книги / Методы и технологии добычи нефти и газа

Positive results of laboratory testing gave occasion to conduct field trials of caustic flooding method in terrigenous reservoirs in Shagirtsko-Gozhanskoe field. Industrial injection of alkaline solution started at four sites with 13 injection and 72 production wells in 1983. Purpose made facility with producing capacity 43 tons/day of dry chemical agent was used for production and dosing water sodium hydroxide solution. In the period from 1978 till 1990 14.1 thousand tons of sodium hydrate were injected. The injection was performed on an intermittent basis, mainly in summer. When there was no alkaline, fresh or sewage water was injected. Average concentration of chemical agent made 0.24 %. Banks dimensions varied in different sites from 1 to 0.17 of reservoir pores volume. Oil production increase in different sites varied from 25 to 1.4 % and made in average 5.6 %. Incremental oil production in all sites made 662.4 thousand tons as of 01.01.1991.

Alkaline solution was injected in carbonate reservoirs in three fields of Perm region (Padunskoe, Opalikhinskoe and Beryozovskoe fields) in summer on an intermittent basis (when there were no alkaline). Average concentration of solution made 0/8 %, bank dimensions – 0.09 of pores volume. Injection of 6 thousand tons of sodium hydrate. Oil production increase in different sites made 0.1 to 3.9 %, incremental oil production made 61 thousand tons.

Results of caustic flooding field trials in Perm region fields were positive in general. Main factors which determine oil production increase during caustic flooding are the following: surface tension decrease at oil and alkaline solution boundary, oil emulsification, change of rock wettability and sedimentation at interaction of alkaline solution with reservoir water.

It was specified that:

1.The best procedure of caustic flooding in terrigenous reservoirs is formation of 1 % alkaline solution banks which size is 0.1 of reservoir pores volume at cyclical injection of alkaline solution and water.

2.Change of hydrogen ions concentration ratio (pH) is the main parameter for caustic flooding process control. On the basis of рН values dynamics the critical pH

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value in the interval of 8–8.5 is defined and when it is exceeded one can face difficulties, which are associated with scaling and sand production.

3. During alkaline flooding it is necessary to envisage measures on prevention of inorganic salts sedimentation – calcium and magnesium carbonates and sulphates.

2.4. Sulfuric Acid Flooding

When strong sulphuric acid is injected, it interacts with reservoir fluid, which causes increase in temperature, carbonates dissolution, oil viscosity decreases, porous medium permeability increases, water viscosity increases at emitting of carbon dioxide gas. Clay swelling ability decreases, oil measure expansion occurs. Bank dimensions make 0.1–0.5 % of pores volume (0.001–0.005 unit fractions), concentration is 90–98 %. Time of injection is 2–3 days. Injection is performed by acid aggregates. Oil production increase makes 10–15 %.

3. METHODS OF MISCIBLE DISPLACEMENT

3.1. Carbonic Acid Injection

Carbonic acid can be used for oil production increase according to three technologies. According to the first technology carbonic acid is injected into the reservoir as an one-off bank in liquefied condition, which is further pushed along the reservoir by carbonated or ordinary water. According to the second technology carbonized water with concentration 4–5 % is injected. The third technology consists in injection of alternating small banks of carbonic acid and water. In any case the total volume of bank and average concentration should be kept.

Oil production increase while displacement of oil by carbonic acid is explained by a number of factors. Mutual dissolution of carbonic acid in oil and hydrocarbons in liquid СО2 are accompanied by oil viscosity decrease, increase in its volume, surface tension decrease at the boundary with water, water viscosity increase, clay swelling ability decrease. Efficiency upgrading is caused by formation of embankment of light hydrocarbons and СО2 at displacement front. Formation of carbonic acid contributes to a number of additional positive factors, such as carbonates

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dissolution, temperature increase. At injection of carbonic acid mutual dissolution of oil and gas cause displacement which resembles miscible displacement.

Technology. Bank volume should make 0.1–0.2 to 0.3 of oil saturated pores volume. Concentration makes 4–5 %. At carbonic acid injection accompanied by flooding СО2/water ratio should be maintained at the level of 1/3. Oil production increase should make 5–10 to 15 %.

3.2. Hydrocarbon Gas Injection

The mechanism of hydrocarbon gas injection resembles carbon acid injection and displacement resembles miscible displacement. Technology. Bank volume should make 0.1–0.3 of oil saturated pores volume. Concentration makes 50–100 %.

3.3. Micellar Flood

The characteristic property of micellar flood is selection of a particular composition of several chemical agents, sequence of their injection, bank dimensions and their concentration for specific geological and physical conditions of oil viscosity and other parameters in laboratory conditions. Usually firstly drilling mud surfactants of 20 % of oil saturated pores volume with concentration 5–10 % are injected. Then micellar flood bank of 5–10 % oil saturated pores volume are injected. Then buffer bank of polymer solution of 40 to 100 % is injected. Then composition of three chemical agents moves along the reservoir of the injected fresh and process water, bank dimensions make 1.5–2 of reservoir pores volume. Oil production increase makes 10 to 100 %.

4.THERMAL METHODS

4.1.Injection of Hot Water and Steam

Enhanced oil recovery at water injection is achieved at the expense of oil viscosity decrease, heat oil expansion and reservoir skeleton as well as intensification of capillary imbibition (for hydrophylic reservoirs). It caused oil mobility, its perme-

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ability to phase increase and increase of driving medium displacement agent coverage of the reservoir, creation of conditions for oil displacement from low permeability pillars. In case of steam injection the aforesaid factors are added by distilling effect, which consists in evaporation of some crude oil under the influence of steam and its transfer along reservoir in vaporous state.

The depth of productive formation bedding is important because when depth increases heat losses in a bore hole increase as well. Net thickness influence heat losses through the upper boundary and bottom boundary: the less is thickness the higher is specific area of heat losses and the higher is relative value of heat losses.

The process monitoring and its adjustment are very important. During injection the following processes should be constantly monitored: injection pressure, temperature at injection and production well heads, dryness factor of heat-transfer medium, oil and water yield change, chemical composition of produced water. Water heating facilities are used for water injection. Steam injection equipment consists of steam boilers, steam supply lines, well head and downhole equipment. Stationary and semiportable steam boilers, portable steam-generating plants are used for steam generation.

4.2. In-situ combustion

There are three types of in-situ combustion. Dry combustion at which 1 m3 of water is injected per 1000 m3 of air. Wet combustion at which 1 to 5 m3 of water are injected per 1000 m3 of air. Super wet combustion at which more than 5 m3 of water are injected per 1000 m3 of air.

To create a combustion source different subsurface heaters, usually electric or gas heaters are used. After heating of bottom-hole area oxidating agent (air) for oil ignition is supplied to the well.

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5.HYDRODYNAMICAL METHODS

1.Nonstationary (cyclical) flooding and fluid withdrawal accompanied by redirection of filtration flows.

2.Increase in injection pressure.

3.Shift of injection front.

4.Spot and selective injection wells location.

5.Additional perforation of production formations.

6.Forced fluid withdrawal.

7.Hydraulic fracturing.

Several years ago hydraulic fracturing was used mainly as a well stimulation

or injection technology, but at present the focus is shifted into the sphere of oil production increase and intake portion of reservoirs, which contributes to the use of additional hardly recoverable reserves in the field.

At present hydraulic fracturing is the most effective geological and technical measures, which provide production and injection multiplication both in low permeability reservoirs and in reservoirs with good permeability. It provides complete coverage and development of new resources and it intensifies development in the whole field.

Since implementation hydraulic fracturing has been one of the main engineering tools of wells productivity increase. The effect is achieved due to:

•Formation of a conductive channel (crack) through the damaged (contaminated) area around the well in order to penetrate beyond the boundaries of this area;

•Channel (crack) propagation deeply in the reservoir for further increase in well productivity;

•Formation of a channel (crack) which allows to change, influence on fluid flow in the reservoir.

List of the existing hydraulic fracturing technologies. Standard hydraulic fracturing. Injection of gel into reservoir and increase in its flow for fracturing, crack propagation at the constant mode of gel injection (2–5 m3/min), crack filling with propping agent and increase in its concentration in gel (to 1 500 kg/m3) which total weight is up to 50 tons. Sphere of application. Productive formations which

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thickness makes up to 15 meters, permeability is more than 0.040 mkm2 and which have small average number of permeable intervals in the section with thick screens (more than 10 m); displacement front not nearer than half of the distance between two wells.

Besides standard hydraulic fracturing there are the following types of hydraulic fracturing technologies: additional fracturing; positive displacement fracturing – injection of gel with propping agent at total weight 50 to 100 tons. Productive formations with thickness up to 20 meters; Selective fracturing; acid fracturing for carbonate collectors accompanied by additional injection of strong acid bank before the stage of crack filling with propping agent.

In general, bore hole is destructed, that is, the rock is cracked affected by hydraulic pressure of working fluid, at that “hydraulic” fracturing occurs. Voltage vector lies in horizontal plane and leads to rock slivering in vertical plane.

Crack formation and propagation at early stages causes increase in reservoir cross section. As the injection is stopped the crack closes and there are no new flow areas. To avoid this fixing agent (propping agent) is added into working fluid of hydraulic fracturing, which are injected into the crack. Propping agent doesn’t move and doesn’t allow the crack to close, which keeps conductive channel during the whole production period open and increases flow area of the collector. Usually sand or any granulated high-tension substitutional product is used as a propping agent.

During the work with carbonate rocks one can use acid, which dissolves the rock, which makes leaching channels, leading in the depth of the collector, as a working fluid of hydraulic fracturing.

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Fig.1. Injection of propping agent into the crack

Depth, m

Fig.2. Voltage Profile

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Concentration, kg/m3

Time, min

Fig.3. Propping Agent Injection Curve

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С хем а обвязки оборудования при ГРП

Fig.5. Equipment hookup at hydraulic fracturing

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