Downhole operation is generally in the wellhead open cases were lower pipe column and downhole accident treatment. Once the fluid column pressure is lower than formation pressure, it is bound to cause the fluid without control jet, it is harmful not only to the formation, and not conducive to the construction, even blowout out of control, causing a greater loss. Usually take well killing process, balance formation pressure, in order to facilitate the operation and construction.
A concept, well killing
Well killing is the use of equipment from the ground into the well injecting liquid density appropriate (i.e. well killing fluid), so that the well fluid column in the bottom cause back pressure and formation pressure balance, the restoration and reconstruction of pressure balance work.
Killing is one of the most basic, the most commonly used process of downhole operation, is the premise of other operating items. Well pressure operation success or failure, directly affects the quality and effect of the well construction. The key is the correct choice of well killing fluid, sufficient to meet the performance requirements of a reasonable construction method and effective construction equipment. The aim is to make the well killing temporarily shut down, so that the reservoir of oil and gas in the can't work in the construction process flow, to facilitate the construction operation. Well pressure to reservoir protection, to comply with the "pressure without spraying, pressure without leakage, pressure of immortality" principle, and should take the following reservoir protection measures:
(1) selects the high quality well killing fluid.
(2) the ground tank clean, operating the pump and pipelines timely cleaning car.
(3) to speed up the construction speed, shorten the working period, completion in time after operation.
(4) before operation to the timely recovery of bottomhole formation pressure for spraying, as soon as possible.
Two, well before the data compression
At present 1 downhole pipe string structure condition
(1) tubing specifications and depth;
(2) specification and depth of downhole tools;
(3) kill manifold, oil pump, pumping specification and combination of screw pump, pumping rod and downhole equipment, specifications and depth of electric submersible pump.
The present situation of 2 well body structure
Investigation of casing specification and in good condition. Consulting the previous construction operations. As a set of variable records should consider the feasibility for spraying pressure well.
3 recent production status
(1) the liquid producing capacity, gas oil ratio, comprehensive water content;
(2) the oil pressure and casing pressure and flow pressure;
(3) static pressure.
Three, the selection of fracturing fluid
Well killing fluid is refers to in the process of downhole operation, used to control the formation pressure of liquid. To hold well, well killing fluid density is not small but not too large. Well killing process is not excessive, otherwise well killing fluid will squeeze into the reservoir, the reservoir the reservoir pollution, even killed. Therefore, the correct choice of fracturing fluid is an important link to ensure the quality of the pressure well. In the actual work is according to the reservoir static pressure size, selection of well killing fluid with different density, the density of the liquid is selected which can meet the requirements of the well to well pressed, do not spray in the operation, but not to damage the reservoir.
1 well killing fluid selection principle
(1) according to the petrophysical property of reservoir damage degree of selection of hydraulic well the lowest.
(2) in conditional circumstance should give priority to solid free well control fluid selection.
Solid free well control fluid is composed of one or more inorganic salts and water prepared containing no solid particulate matter well killing fluid containing dissolved salts, usually around 20%. Solid free well control fluid has many different types, according to operation requirements, the relative density can be selected in the 1.06 to 2.30 range. Well killing fluid inorganic salt used the relative density can be formulated as follows:
Well killing fluid of potassium chloride can be prepared for a maximum of 1.17 relative density;
Well killing fluid in sodium chloride can be prepared for a maximum of 1.20 relative density;
Well killing fluid and sodium bromide can be prepared for a maximum of 1.50 relative density;
Well killing fluid the calcium chloride can be prepared for a maximum of 1.39 relative density;
The relative density of well killing fluid the calcium bromide can be prepared for a maximum of 1.39~1.70;
Well killing fluid and calcium bromide and calcium chloride can be prepared by different ratio of relative density to a maximum of 1.33 ~ 1.80;
The relative density of well killing fluid and zinc bromide, calcium bromide and calcium chloride can be prepared by different ratio of 1.80~2.30 maximum.
2 well killing fluid density selection
Well killing fluid relative density according to the following formula:
Alpha =P * 102/H * (1+K)
Type a relative density of alpha killing fluid;
P well within the past 3 months, the measured static pressure value, MPa
H central reservoir depth, m;
K added amount, the general operation and the construction of K=0 ~ 0.15; workover construction K=0.15 ~ 0.3.
3 well killing fluid determined
Well killing fluid is calculated by the following formula:
V=1/4 * n D2h (1+K)
Type V - well killing fluid dosage, m3;
D casing diameter, m;.
H - pressure well depth, m;
K added amount, K=0.15~0.3.
Four, the choice of the way of the pressure well
In the pressure well before, except for well killing fluid density and the nature of the selection of appropriate, to the different conditions of the wells, selection pressure well appropriate manner is an important factor to guarantee the success of the pressure well.
At present, well killing method commonly used field on circulation method, filling method and squeezing method three.
1 circulation method
Recycling method is now the most widely used in oil well repair application. The well killing fluid is prepared with the pump into the well and the circulation, the relative density of liquids is smaller in the wellbore wells fracturing fluid replacement, so that the original be wellbore oil, gas, water filled by killing fluid filled. Well killing fluid column back pressure at the bottom of a well balance of generation, reservoir pressure, so that the formation of oil and gas in no longer enter the wellbore, which will be well down.
The key circulation killing is to determine the relative density of killing fluid appropriate control and the bottom hole pressure. Can be divided into the reverse circulation kill and kill two methods of normal circulation.
1) reverse circulation method
The reverse circulation kill is well killing fluid from the oil pump casing annular space man well. The well fluid column up to process and cycle from tubing wellhead.
The reverse circulation kill is used in high pressure, large output of oil and gas wells. Because of the reverse circulation kill, the fluid flow is from the oil pipe column casing annular space large cross-sectional area, low flow rate into the cross-sectional area is small, high velocity in. Based on the principle of hydraulics, the displacement of certain conditions, when the well killing fluid from the pump casing annular space column and into, descending velocity pressure wells and low friction loss is small, the pressure drop is small, resulting from the back pressure is relatively large on the bottom. Visible, the reverse circulation kill from the beginning to produce large bottom hole pressure. Therefore, for high pressure, large output wells, using the reverse circulation kill method is not only easy to succeed, and the pressure after the well, even if the oil layer is slight damage, can also be used in production of oil well pressure, high yield themselves to relieve; instead, if well pressure on low pressure wells using reverse circulation method, it will produce the larger the bottom hole pressure, easy to cause the production layer damage, and even the emergence of formation breakdown phenomenon. The reverse circulation kill exclusion liquid flow time is short, the ground pressure wells, the success ratio of well killing incremental less advantages.
2) positive cycle method
Positive circulation pressure well killing fluid column will be pumped into the well to replace the well fluid from an oil pipe, oil casing annular space, up from wellhead to the circulation process.
Is the circulating pressure well suitable for low pressure and larger output of oil wells. In the displacement of certain conditions, when the well killing fluid from the tubing pump into, descending velocity well killing fluid fast, large friction loss, pressure drop is big, the back pressure generated is relatively small on the bottom. So, for the low pressure wells, using the normal circulation well killing method not only can achieve the purpose of pressure wells, can also avoid the leakage formation. In the use of positive circulating pressure well, should be first in the wellbore of gas discharge. Because such wells generally low pressure, high gas venting, suddenly, can cause temporary stopping spraying. Then, immediately from the tubing in the well killing fluid pumping, this possibility, well killing fluid gas is small, does not cause leakage, so the pressure well can be successfully. But the pressure well for high production wells, gas wells and high pressure wells, the success rate of the reverse circulation kill lower than.
2 perfusion method
Perfusion is to wellbore perfusion a fracturing fluid, fracturing method of wellbore fluid column pressure to balance formation pressure. Simple workover this method is used in the bottom hole pressure is not high, well repair work is not difficult, small workload, workover time short, such as the replacement of oil well production tree total gate, lift the drill pipe sticking accident near the well mouth, welding well, replacing the four flange and other operations. Its characteristics are not contact well killing fluid and reservoir, basic ruled out reservoir damage. The well killing method has the advantages of simple equipment, convenient operation, the oil well production quickly returned to normal.
The 3 squeezing method
Squeezing method is in neither with circulation method, and not with the perfusion pressure wells used in the case, such as downhole sand plug, paraffin blockage or because of some accident cannot cycle high pressure wells. The method is well wellhead pressure only well killing fluid import and export did not return, on the ground with high pressure well killing fluid into the wellbore wells, oil, gas and water squeezed back formation, in order to achieve the purpose of well pressure.
Squeezing method is different from the perfusion method, it is the use of high-pressure pump to pump into the well killing fluid in the well, and the wells of the oil, gas and water squeezed back formation. It is different from the circulation method in pressure well only when no export import. Well killing method the disadvantage of this is: in the high pressure will wellbore fluid (oil, gas, water) to squeeze back formation at the same time, also has the possibility to be well within the dirt (such as sand and mud) into the stratum, thus cause downhole oil reservoir blockage, and the pollution of oil layer.
Five, pressure wells construction
Well killing process is relatively simple, but the construction is relatively complicated, should be very cautious, otherwise, not only the pressure well no, will bring damage to the reservoir. The right to determine the pressure wells, in strict accordance with the killing process operation, maintain and deploy a good well killing fluid properties, timely admission all information is an important condition for the success of the pressure well.
1 keep the liquid density and
Due to the influence of natural gas in formation, may occur well killing fluid gas pressure in the process of well drilling fluid density, reduce pressure, resulting in decreased back pressure fluid column in the bottom of the well produced, when the bottom hole pressure drops below the formation pressure, will blowout. Therefore, in order to prevent blowout, must be within a certain period of time will be well has gas invasion liquid out, in order to maintain the fluid column in the bottom of the back pressure will be produced, well pressed.
2 Export Control
Ensure that import the displacement of more than exports of the overflow traffic, using high pressure pressure pressure well, let the wells in gas bearing fluid is gradually replaced by the fracturing fluid.
3 to prevent the leak and killing fluid into the reservoir
As in the killing process found in wellhead pressure is low or there is a downward trend, while the occurrence of well killing fluid pump into the amount of discharge amount less phenomenon, it shows well leakage. Especially some formation water absorption capability is very strong, well pressure at the beginning of the pump pressure is very high, capacity big, very easy to cause the pressure leakage phenomenon, the result makes well killing fluid enter reservoir. If the well has been pressed, still continue to high pressure squeeze in the well killing fluid, also can make the well killing fluid into reservoir. So, in kill operation, correct judgment whether pressed well is an important work. Well pressed characteristic mainly has following several points:
(1) the wellhead pressure is nearly equal to the import and export;
(2) is equal to the amount of export import displacement overflow;
(3) the relative density of well killing fluid import export relative density is approximately equal to;
(4) export no bubbles, stop the pump head without overflow.
4 preventing blowout
In kill operation, wellhead pump pressure stable, liquid volume pump human fluid volume and wellhead back out of roughly the same, import and export the liquid density is almost the same, return no liquid bubble. Shut in well 30min head without overflow, no abnormal sound in the wellbore, these are well rested method to judge whether. If the following conditions are blowout omen:
(1) the import of small displacement, the export of large overflow quantity, exports increased bubble overflow;
(2) well killing fluid inlet relative density, relative density of small export, the relative density of a continuous downward trend;
(3) export jet velocity increases gradually;
(4) after stopping pump inlet pressure increased.
In case of the above phenomenon, should immediately killing fluid circulation and adjustment well killing fluid properties (such as improving density etc.), to take timely BOP measures necessary to ensure the safety.
Item 5 pressure well construction should be paid attention to in the
Regardless of the method well pressure must pay attention to the following questions:
(1) according to the design requirements, the preparation of well killing fluid in accordance with the conditions of. In general no obvious leakage layer well prepared, the amount of liquid is usually 1.5~2 times of wellbore volume.
(2) the import pipeline pressure test pressure wells must achieve the expected 1.2~1.5 times of pump pressure, no thorns no leakage. High pressure and choke line shall be used for steel pipe lines, the prohibition of the use of elbow, hose and the low pressure pipeline, and fixed firmly.
(3) the circulating pressure well operation, tap (or swivel joint), hose to be tied to a safety rope.
(4) well before the pressure of gas oil is relatively large or high pressure wells, should first use of choke control exhaust, reoccupy clear water pressure well circulation degassing, pressure well hydraulic well and then use the high density.
(5) the import and export pressure drilling fluid properties, displacement should be consistent. Requirements of the import and export pressure of drilling fluid density difference is less than 2%, as far as possible to increase the displacement of the pump (not less than 0.5m3 / min) circulation pressure well, and well repairing pump suction line to install the filter. When the import volume of more than 1.2 times the wellbore volume is still not returned and a large amount of leakage occurs, should stop construction, consult the relevant departments, take effective measures.
(6) pressure well midway generally should not stop the pump, outlet should properly control the displacement, do well killing fluid can not leak and not by the gas invasion. Can stop the pump for liquid and import performance agreement return well within the. If after the pump is stopped still found some spillover or spray potential, should exhaust gas recirculation, or by using the method of well shut in well stability, so that the gas separation, and then open the well emptying inspection effect.
(7) the highest pressure wells pump pressure shall not exceed the reservoir absorption threshold pressure (with the exception of squeezing method). In order to protect the reservoir pressure, avoid long well, must be continuous construction, reduce the well killing fluid to reservoir pollution.
(8) squeeze liquid injection method pressure of injection wells depth, the control should be at the top of the reservoir above 50m. Kansai after a period of time, well open the inspection effect.
(9) if the pressure well failed, must analyze the reasons, not blindly increase or decrease the well killing fluid relative density.
Six, the influence factors of the success or failure of the pressure well
The factors that influence success or failure to kill the well. In the site construction is often due to a hand in the construction problems will cause pressure well failure or damage to stratum, therefore, understanding the factors affecting pressure well is an important part of the pressure well construction. Effect of pressure well success or failure factors are mainly the following aspects.
1 well killing fluid performance influence
Well killing process, various conditions in the borehole and formation are constantly on the fracturing fluid for a role, prompting the well killing fluid suitable performance in constant change, affecting the success rate of fracturing well. The main reason of pressure damage drilling fluid properties is the following "four invasion":
(1) the water invasion: in kill operation, water intrusion make well killing fluid property damage. Well killing fluid by water invasion, its viscosity decreases, the decrease in density, should adjust its performance.
(2) gas: formation (oil well) within the large natural gas mixed with well killing fluid in the wellbore, occupy the volume, the well killing fluid density decreases, viscosity increases, causing the pressure well difficult. The occurrence of this situation, is the blowout "warning signal", should immediately adjust the pressure of drilling fluid properties.
(3) calcium (cement penetration): strata in the gypsum invasion well killing fluid, causing a modified sodium base clay properties of well killing fluid in the transformed into calcium base clay, called calcium. The viscosity and shear force to reduce the pressure of well liquid calcium, water loss increases; cement penetration is due to cement intrusion made modified well killing fluid performance deteriorated, the increase of viscosity, liquidity, water loss increased, causing pressure well difficult, serious when can damage the producing layer. At this time, can join the lignite alkali, tannic acid powder, take a precipitation recovery well killing fluid properties.
(4) saline intrusion: formation of salt (water) invasion, well killing fluid property change, well killing fluid viscosity increase after saline intrusion, easy to gas invasion caused the decrease in density, well blowout accident will be serious. Well killing fluid salt water intrusion is pre improve measures to prevent the killing fluid density, the brine layer down, and join processing agent to stabilize its performance, so that salt water intrusion will not happen.
2 the equipment performance influence
Infusion equipment for pressure wells must be in good condition, if the pump injection equipment performance is not good, such as the pump suction water or displacement is insufficient, make well killing fluid can not be continuously injected into the well, will cause the killing operation failed. Water shortage reason may be water line was well killing fluid in the dirt blocking the pipeline construction in winter due to ice or. The pump displacement main reason may be the small pump performance is not up to the requirements, or the pump itself is in poor condition caused; killing operation time is too long will make the equipment fatigue failure, is the cause of the failure of the pressure well. Therefore, in killing before operation, carefully check the construction of the equipment status, avoid these problems appear in the construction.
Effects of 3 construction process
Effects of cyclic pressure well construction process mainly has three aspects:
(1) the well condition is unknown. Such as mastering the wellhead pressure, underground wax, wellhead ejecta composition, gas oil ratio, and the surrounding communicated well situation is not clear, resulting in unexpected problems occurred in pressure during well caused by surprise, the well pressure operation failed.
(2) the construction preparation is not sufficient. Before the construction, no Beizu well killing fluid, appear without well killing fluid supplement problems, resulting in the killing operation failure; pressure well before the pipeline without inspection, construction pipeline rupture caused failure of construction; the liquid tank is insufficient, occurring in the construction of tank phenomenon will cause pressure well failed.
(3) the improper technical measures. As the pressure in the process of well wellhead valve control properly, affect the construction of. Such as wellhead outlet valve is too large, will make the pressure and fluid loss, caused not only a waste, but also make the killing is not successful; the outlet valve is too small, in kill operation makes lots of well killing fluid into formation damage. Well killing fluid density, pressure well construction choice improper way would affect the success rate of fracturing well.