What does water injection mean in oilfield development?

Water injection is the most important way of oilfield development, and it is also the most widely used measure to improve crude oil recovery and oil recovery. In the middle and late stage of oilfield development, water injection is the premise of stable and increased production and maintaining normal production. Water injection is a secondary oil recovery method. Water is injected into the formation through water injection wells to displace underground crude oil to production wells, so as to improve the recovery ratio of crude oil after primary oil recovery. Injected water plays a dual role in displacing crude oil and supplementing formation energy, which urges oil wells to produce more crude oil. Most oil fields in China adopt early water injection development, and have now entered the stage of high water cut. According to the requirements of oilfield development, it is the basic task of water injection project to ensure the quality, quantity and effective injection of injected water.

1. Water source In the initial stage of water injection project planning, it is necessary to find and select the water source that is most suitable for reservoir characteristics. According to the water quality standard of injected water, water treatment, anti-corrosion and construction cost are comprehensively considered for selection. The basic principles for finding water injection sources are:

(1) Sufficient and stable water supply to meet the needs of water injection, auxiliary production water, domestic water and other water.

(2) The water quality is relatively good, and the water treatment process is simple, economical and feasible.

(3) Give priority to oily sewage to reduce environmental pollution.

(4) Consider the secondary or multiple use of water to reduce the waste of resources.

The types of water sources are groundwater, surface water, oily sewage, seawater and mixed water.

Shallow underground fresh water is generally located in the alluvial layer of river bed, with stable water quantity and water quality not affected by seasons. Deep groundwater has high salt content, and deep water intake can reduce the influence of bacteria.

Surface water is mainly fresh water in rivers, lakes and reservoirs, with low salinity, high sediment content, sufficient dissolved oxygen, rich biological reproduction, peculiar smell and colloid content, and the water quantity is affected by seasonal changes.

Oily sewage is generally alkaline, low in hardness, low in iron content, high in salinity, high in oil content, rich in colloid and complex in suspended solids, and can only be discharged after water quality treatment. With the increase of produced water in oil layer, oily sewage has become the main source of oilfield water injection.

Seawater is rich in resources, high in oxygen content, high in salt content and strong in corrosiveness, and suspended solid particles change with seasons. Seawater is generally used in gulf coast or offshore oil fields. When drilling shallow water source wells along the coast, the natural filtration of strata can reduce mechanical impurities.

Using two or three water sources at the same time is called mixed water, especially when oily sewage is mixed with other water sources. In areas with severe water shortage, domestic sewage can be mixed with oily sewage or other water sources.

Second, the water quality is the specified index of the injected water quality, which indicates the composition and content of minerals, organic substances and gases allowed in the injected water, as well as the content and particle size distribution of suspended solids.

1. Causes of reservoir damage Poor quality of injected water will cause reservoir damage, resulting in decreased water absorption capacity and increased water injection pressure. The main causes of injuries are as follows.

1) insolubles cause reservoir blockage, and mechanical impurities and bacteria contained in injected water will block the reservoir. The propagation of bacteria makes the viscosity of fluid rise, and inorganic precipitation is derived. Dissolved oxygen, H2S and other corrosion products precipitate on the metal, which will block the seepage channel. Oil and its emulsion can also block the throat, showing liquid lock and emulsion droplets adsorbed on the surface of the throat.

2) The injected water is incompatible with the formation water, which may directly react with the formation water to generate CaCO3, CaSO4, BaSO4, SrSO4 and other precipitates. CO2 dissolved in water can react with Ca2+, Fe2+, Ba2+ and Sr2+ to generate corresponding carbonate precipitation.

3) The incompatibility between injected water and reservoir rocks and minerals is sensitive to salinity, which will cause the expansion, dispersion and migration of reservoir clay. The damage degree depends on the type and content of clay minerals, reservoir permeability and salinity of injected water. Fresh water usually causes more serious clay swelling than salt water. The more the minimum particle content in clay, the greater the expansibility. In addition, water injection will also cause emulsion inversion.

4) The change of injection conditions and lower injection speed are beneficial to scaling and bacterial growth; High speed will aggravate corrosion, particle shedding and migration. During water injection, the formation temperature gradually decreases, the fluid viscosity gradually increases, the seepage resistance gradually increases, and the water absorption capacity gradually decreases. Water temperature will affect the solubility of minerals and gases, which will lead to scaling. The decrease of temperature is beneficial to the formation of exothermic precipitation and the precipitation of wax. Pressure changes will lead to stress sensitivity, structural damage and precipitation of oil layers. The change of pH value will cause the particles to fall off, disperse and precipitate. The higher the pH value, the greater the scaling tendency.

Objective The characteristics of reservoir and its fluid are the potential factors of reservoir damage. The quality of injected water is the external condition that induces reservoir damage, and it is also the key to the success or failure of water injection. Therefore, improving water quality can effectively control reservoir damage.

2. The injected water with unqualified water quality leads to the decrease of water absorption capacity, the increase of injection pressure, the imbalance of injection and production, and the decrease of crude oil production. The basic requirements of injected water quality are: the water quality is stable and does not react with formation water to produce precipitation; Does not cause hydration expansion or suspension of clay minerals in the oil layer; Low corrosion and low suspension; The mixed water source should ensure its good compatibility.

In order to make the injected water meet the above requirements, the following points should be done.

1) Controlling suspended solids is to strictly control the particle size and concentration of solid substances in water according to the pore structure and throat median of reservoir rocks. The injected water needs to be carefully filtered in low permeability layer to reduce the damage to oil layer.

2) Controlling dissolved oxygen, corrosive CO2 and H2S in corrosive medium is the fundamental reason for corrosion of water injection equipment and pipeline steel. The existence of a large number of iron ions in water is a sign of corrosion. Oxygen will accelerate the corrosion rate. By limiting the gas content, the scale and speed of corrosion can be controlled, the life of water injection system can be prolonged, the blockage of formation by corrosion products can be reduced, and the oil production cost can be reduced. Therefore, the content of corrosive medium and the total corrosion rate must be strictly controlled.

3) Control most of the injected water with oil content as oily sewage. The polymerization, aggregation and adsorption of oil will bring many adverse effects on reservoir permeability.

4) Controlling bacterial content Sulfate-reducing bacteria, saprophytic bacteria and iron bacteria are the most serious hazards in oilfield water injection in China. Under certain conditions, the reproduction speed of bacteria is amazing, and the population can be doubled in half an hour. Sulfate reducing bacteria can reduce sulfate to sulfide under anaerobic conditions, and the generated H2S corrodes steel and forms FeS precipitate. Iron bacteria can secrete a large amount of Fe(OH)3 and promote the oxidation of ferrous iron to Fe3+, which also provides a local anaerobic zone for the reproduction of sulfate-reducing bacteria. Saprophytic bacteria can get energy from organic matter, and its harm is similar to that of iron bacteria. A large number of viscous substances secreted by bacteria strengthen scaling, block the pores and throats of oil layers and increase the flow resistance of pipe network.

5) The hazards of controlling pipe wall scaling are equipment wear, corrosion and flow obstruction; Scaling in reservoir seepage channel will seriously affect water absorption capacity. The injected water is incompatible with reservoir rocks, minerals and formation water, resulting in precipitation. The mixing of two kinds of water may also produce precipitation. Precipitation is the premise of scaling. Calcium ions can quickly combine with carbonate or sulfate to form scale or suspended solid particles. Magnesium ions and carbonates can also cause precipitation. Barium ions react with sulfate to produce extremely insoluble barium sulfate. Controlling the flow rate, pH value and other conditions can prevent the formation of scale.

Third, water treatment Most of the source water needs to be treated. Some water sources only need simple treatment or even no treatment, while some low permeability reservoirs need high water quality treatment technology.

1. water treatment measures 1) sedimentation is to let water stay in the sedimentation tank for a certain period of time, so that the suspended solid particles in it can be precipitated by their own gravity. Sufficient setup time and setup speed are the key. Adding a circuitous baffle in the sedimentation tank can change the flow direction, increase the process and prolong the sedimentation time, which is beneficial to the coagulation and sedimentation of particles. Flocculants can react with suspended solids in water physically and chemically, so that fine particles can be condensed into large particles, and the settling speed can be accelerated. After precipitation, the content of suspended solids in water should be less than 50 mg/L.

2) Filtration is an important link in water quality treatment. The water from the sedimentation tank often contains a small amount of fine suspended solids and bacteria, which need to be removed by filtration. Even the groundwater without precipitation needs to be filtered.

Filtering can remove suspended matter or iron and partially remove bacteria. The iron in groundwater is mainly ferrous ion, which is easily hydrolyzed to generate Fe(OH)2, and then oxidized to generate Fe(OH)3 precipitate. The mechanical impurity content after filtration should be less than 2 mg/L. There are many kinds of filters, as shown in Figure 7- 1, pressure manganese sand iron removal filter tank.

Figure 7- 1 manganese sand iron removal filter tank

1- tank; 2- filter material layer; 3— Cushion; 4- Water collection and distribution pipeline; 5— Water inlet pipe; 6- backwashing drainage pipe; 7- outlet pipe; 8- backwashing water inlet pipe; 9— Automatic exhaust valve; 10-exhaust pipe 3) There are many ways to sterilize and control bacteria in water, but none of them is universally effective. Bacteria have strong adaptability and will produce drug resistance, and various methods should be used alternately. In addition to chemical methods, ultraviolet radiation can also kill sulfate-reducing bacteria. Cleaning the pipe network and wells helps to reduce the harm of bacteria.

4) A certain amount of oxygen is always dissolved in degassed surface water and seawater. Some water sources contain carbon dioxide and H2S, and these corrosive gases should be removed. The vacuum deaerator tower is shown in Figure 7-2. Low pressure makes dissolved gas fully overflow, and low temperature is not conducive to degassing. A multi-stage process can be used to reduce the oxygen content. Dissolved oxygen in water can be extracted by washing natural gas or inert gas with water in countercurrent, as shown in Figure 7-3. Under acidic conditions, both H2S and CO2 can be removed by vacuum deoxidation or gas stripping deoxidation. Chemical reagents can also remove oxygen and can be used in combination with the first two methods.

Figure 7-2 Vacuum Deoxygenation Tower

Figure 7-3 Counter-current Stripping Deoxygenation Tower

5) The degreased produced water contains a small amount of oil droplets with small diameters, which are in three dispersion states: floating oil, dispersed oil and emulsified oil. As long as the oil slick is allowed to stand for a period of time, it can surface; If the standing time is enough, the dispersed oil can also float to the surface. Oily emulsion is the main treatment object. The gravity degreasing device mainly provides enough residence time for the aggregation and separation of oil droplets. Gas flotation method is to inject a large number of small bubbles into water. Bubbles attach to suspended oil droplets, making them lighter and easier to rise to the water surface.

6) When exposed water contains a large amount of unstable supersaturated carbonate, such as bicarbonate, magnesium bicarbonate, ferrous sulfate, etc., the temperature rises after being injected into the oil layer, which may cause carbonate precipitation and block the oil layer. Exposure can precipitate carbonate in advance.

When treating drinking water or having specific water quality requirements, it is necessary to soften and remove calcium ions and magnesium ions, and desalt and remove various dissolved salts.

2. There are two types of water treatment systems in the water treatment process: closed and open. Closed water treatment system is a system that completely isolates oxygen, which is used for systems that contain no air or little oxygen and hardly need chemical treatment. When the water source is saturated with oxygen or H2S and CO2 needs to be removed by ventilation, the open system is selected. Treatment equipment and process measures should be selected according to the water quality index, and the process should be arranged safely, economically and scientifically, and the treatment process can be changed appropriately according to the specific site, water source and water quality.

1) the main problem of produced water treatment is to remove oil, bacteria and suspended particles from the water. At present, gravity separation tank is mostly used for oilfield sewage treatment, and the treatment process is shown in Figure 7-4.

Figure 7-4 Flowchart of Oily Sewage Treatment

1-oil removal tank; 2- filter tank; 3- Buffer water tank; 4- water delivery pump; 5- Clean the water tank; 6— High pressure water injection pump; 7— Oil transfer pump; 8— Dirty oil tank; 9— Sewage recovery tank; 10- recovery water pump; 1 1- coagulant dissolving tank; 12— dosing pump; 13- bactericide dissolving tank; 14 —— adding bactericide pump 2) The groundwater treatment process has high salinity, mainly containing iron, manganese ore and suspended matter. Manganese sand iron removal filter can remove iron and most suspended solids. The treatment process is as follows: pump water from the water source well with a deep well pump, treat it with a manganese sand iron removal filter tank and a quartz sand filter tank, pass through a buffer water tank, and then pump it to the water delivery pipeline.

3) The content of sediment and dissolved oxygen in surface water treatment process is high. The emphasis of treatment is deoxidation and removal of suspended solids. The treatment process is as follows: water diversion structure → water intake pump house → liquid medicine mixing tank → reaction sedimentation tank → filtration tank → clear water tank → suction tank → water delivery pump house, and then delivered to water delivery pipeline after measurement.

4) The seawater treatment process contains a lot of oxygen and suspended matter. The treatment process mainly consists of a three-stage purification combined device and a two-stage deoxygenation process.

Low permeability reservoirs require higher water quality. In addition to basic treatment, fine filtration and deep strengthening treatment are also needed. For ultra-low permeability reservoirs, too high water quality standard will increase the treatment cost, so gas injection can be used to maintain formation pressure.

4. The performance of water injection wells is similar to the inflow performance of oil wells. The performance of water injection wells is to study the water absorption capacity of water injection wells and its influencing factors. The indicator curve of water injection well can be used to analyze the change of formation water absorption capacity and judge the condition of downhole tools.

1. Indicating curve of water injection well The indicating curve of water injection well refers to the relationship curve between injection pressure and water injection volume under the condition of stable flow. The indicator curve of each sublayer is the relationship between injection pressure and injection quantity, which can be obtained by throwing ball test. The measured indicator curve is straight and broken. In Figure 7-5, the linear incremental curve 1 reflects the water absorption law of the formation. Vertical curve 2 shows that the reservoir permeability is extremely poor, the water nozzle is blocked or the test is faulty. Descending curve 3 and starting curve 4 are abnormal indication curves. Curve 5 is upturned, reflecting poor formation connectivity, difficult diffusion of injected water, increased resistance, increased pressure and decreased injection speed. Curve 6 is a downward folding type, indicating that under high water injection pressure, the new reservoir begins to absorb water or micro-cracks are generated in the formation, resulting in an increase in water absorption of the reservoir.

Figure 7-5 shows the shape of the curve.

The reciprocal of the slope of the indicator curve is the water absorption index, which represents the daily water injection quantity under the unit bottom hole pressure difference of the water injection well and describes the water absorption capacity of a single well or single layer of the water injection well. The water absorption index per unit oil layer thickness is called specific water absorption index or water absorption index per meter. The ratio of daily water injection to wellhead injection pressure is called apparent water absorption index.

2. The water injection profile vividly describes the layered water absorption capacity of water injection wells. Isotope carrier method is often used to measure water absorption profile, and solid carrier adsorbed with radioactive isotope is added into water to make activated suspension. When the injected water enters the deep stratum, the solid carrier is filtered on the stratum surface. Solid carrier has strong adsorption and uniform suspension, so it accumulates in the area with large water absorption and high radioactivity. Before and after injecting the activated suspension, radioactive logging was carried out, and the two measured radioactive curves were superimposed to obtain the water absorption profile. The abnormal curve is the water absorption horizon (Figure 7-6). The percentage of abnormal area of each layer to the abnormal area of the whole well is the relative water absorption, that is, the percentage of water absorption of small layers to the whole well. Well temperature method can also be used to determine the water absorption horizon.

Figure 7-6 Water Absorption Profile

3. The analysis of the indicator curve shows that the indicator curve reflects the water absorption capacity and law of the formation. By comparing the indicator curves measured in different periods, we can understand the change of water absorption capacity of oil layers. In Figure 7-7 ~ Figure 7- 10, curve I is the first measured curve, and curve II is the measured curve after a period of time.

Figure 7-7 shows that the curve moves to the right.

Figure 7- 10 curve moves down in parallel.

(1) indicates that the curve moves to the right and the slope decreases, indicating that the water absorption index increases and the water absorption capacity of the formation increases (Figure 7-7).

(2) The exponential curve moves to the left and the slope becomes larger, indicating that the water absorption index becomes smaller and the water absorption capacity of the formation becomes worse (Figure 7-8).

Figure 7-8 shows that the curve moves to the left (3) shows that the curve moves up in parallel, which is caused by the increase of formation pressure. The constant slope means that the water absorption capacity has not changed (Figure 7-9).

Figure 7-9 Curve moves up in parallel.

(4) The indicator curve moves down in parallel, which is caused by the decrease of formation pressure, and the water absorption capacity remains unchanged with the same slope (Figure 7- 10).

Generally, only the water injection volume of the whole well is measured during normal water injection. The recent stratified test data can be used to sort out the stratified indicator curve, obtain the water absorption of each layer and the water injection of the whole well under the recent normal water injection pressure, calculate the relative water injection of each layer, and then distribute the measured water injection of the whole well to each interval in proportion.

Verb (abbreviation of verb) water injection technology It is the main content of water injection technology to inject water from water injection wells to specific oil layers with good quality and quantity. Oilfield water injection system includes oilfield water supply system, oilfield water injection surface system, wellbore flow system and reservoir flow system.

1. Injection system Injection system includes surface water injection system and wellbore flow system. It consists of water injection station, water distribution room, wellhead, underground water distribution pipe string and corresponding pipe network.

Some wells are water injection wells specially drilled for water injection, and the temptation to convert low-yield wells, extra-high water-bearing wells and marginal wells into water injection wells is also great. The wellhead equipment of water injection well is the Christmas tree for water injection. Downhole structure should be simple, generally only string and packer are needed. A plurality of water injection wells form a water injection well group, and water is distributed by the water distribution room. A booster pump and a filtering device can be added to the wellhead or the water distribution room, and each water injection well is generally metered in the water distribution room.

Water injection station is the core of water injection system. The basic flow in the station is: incoming water enters the station → metering → water quality treatment → water storage tank → pumping out. The water storage tank has the functions of storing water, buffering pressure and isolating. Water injection station can distribute water to single well or water distribution room. The diameter and length of water injection pipe network directly affect the water injection cost.

2. Layered water injection The core of layered water injection is to control the water absorption of high permeability layer, strengthen the water absorption of middle and low permeability layer, make the injected water advance evenly and prevent single layer surge. Downhole pipe string includes fixed water distribution pipe string (Figure 7- 1 1), movable water distribution pipe string and eccentric water distribution pipe string. The water distributor generates a certain throttling pressure difference to control the water injection quantity of each layer. The core of layered water distribution is to select the underground water nozzle, and adjust the water distribution of each layer through the size and throttling loss of the water nozzle, so as to achieve the purpose of layered water distribution.

Figure 7- 1 1 Fixed water distribution pipe string

3. Water injection technical measures After the oil layer entered the middle and high water cut stage, the plane contradiction, interlayer contradiction and intralayer contradiction became increasingly prominent. In heterogeneous oil fields, due to the differences in properties, the water absorption capacity of each interval is very different, and the water injection profile of water injection wells is extremely uneven. After fracturing, the high permeability layer absorbs more water, and the oil well seriously produces water; The medium and low permeability layer absorbs less water, the formation pressure drops rapidly, and the oil well production is difficult. It is necessary to adjust and plug the high permeability layer to reduce the water absorption capacity; Transform the low permeability layer and reduce the flow resistance. Therefore, improving the water absorption profile to achieve water absorption balance can improve the volume sweep coefficient of injected water.

Pressurized water injection is a technical measure to improve the bottom hole water injection pressure. High pressure causes tiny cracks in the formation, which flow in small pores and absorb water in low permeability layers. Increasing the injection pressure properly can increase the water absorption of each layer in a balanced way.

Pulse nozzle pressurization is to make the water flow produce large pulsation and form high frequency water jet. High-frequency pressure pulse can loosen and fall off pollutants near the well; Disperse solid particles and heterogeneous droplets to prevent blockage, remove blockage and increase injection. Pulse nozzle pressurization has strong applicability, and it does not need to change the original water distribution and testing process, nor does it increase investment.

Periodic water injection is also called intermittent water injection or unstable water injection. Periodically changing the water injection quantity and injection pressure forms an unstable state, resulting in liquid exchange between layers or fractures with different permeability and bedrock. The greater the permeability difference, the stronger the liquid exchange capacity and the better the effect. This method can reduce the comprehensive water content.

There are three methods of plugging adjustment: mechanical method is to plug the ultra-high water absorption interval with packer or current-limiting perforation; The physical method is to plug the high permeability interval with solid particles, heavy oil or foam; Chemical method is the most widely used method in this field, and its mechanism is different. In order to meet the needs of different water injection wells, various profile control technologies are constantly emerging.

The injected water with low salinity will break the original relative balance of the stratum and lead to the hydration and expansion of clay. Salinity gradient water injection is to gradually reduce the salinity of injected water. The smaller the gradient, the smaller the impact on clay minerals and the smaller the damage to the formation.

Strong magnetic treatment can change the properties of injected water, and the effect of inhibiting clay swelling and scaling prevention is very obvious. Anti-swelling agent slug can also be injected to inhibit the hydration expansion of clay. Comprehensive application of clay anti-swelling technology can increase water absorption, reduce injection pressure and greatly improve treatment effect. Various water injection process measures have their specific adaptability. The continuous development of new water injection technology will continuously improve the effect of oil field water injection development.