viscosity, to locally increase the reservoir pressure, to improve the filtration conditions and to increase the oil inflow to the well.
The mechanism of the processes occurring in the formation is quite complicated and accompanied by the same phenomena as in the steam treatment, but in addition to this in this case there occur a countercurrent capillary filtration and redistribution of the reservoir liquid when the well is shut in. During injection the steam penetrates into the most permeable reservoir layers and large pore zones. While soaking in the heated zone of the formation there is an active redistribution of saturation due to capillary forces: hot condensate replaces low-viscosity oil in the small pores and low permeable layers and forces it to the larger pores and higher permeable layers.
Such redistribution of oil and condensate saturation in oil reservoir is the physical basis of the process of oil extraction using cycling steam treatment. Without capillary exchange of oil and condensate during cycling steam soaking the impact would be minimal and limited to the first cycle only.
|3.2. Gas EOR|
Air-injection. The method is based on air-injection and subsequent air-transformation into effective displacement agents due to Low-Temperature Oxidizing process. As a result of Low Temperature Oxidation reactions high performance gas displacement agents, containing Nitrogen, Carbon Dioxide and Light Hydrocarbon are formed (Pic. 6).
The advantages of the method include:
Х use of air, that is an inexpensive agent;
Х use of the natural energy of the formation, i.e. high formation temperatures (over 60-70 o—) for the spontaneous initiation of intraformational oxidation processes and creation of an efficient displacing agent.
The rapid initiation of active intraformational oxidative processes is one of the most important consequences of the use of formation thermal energy for the implementation of air injection in the light oil fields.
The intensity of oxidation reactions grows rapidly within the temperature increase. Carbon dioxide injection. Carbon dioxide dissolves in water much better than hydrocarbon gases. The solubility of carbon dioxide in water increases with increasing of pressure and decreases with increasing of temperature.
When dissolved in water, carbon dioxide viscosity increases slightly and this increase is insignificant. With the mass content of 3-5% carbon dioxide in water its viscosity increases only by 20-30%. Formed by dissolving CO2 in water, carbonic acid N2CO3 dissolves some types of the rock cement increasing reservoir permeability. Clay water swellable also reduces because of the carbon dioxide. Carbon dioxide dissolves in oil 4-10 times better than in water, so it can pass from the aqueous solution into the oil. During the transition interfacial tension between oil and water becomes very low greatly improving the oil displacement process.
Carbon dioxide in water contributes to the washing -off of the oil film which covers the primary rocks, and reduces the possibility of the water film breaking. As a result, drops of oil at a low interfacial tension roam freely in the pore channels, and the oil phase permeability increases.
When CO2 dissolves in oil viscosity of oil decreases, its density increases, while the oil volume increases significantly: the oil swells.
1,5-1,7 times increased oil volume with dissolved CO2 in it makes a particularly large contribution to oil recovery improvement in the low-viscosity oil reservoirs. In displacing high-viscosity oil the major factor that increases the rate of displacement is a decrease of oil viscosity due to dissolving CO2 in it. The larger the initial value of oil viscosity, the stronger is this decrease.
When reservoir pressure is above the pressure of full miscibility of formation oil with CO2, carbon dioxide will displace oil as an ordinary solvent. In this case three zones occur in the formation: an original formation oil, a transitional zone (from the properties of the original oil to the properties of the injected agent) and a zone of pure CO2. If CO2 is injected in the already water flooded formation, oil that displaces formation water, occur before the CO2 zone.
The volume expansion of oil due to the influence of dissolved CO2 on it, together with the change of viscosity of liquids (a decrease in oil viscosity and increase in water viscosity) are the main factors determining the efficiency of carbon dioxide use in oil extraction in general and extraction of oil from flooded reservoirs in particular.
Nitrogen, flue and other gases injection. This EOR method is based on the combustion of solid propellants in liquid without any air-tight chamber or containment. It combines thermal, mechanical and chemical effects, namely:
a) combustion gases formed under high pressure (up to 100 MPa) displace liquid from the well into the formation which extends natural and creates new cracks;
b) heated (up to 180-250 oC) powder gases when penetrating into the formation melt the wax resins and asphaltenes;
c) gaseous combustion products are composed mainly of hydrogen chloride and carbon dioxide. Hydrogen chloride in the presence of water forms a weakly concentrated solution of hydrochloric acid, carbon dioxide when dissolved dissolving in oil reduces oil viscosity and surface tension which results into increase of well productivity.
|3.3. Chemical EOR|
These methods are first of all suitable for enhanced oil recovery from the heavily depleted, flooded formations with scattered, irregular oil saturation.
The methods are applied in the deposits with low viscosity oil (no more than 10 mPa*s), low salinity water, where productive formations are represented by carbonated collectors with low permeability. (Pic. 7).
Surfactant flooding (including foam) Flood displacement is aimed at reducing the surface tension at the oil-water border, increasing oil mobility and improving its displacement by water. Due to improving the wettability of rocks, water is better absorbed into the pores filled with oil. As a result water faster moves in the formation and displaces more oil.
Polymer displacement. During polymer flooding a high molecular chemical reagent Ц polymer (polyacrylamide) is dissolved in water. This reagent has the ability even at low concentrations to significantly increase water viscosity reducing its mobility and thus increase the coverage of reservoirs flooding.
Polymers are УthickeningФ the displacement water. This reduces difference between oil and water viscosities and as a result effectively prevents water breaking through oil due to viscosity difference or heterogeneity of the formation physical characteristics.
In addition polymer solutions of high viscosity displace not only oil, but also water from the porous medium. Therefore they interact with the skeleton of the porous medium, i.e. rock and its cementing substance. This causes the adsorption of polymer molecules which fall out of solution on the surface of the porous medium and cover the channels or impair filtration of water. The polymer solution preferably enters highly permeable layers and at the expense of increase in viscosity of the solution and reduce in conductivity of the medium there is a significant decrease in the dynamic heterogeneity of fluid flow and, consequently, increase in the coverage of reservoirs by water flooding.
Alkaline displacement. The EOR method of alkaline displacement is based on the interaction of alkalis with formation oil and rock. Oil interacts with organic acids, resulting into the formation of surface-active substances that reduces surface tension at the interface of oil-alkaline solution and increases rocks wettability. Alkaline solution is one of the most effective ways to reduce the contact angle of water wetting of rock, i.e. hydrophilization of porous medium which leads to increased rate of oil displacement by water.
Chemical reagents displacement (including micellar-polymer flood, etc.). Micellar solutions are transparent and semitransparent liquids. They are mostly homogeneous and stable to phase separation, while the oil emulsions in water or water in oil are not transparent, heterogeneous in globule structures, and have phase instability.
The mechanism of oil displacement by micellar solutions is determined by their physical and chemical properties. Due to the fact that the interfacial tension between the solution and the formation fluids (oil and water) is very low, micellar solution displaces oil and water eliminating the capillary forces effect,. When scattered oil globules coalesce into a continuous phase a zone of high oil saturation (oil shaft) and a zone of high water saturation (water shaft) beyond it can be seen. Oil shaft displaces (collects) only oil flowing through the water. In the area of oil the shaft the speed of oil filtration is higher than the speed of water filtration. Micelle solution, following the water shaft, displaces the water with a density dependent on interfacial tension between the phases.
The same filtration mechanism can be observed during the displacement of residual (immobile) oil from the flooded homogeneous porous medium.
Microbiological treatment. These technologies are based on biological processes with the use of microbial targets. During the process microorganisms delivered into the formation, metabolize petroleum hydrocarbons and generate the following oil displacement useful products:
Х Alcohols, solvents and weak acids, which lead to a decrease in viscosity, oil fluidity temperature, as well as remove paraffins and heavy oil from porous rocks, increasing the permeability of the latter.
Х Biopolymers, which when dissolved in water, increase its density and facilitate oil recovery.
Х Biological surface-active substances, which make oil surface more slippery, reducing rock friction.
Х Gases that increase pressure inside the formation, and help to push oil to the well bore.
|3.4. Hydrodynamic EOR|
Hydrodynamic EOR methods can intensify the current oil production, increase the degree of oil extraction, as well as reduce the volume of water to be injected into the reservoir. As a result the produced liquid water cut is reduced and the reservoir energy is focused on oil production. (Pic. 8).
Integrated technologies. Integrated technologies make in a separate EOR group and do not belong to the conventional water-flood methods applied to maintain oil formation pressure. These methods are aimed at selective intensification of oil production .
The production growth is achieved due to vertical flows in unhomogeneous formation through low-permeable girts from low-permeable layers into high-permeable layers due to a special injection programs utilizing unsteady effects. (Pic. 9)
Barrier flooding. Development of oil and gas fields is often accompanied by breakthrough of gas to the face of producing wells, which greatly complicates their operation due to high gas factor. The essence of the barrier flooding is that the injection wells are located in the zone of gas-oil contact. Water injection and oilgas production are synchronized in such a special way to exclude the mutual cross-flows of oil in the gas formation area, and gas in the oil area.
Non-stationary (cyclical) flooding. During injection cyclic impact and the change in reservoir fluid flow directions different pressure is created in the layers with heterogeneous pore sizes, zone permeability and different oil saturation. This is achieved by changing the volume of water injection or selection of the injected liquid type.
As a result of such non-stationary, time-varying effects of periodic waves of high and low pressure occur in the reservoir. Layers, zones and areas of low permeability and oil saturation have low conductance, and the speed of pressure spread in them is much lower than in the high permeability, saturated layers, zones and areas. Therefore, opposite pressures occur between oil-saturated and flooded areas. When the pressure in the formation rises, i.e. with the increase of water injection positive pressure difference occurs, i.e. the pressure is higher in the flooded areas and lower in oil-saturated areas.
Redistribution of fluids in non-uniformly saturated formation under the action of alternating pressure changes takes place.
Accelerated production. Accelerated production of liquids is usually applied at late stages of oil field development, when water cut is more than 75%. This increases oil recovery due to an increase in pressure differential and filtration speed. Areas previously not covered by the flooding are now involved in the development process as well as in the membrane separation of oil from the surface of rocks.
|3.5. Oil Production Intensification|
Hydraulic fracturing . Hydraulic fracturing is creating cracks in the rocks surrounding the borehole, due to the pressure at the well bottom as a result of viscous fluid injection into the rock. Viscous liquid is injected into the well at a rate which ensures the creation of downhole formation cracks. These cracks have vertical and horizontal orientation. The length of the crack sometimes reaches tens of meters, the width of it is usually about few mm or cm. After fracturing a mixture of viscous liquid with solid particles in it to prevent the closing of cracks under the action of rock pressure is pumped into the well. Hydraulic fracturing of formation is performed in low-permeability formations where the individual zones and intercalations are not engaged into active development which reduces possible oil recovery . Cracks, crossing the poorly drained areas and interlayers provide better production As a result oil is more easily flowing from the formation first into a hydraulic fracture and then to the well, thus increasing oil recovery. (Pic.10)
Horizontal wells. The use of horizontal drilling has well established itself in the following increasing number of unprofitable wells: wells with marginal or watering oil and inactive emergency wells in the transition to more advanced stages of oil field development, when watering or reduce in formation pressure in many developed areas (especially in lithologically heterogeneous, difficult formation zones) is ahead of reserve recovery under the existing density of the production well grid.
Electromagnetic treatment. This method is based on the use of internal sources of heat, activated as a result of high frequency electromagnetic fields stimulation. The zone of influence is determined by the method of creation (in one well or between several of them), intensity and frequency of the electromagnetic field, as well as the electrical properties of the formation. In addition to the thermal effects electromagnetic exposure results in oil de-emulsification, reduce the onset temperature of paraffin crystallization and the appearance of additional pressure gradients due to the force effect of electromagnetic fields on the formation fluid.
Wave treatment. There are several types of wave and termowave (vibration, shock, pulse, thermoacoustic) treatment of the oil-bearing formations and first of all on its bottom-hole zone.
The main purpose of the technology is to develop low permeability and isolated formation areas by the influence of elastic waves which damp in high permeability formation areas but travel considerable distances and with sufficient intensity to put on production low permeability areas of the formation.
Application of these methods results in a noticeable intensification of filtration processes in formation and for some period of time increase oil output. The positive effect of wave action is found both in directly treated well and in some wells situated at a considerable distance from the waxe source up to hundreds of meters or more, i.e. the wave treatment can be realized both locally and at a distance.
All the methods mentioned above are characterized by varying potential of enhanced oil recovery.
For example, Russian oil recovery index using thermal methods is about 15-30%, gas methods is around 5-15%, chemical methods is about 25-35%, physical methods is around 9-12%, hydrodynamic methods make 7-15%. (Pic. 11).
|3.6. Petros EOR|
|4. EOR results|
According to the aggregated data with the application of modern methods of enhanced oil recovery the oil recovery index usually reaches up to 30-70%, whereas within the original mode of development using only primary reservoir potential energy an average oil recovery index is about only 25%, while using the secondary production methods, i.e. flooding and gas injection to maintain formation energy, the index reaches up to 25-40%. EOR methods make it possible to increase the world's recoverable oil reserves 1,4 times, i.e. up to 65 billion tons, the average value of this ratio is to increase the final recovery rate from 35% to 50% by 2020 with the prospect of further growth. In 1986 worldТs oil production with the use of EOR methods was about 77 million tons. Now it is increased up to 110 million tons. According to Oil and Gas Journal, by 2006 301 EOR project were implemented in the world except CIS countries. We also note that according to experts, the use of modern methods of enhanced oil recovery leads to a significant increase in oil recovery index, for example, increasing oil recovery by only 1% in Russia will help to produce additional 30 million tons of oil annually.
Thus, the international experience shows that the demand for modern EOR methods increases every year and their potential grows impressively. This is facilitated by the fact that the cost of oil production using modern EOR methods is continuously decreasing and is quite comparable with the cost of oil production using traditional methods.
|5. Experience of using EOR methods in the world|
World oil consumption is growing continuously; over the past 20 years the average increase in oil consumption reached up to 1.45% per year. During this period there were years when oil production fell, but the general trend was to increase oil production.
|World Oil Production, February 2010|
|World Oil Production with the use of EOR methods, 2008|
1. Surguchev ћ.L. Secondary and tertiary methods of EOR.
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