First, producing formation and completion are to connect the oil and gas formation with the wellbore, thus providing an oil flow channel to ensure that oil and gas flow from the formation into the bottom hole. Any phenomenon that restricts the flow of oil and gas from around the wellbore to the wellbore is called "pollution" to the formation damage. Practice has proved that the process of drilling the production layer will cause damage to the oil and gas layer more or less. Therefore, protecting oil and gas reservoirs is the primary problem faced by completion. In the past, the world oil price was low and oil sources were abundant, which largely ignored the protection of oil and gas reservoirs. Since the energy crisis occurred in some western countries in the mid-1970s, preventing reservoir damage and maximizing the productivity of oil and gas wells have risen to an important position and become one of the most important hot topics in current drilling technology.
1. Reasons for reservoir damage Since the study of reservoir damage mechanism, there have been various opinions. Recently, permeability theory holds that formation damage is usually related to the migration and plugging of solid particles in drilling fluid, as well as chemical reaction and thermodynamic factors. It is difficult to fully grasp the mechanism of reservoir damage under complex conditions. Therefore, most of the current research results can only guide production practice qualitatively, and there is still a certain gap from quantitative evaluation.
The commonly used drilling fluid for drilling production wells is water-based mud. Because the pressure of drilling fluid column is generally greater than the formation pressure during drilling, under the action of pressure difference, water and clay in drilling fluid will invade oil and gas reservoirs, causing various damages to oil and gas reservoirs.
1) makes the pay zone clay swell. As we all know, there is usually an extremely thin clay film around oil sands particles. There are many micropores between sand grains, and there are many thin clay interlayers in the oil and gas layer. Under the invasion of free water in drilling fluid, the clay components around sand particles will expand in volume, which will narrow the oil and gas flow channel and reduce the oil and gas production capacity.
2) Destroy the continuity of oil and gas flow. When the oil and gas saturation of oil and gas reservoirs is high, oil and gas flow continuously in pores. A small amount of raw water sticks to the hole wall and fixes tiny loose particles, which will not be washed away at a considerable oil and gas velocity. When a large amount of drilling fluid filtrate invades, it will destroy the continuity of oil and gas flow, and the single-phase flow of crude oil or natural gas will become oil-water two-phase flow or gas-water two-phase flow, which will increase the flow resistance of oil and gas. Once water becomes a continuous mobile phase, as long as the flow velocity is slightly higher, it will wash away the loose particles that were originally stable on the particle surface and accumulate in the narrow part, blocking the flow channel and seriously reducing the permeability.
3) Water lock effect is generated, which increases oil and gas flow resistance. The filtrate of drilling fluid penetrating into oil and gas reservoir is discontinuous, but in the state of oil and gas separation with small water plug. Oil-water emulsion will be formed in some places. Because of the systolic pressure on the curved surface, the resistance of oil and gas flowing into the well will be greatly increased.
4) Precipitation is generated in formation pores.
2 drilling fluid type The damage of drilling fluid type to the production layer. This clear water is suitable for fractured oil and gas reservoirs. The lowest low solid (no solid) drilling fluid is small, the water-in-oil emulsion is small, the oil-based drilling fluid is high, the crude oil is small, and the air (natural gas) is small. The bottom hole completion mode refers to the communication mode and bottom hole structure adopted between the production layer and the bottom hole after drilling. From the perspective of oil and gas production, the requirements for various completion methods are as follows:
(1) The communication condition between oil and gas reservoir and wellbore is the best, and the damage to oil and gas reservoir is the least. (2) The seepage area between oil and gas reservoir and wellbore should be as large as possible, and the resistance of oil and gas inflow should be minimum; (3) Effectively block the oil layer, gas layer and water layer to prevent the interlayer from interfering with each other; (4) Effectively control sand production in oil layers, prevent borehole wall collapse, and ensure long-term stable production of oil and gas wells; (5) It can meet the requirements of layered water injection, gas injection, fracturing, acidification, artificial lifting and underground operation; (6) Heavy oil recovery can meet the requirements of steam injection thermal recovery; (7) Having the conditions for sidetracking in the later stage of oilfield development; (8) Simple process and low cost.
After the completion of oil and gas wells, the bottom hole structure is not easy to change. Therefore, reasonable and effective bottom hole completion methods should be carefully selected according to the specific conditions of oil and gas reservoirs and practical experience in various places. At present, the commonly used bottom hole completion methods at home and abroad are open hole completion, perforation completion, slotted liner completion and gravel packing completion.
1. Open-hole completion method is called open-hole completion method, which directly exposes oil and gas reservoirs without casing pipes. After the wellbore above the oil and gas layer is cemented, the oil and gas layer is opened with a smaller bit, which is called early open hole completion. Fig. 5- 1 1 is a schematic diagram of early open hole completion of vertical wells. In the later open hole completion, after drilling directly through the oil and gas reservoir without changing the bit, the cementing operation is carried out in the interval above the oil and gas reservoir. Fig. 5- 12 is the schematic diagram of open hole completion in the later period of vertical well. The biggest advantages of open hole completion are direct communication between oil and gas reservoir and bottom hole, large circulation area, small flow resistance, simple construction, low cost and high output.
Figure 5- 1 1 Initial stage of open hole completion
Figure 5- 12 Late Stage of Open Hole Completion
Wells completed by open hole completion are easy to collapse, and oil and gas sand production cannot be controlled. Generally, it is only suitable for single oil-gas layer with hard and dense rock formation and no oil-gas-water layer. Multi-reservoir wells with similar oil and gas properties can also be used, but it is impossible to produce by layers. Open hole completion method is an early completion method. With the emergence of efficient and powerful perforation technology for oil and gas wells, the advantages of open hole completion are not as prominent as in the past. Open hole completion can be used for vertical wells, directional wells and horizontal wells. There are many variants of open hole completion methods to improve their adaptability.
2. Perforation completion method Perforation completion method is the most widely used completion method at home and abroad. It can be used in vertical wells, directional wells and horizontal wells. Perforation completion includes casing perforation completion and liner perforation completion.
Casing perforation completion is to drill through the oil and gas reservoir to the designed depth with a bit of the same size, run the casing to the bottom of the oil and gas reservoir and inject cement for cementing, and then use a perforator to shoot through the casing and cement ring and inject it into the production layer to a certain depth. Oil and gas can flow into the well through the holes formed by perforation. Fig. 5- 13 is a schematic diagram of casing perforation completion in vertical wells.
Figure 5- 13 Casing Perforation Completion
Liner perforation completion is to run the technical casing for cementing when drilling to the top of the oil and gas reservoir, then drill through the oil and gas reservoir to the designed depth with a smaller bit, and run the liner with a drilling tool and hang it on the technical casing (the overlapping section between the liner and the technical casing is generally not less than 50m). The liner is cemented and then perforated. The structure of oil and gas reservoir is exactly the same as that of perforation completion. Fig. 5- 14 is a schematic diagram of liner perforation completion in vertical wells.
Figure 5- 14 liner perforation completion
The advantages of perforation completion method are:
(1) can effectively support the loose and fragile production layer; (2) The oil layer, gas layer and water layer can be effectively sealed to prevent gas channeling and water channeling; (3) Various layered technical measures such as layered testing, layered mining and layered acidification can be carried out; (4) Non-tubing completion and multi-tubing completion can be carried out.
(5) Except open hole completion, it is more economical than other completion methods.
The main disadvantages of perforation completion method are: drilling fluid and cement slurry have serious damage to oil and gas reservoirs during drilling and cementing; Due to the limited number and depth of perforation holes, the communication area between oil and gas reservoir and bottom hole is small, and the resistance of oil and gas flowing into the well is large.
3. Slotted liner completion method Slotted liner completion method is a completion method of running slotted liner in open hole completion. Corresponding to open hole completion, slotted liner completion is also divided into early and late stages. Pre-slotted liner completion refers to casing cementing when drilling to the top of the oil and gas reservoir, then changing a small bit to open the oil and gas reservoir, and finally putting a pre-drilled or slotted liner under the ground below the exposed part of the oil and gas reservoir, and hanging and fixing the liner on the upper casing with a slip packer. Fig. 5- 15 is the schematic diagram of vertical well pre-cut seam liner completion. The later slotted liner completion is to directly drill through the oil and gas reservoir, and then cement the interval above the oil and gas reservoir. Fig. 5- 16 is the schematic diagram of slotted liner completion in the later period of vertical well. Oil and natural gas can only flow into the oil well through holes or gaps in the liner. The slotted liner completion method can prevent sand and protect the borehole wall, but it cannot be exploited in layers. Simple process, convenient operation and low cost, it is mostly used in medium-coarse sandstone reservoirs with low sand production, and can be used in vertical wells, directional wells and horizontal wells.
Figure 5- 15 Early Slotted Liner Completion
Figure 5- 16 Late Slotted Liner Completion
4. Gravel packing completion method Gravel packing completion method is generally used in formations with loose cementation and serious sand production. This method can effectively protect the shaft wall and solve the problem of sand control, but the construction technology is complicated. Gravel packing completion methods are divided into open hole gravel packing completion and casing gravel packing completion.
Open-hole gravel packing completion is to drill the production layer after casing is run to the top of the oil and gas layer for cementing, then enlarge the hole of the oil and gas layer with a downhole reamer, then run a wire-wound screen, and circulate the pre-selected gravel into the well and fill it at the bottom of the well. The advantages of open-hole gravel packing completion are large flow area and small flow resistance, but the disadvantage is that it cannot be mined in layers. Fig. 5- 17 is the schematic diagram of open hole gravel packing completion.
Casing gravel packing completion is casing cementing and perforation after drilling oil and gas reservoirs. After cleaning the perforation, run the wire-wound screen and fill the gravel. This method can be used for layered production. At present, casing gravel packing completion mostly adopts high density packing, which has high efficiency, good sand control effect and long validity period. Fig. 5- 18 is a schematic diagram of casing gravel packing completion.
Figure 5- 17 Open hole gravel packing completion
Figure 5- 18 Casing Gravel Packing Completion
Gravel packing completion method can be used for vertical wells and directional wells. However, it should be used with caution in horizontal wells, because horizontal wells are prone to sand sticking and gravel packing failure can not prevent sand.
3. Well completion wellhead device In the process of testing and production of oil and gas wells, there must be an absolutely reliable wellhead device, so that in-well operation and oil and gas production can be carried out in a controlled and planned way. Well completion wellhead device is a wellhead device installed on the ground to suspend and place various well strings, control and guide the outflow of oil and gas in the well or the injection of surface fluid. Well completion wellhead devices usually include casing head, tubing head and Christmas tree.
The types of wellhead devices for completion should be determined according to the characteristics of oil and gas reservoirs. The wellhead device of low-pressure oil and gas well is relatively simple, as long as the annulus is sealed and the tubing head and Christmas tree are installed. For high-pressure oil and gas wells, sufficient strength and reliable sealing are required. At the same time, it must meet the requirements of safety testing, acidizing fracturing and oil and gas production. For oil and gas wells containing hydrogen sulfide, sulfur-proof wellhead devices should be adopted to ensure safe production.
1. Casing head If the oil and gas reservoir pressure is low and the casing cementing cement of each layer returns to the wellhead, there is no need to install the casing head, just seal and weld the annular space with an annular iron plate, and the Christmas tree is directly installed on the flange of the tubing head.
For oil and gas wells with high requirements, casing heads are usually installed after cementing to seal the annulus between the two casings, hang the second casing string and bear part of the gravity. When drilling, the casing head can be used to install the wellhead BOP.
The thread at the lower end of the casing head is connected with the process casing, and the oil layer casing sits on the slope of the casing head through slips. There is an oil-resistant seal pressed by a steel washer on the slip to seal its annular space. The flange at the upper end of the casing head is used to connect the tubing head.
If the cement does not return to the wellhead, the free casing string is above the cement consolidation point. When the temperature and pressure in the well change, the casing length will be extended or shortened, which will cause the stress of casing string itself and casing head to change. The factors that affect the stress of free casing string in the well include the gravity of casing itself, temperature change, density change of drilling fluid, oil and gas or injected fluid in the well, liquid level change in casing string, etc. When installing casing head, these influences should be analyzed and calculated to determine a reasonable initial tension value of casing string. Ensure that the lower part of the free casing string will not be damaged by compression, bending and instability; The upper casing string should be able to bear the maximum tensile load without thread slippage or casing fracture. At present, there are mature calculation methods to ensure that the free casing string is in a favorable stress state during oil well exploitation and will not be damaged.
2. Tubing head Tubing head is used to seal the annular space between tubing and production casing, hang tubing string and install Christmas tree. At present, most high-pressure oil and gas wells use tubing heads composed of special four-way and tapered tubing hangers.
After cementing the casing, install the tubing head cross on the flange of the casing head. After running the tubing, connect the tapered tubing hanger to the upper end of the tubing string, then send it to the tapered seat of the special four-way with the lifting nipple, and hang the tapered tubing tightly with the jacking thread. The annular space between tubing and reservoir casing is sealed by tubing hanger and its sealing ring and O-ring.
When installing the tapered tubing hanger, care should be taken not to jerk or collide with its sealing part.
3. The Christmas tree is an assembly consisting of various gate valves, four-way or three-way valves and throttle valves. The Christmas tree is installed on the tubing head to control the oil and gas flow, carry out planned safety production, and complete testing, fluid injection, acidizing and fracturing operations.
Four. Completion technology Completion technology varies with the completion method of oil and gas wells. Routine work includes perforation, tubing running, wellhead installation, oil and gas diversion, completion test, acidification and production.
1. Perforation At present, most wells at home and abroad are completed by perforation. Shaped charge perforators (that is, perforating guns) are widely used to complete perforating operations. After the perforating gun is loaded with perforating charges, it is transported to a predetermined position in the well. When the shaped charge is detonated, it can produce high-temperature, high-pressure and high-speed jet, which will rush to the target position.
Perforating charge explosion is a rapid physical and chemical thermal reaction, and the temperature is as high as 3000 ~ 5000℃. Due to the extremely high temperature, extremely hot gaseous substances are generated, which rapidly expand to 200 ~ 900 times of the original volume, and instantly convert potential energy in a strongly compressed state into kinetic energy. The velocity of kinetic energy shock wave can reach 200 ~ 800 m/s, which makes the pressure around the explosion point rise sharply, reaching thousands to tens of thousands of MPa. Explosives are made into conical grooves by using the directional characteristics of explosion. Because of its focusing effect, the shaped charge jet has the highest density and the strongest penetration ability at the focus, and it is easy to penetrate the casing wall and cement ring, forming holes with a certain depth in the formation.
When perforating, the bottom hole pressure is greater than the reservoir pressure, which is called positive pressure perforation. The residue and debris after positive pressure perforation are difficult to be discharged from the formation, which will cause perforation channel blockage and do great damage to oil and gas reservoirs. When perforating, the bottom hole pressure is less than the formation pressure, which is called negative pressure perforation. After negative pressure perforation, the formation fluid can immediately flow to the bottom of the well under the action of pressure difference, so as to bring the residue out without polluting the pay zone. Negative pressure perforation is a new perforation technology developed in recent years and has been widely used in production.
Modern perforation technologies include cable-driven casing gun perforation, cable-driven through tubing perforation, tubing-driven perforation, tubing-driven perforation combination, high-pressure jet sand blasting perforation, directional perforation, ultra-high pressure positive pressure perforation, coiled tubing-driven perforation and other technologies.
The process of tubing perforation is as follows: run the tubing into the well, and install the well sealer, fall preventer, blowout preventer and blowout preventer box on the Christmas tree. Put the perforating gun, cable connector and downhole instrument into the BOP and connect the cable. After the installation is completed, open the fall preventer and well sealer, and lower the perforating gun into the oil shoe with the help of cable. Radioactive logging calibrates the well depth, and then aims at the horizon to detonate perforation. Then pull out the cable, and immediately close the main gate of the Christmas tree when the perforating gun and downhole instruments enter the BOP. Release the pressure in the BOP, and then remove the device above the Christmas tree.
Through-tubing perforation has the advantage of negative pressure perforation, which is especially suitable for workover and new formation without stopping production, avoiding well shut-in and tubing tripping. However, due to the limitation of tubing diameter, high borehole density and deep penetration cannot be achieved, and the pay zone thickness drilled at one time is limited. At present, it is mostly used in offshore oil and gas wells and non-stop production wells.
The tubing conveying perforation technology is to connect the perforating gun to the tubing string and send the perforating gun to the perforation position with the help of tubing. Perforation fluid is used to create negative pressure environment before perforation. Set the tubing string, install the well sealer, and after the radioactive logging depth is corrected, detonate the horizon perforating charge, and then test the oil after releasing the gun. There are many ways of initiation, such as throwing rod initiation, tubing pressurization initiation, annular pressurization initiation, electric initiation, etc. The impact initiation of iron bars from oil pipes is the simplest and most commonly used.
Tubing conveying perforation technology has the characteristics of high hole density, deep penetration, good effect of negative pressure well washing and high safety. It is especially suitable for deviated wells, horizontal wells and heavy oil wells, and must be used in high pressure formations and gas wells.
2. Running tubing tubing is a channel for underground oil and gas to flow to the ground, and it is also a tool for well washing, well killing, acidification and fracturing. Tubing is a seamless steel pipe made of high-quality steel, which is connected into tubing string through coupling. The tubing shoe at the bottom of the tubing string is a tubing nipple with a small inner diameter, which is used to prevent tools such as downhole pressure gauges from falling into the bottom of the well.
The screen running in the oil layer is a slotted or perforated tubing. The length is generally 6 ~ 8m, and the aperture is 12mm. The total area of drilling holes is larger than the cross-sectional area of tubing, so as to increase the oil and gas circulation path, prevent larger drilling cuttings from entering tubing, and make up for the influence of small cross-sectional area of tubing shoes on production.
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Because the annulus between tubing string and casing is sealed by tubing hanger, the oil and gas flowing into the well from the formation can only enter the screen and rise to the ground along the tubing. The Christmas tree is connected with the surface production pipeline to export oil and gas from the well in a controlled manner.
3. After guiding the oil and gas to flow along the tubing and installing the wellhead device, the next step is usually to guide the oil and gas to flow. For oil and gas wells that can't self-gush due to high liquid column pressure in the well, the liquid column height or fluid density in the well should be reduced as much as possible to reduce the liquid column pressure and induce oil and gas to flow into the well. The commonly used methods are jet replacement method, fishing induced jet method, swabbing induced jet method and gas lift method.
1) Displacement injection method is called displacement injection method, which uses low-density liquid such as crude oil or clean water to displace drilling fluid in the well, reduce the pressure of liquid column and induce oil and gas to flow into the well. Clean water is injected into the well from the tubing to gradually replace the drilling fluid in the well. For high-pressure wells or deep wells, in order not to cause excessive pressure change in the well, light drilling fluid can be used instead of heavy drilling fluid first, and then clean water can be used instead of light drilling fluid to ensure the safety of the well.
2) The fishing induced injection method is to fish out the liquid in the wellbore one by one with a special fishing tube to reduce the height of the liquid column and induce oil and gas to flow into the well. This method is generally used when the spray is still ineffective after replacement.
A variation of fishing and induced injection method is called drilling tool drainage method. The bottom hole drilling tool with back pressure valve can be regarded as a long fishing sleeve, which can make the liquid level in the well drop rapidly 1000 ~ 1500m.
3) Swabbing method Swabbing method is actually to put a special sucker rod in the tubing string, and use the local vacuum formed by the sucker rod moving up and down in the tubing to gradually pump out some clear water in the well, thus reducing the height of the liquid column in the well and achieving the purpose of induced injection.
Swabbing method can reduce the height of liquid column in the well to a very low level. When the suction cup descends, the valve is opened, and water flows into the oil pipe from the water hole in the central pipe of the suction cup; When the pump is lifted, the valve is closed, and the water column pressure in the oil pipe makes the rubber expand and seal it against the inner wall of the oil pipe. As the sucker moves upward, the water column above the sucker is discharged from the wellhead. For wells that still can't flow spontaneously after injection displacement, swabbing method can be used to induce injection.
4) The principle of gas lift method is similar to that of displacement injection method, except that it is not clear water but compressed air that is displaced into the well. Gas is injected from the annulus, not through the tubing. Because of the low gas density, as long as the damage to oil and gas reservoirs is not very serious, the purpose of induced injection can be achieved after gas lift. In some conditional areas, high-pressure natural gas from adjacent wells can be used instead of compressor for gas lift. Gas lift can also be used to induce injection into wells where alternative injection is ineffective.
4. Completion test The main task of completion test is to measure oil and gas production, formation pressure, bottom hole flowing pressure and wellhead pressure, and get all data of oil, gas and water, which provides reliable basis for oil and gas exploitation.
1) Determination of oil and gas production After the oil, gas and water produced by oil and gas wells enter the separator, the gas is discharged from the upper part through the separation umbrella, and the oil and water settle. The liquid level in the glass communication pipe can reflect the change of oil-water level in the separator. By recording the time required for the liquid level in the glass tube to rise to a certain height, the liquid production of each well can be calculated, and the oil-water content can be obtained through sampling and analysis.
The output of natural gas is usually measured by throttle flowmeter. The orifice diameter of flowmeter should be suitable for the production range of natural gas.
2) Shut-in under formation pressure and bottom hole flowing pressure. After the pressure in the well is stabilized, the bottom hole pressure measured by the downhole pressure gauge is the formation pressure. Formation pressure can also be calculated by shut-in wellhead pressure and liquid column pressure. For the formation with poor permeability, it takes a long time to shut in and restore the pressure in the well. In order to save time, formation pressure can be inferred according to the law of pressure recovery in a period of time.
Bottom hole flowing pressure refers to the bottom hole pressure measured during stable production. In case of tubing production, bottom hole flowing pressure can be calculated from casing pressure and annular liquid column pressure.
3) Wellhead pressure The wellhead pressure of oil and gas wells includes oil pressure and casing pressure. Oil pressure reflects the pressure in wellhead tubing, and casing pressure reflects the annular pressure between wellhead tubing and casing. The oil pressure and casing pressure are different in the production process, and the shut-in pressure should be equal after stabilization. These two pressure values can be read by pressure gauges on the ground.
4) The sampling of oil, gas and water is to analyze and evaluate the fluids in the pay zone. Therefore, it is required that the samples taken are representative and undistorted. Generally speaking, samples are collected at the wellhead. Sometimes, in order to keep the original state of oil and gas underground, it is necessary to lower the downhole sampler to the bottom of the well for sampling and sealing, and then take it to the ground for laboratory analysis.
Reflections on examination questions
1. What is the function of drilling? 2. What are the technical characteristics of modern rotary drilling rig? 3. What does the wellbore structure include? 4. How many stages has drilling technology gone through? What are the characteristics? 5. What systems does an oil rig consist of? What is the function of each system? 6. What types of BOPs are there? What are their uses?
7. What are the main components of the drill string?
8. Why is the Kelly made into a square? 9. What are the uses of auxiliary drilling tools such as centralizers, shock absorbers and jars? 10. What are the main components of common three-cone bits? 1 1. What kind of diamond bit is used in oil drilling? Under what conditions are they used? 12. What is the function of drilling fluid? 13. What are the components of water-based drilling fluid? What kind of system does it belong to?
14. What are the basic elements of drilling fluid performance?
15. What is the relationship between drilling fluid density and drilling work? 16. How to optimize the drill bit?
17. What is the deviation control standard? 18. What are the characteristics of the killing cycle?
19. What are the types of conventional borehole trajectories?
20. What external forces mainly affect the casing string in the well? What are the basic principles for designing casing strings? 2 1. What is the basic composition of casing string?
22. Describe the basic process of cementing.
23. What protective measures are often taken when drilling oil and gas reservoirs? 24. What are the commonly used completion methods? 25. What are the main methods to induce oil and gas flow? 26. What are the components of the well completion wellhead device? What are their main functions?