Drilling waste treatment is for processing and recycling drilling solid generated in the process of drilling, and reaching the environmental emission standards of the countries and regions eventually, it can effectively avoid the pollution of environment, also can save drilling fluid materiasl and reduce the cost of drilling fluid. At present, there are various types of drilling waste treatment methods and devices which have the different treatment effects. But for the domestic and foreign major site conditions in China, drilling waste removal is the most important and the most commonly used of drilling waste treatment methods.
Dry drilling waste removal refers to dewatering drilling waste by special equipment, and make the content of oil and water in the drilling waste decreased effectively, so that the drilling waste reached the statutory emission standards. Two kinds of key equipments in the dry process of drilling waste are as following:
The DC Machinery TLL-A vertical drilling cuttings dryer is a kind of equipment which make the solid-liquid mixtures separation by centrifugal force generated by the mechanical rotation. It is adaptable, better treating efficiency, stable working, having long service life and can effective recyling the drilling fluid in drilling cuttings. It is the preferred equipment of dry drilling cuttings, recycle drilling fluid and especialy recycle oil base drilling fluid.
Dry shale shaker is also suitable for the recovery of oil base, water base and synthetic base drilling waste in drilling cuttings, and we can make the choice of different configurations of dry vibration sieve numbers according to the site conditions,.
DC dry shale shaker is a kind of linear motion shale shaker. On one side, the dry shale shaker used to recover the mud adsorptive by the cuttings that excluded by mian shale shaker and hydraulic cyclone, which can effectively reduce the mud waste, thereby reducing the cost of drilling; on the other hand the liquid holdup / oil content in the cuttings is reduced to 10%, thereby reducing the pollution to the environment.
The mineral composition of shale is more complex, in addition to the common clay minerals such as kaolinite, montmorillonite and illite,it is also mixed with quartz, feldspar, mica, calcite, dolomite, pyrite, apatite and other minerals. Clay minerals are easily expanded when encountering water, and affect the normal drilling, cause the underground complex such as wellbore collapse. It will cause the pollution of drilling fluid to post production and reduce the capacity at the same time, which is the main reason for the use of oil-based mud.
We can try to use water-based mud if it is vertical wells. But for shale gas, the development benefits of vertical shaft are far less than horizontal wells, shale gas development mainly adopts horizontal well and most of them are along the minimum principal stress direction, as it is easy to collapse when shale hydration expansion occurred, it is more easier to collapse in this direction. The anti collapse performance of horizontal well oil based mud is indeed better than water-based drilling fluid.
- Using the water-based mud in the drilling process, the shale itself is a kind of brittle mineral, it will expand when encountering water, this will easily lead to the wellbore collapse and affect the quality of drilling.
- The non organic matter pores of the shale may be the water wetting phase, and the gas in the shale pores can be easily expelled and replaced by water and it will affect the later drilling quality
- Water based mud is prone to cause blockage, the shale layer itself has very low permeability, and causing capacity changes;
- Oil based mud can play a supportive role to shale, due to the small pore stopped by capillary pressure, the oil molecules is difficult to enter into the organic matter and inorganic pore of shale.
So for drilling, using the oil base mud that equipped with a certain proportion of proppant as the drilling fluid and this is crucial to the horizontal well, rotary steering, wellbore trajectory and the late cementing, it should be a comprehensive integration technology.
The pressure difference between inlet and outlet of the cyclone, which is working pressure, is one of the important working parameters. Increasing the pressure difference of the inlet and outlet, in fact, increases the flow rate of the liquid, so that the time of the liquid in the hydrocyclone is less.
Since the time of the liquid in the hydrocyclone is basically the separation time of the solid phase, the residence time is reduced, and the separation process is not sufficient.
Moreover, due to the reduction of residence time, the wear of the cone can be accelerated. Reduce the export pressure, resulting in the decline of the swirl velocity, centrifugal force is not enough, despite an increase in the liquid residence time in the cone, also cannot achieve full separation effect. Therefore, cyclone will have an optimum working pressure.
In the solid control system, general layout and process sequence is shale shaker, degasser, desander( desilter or mud cleaner), centrifuge.
The drilling fluid which returns from the wellhead that contains a large number of cuttings will flow to the first level purifying equipment—the entrance of shale shaker through the wellhead elevated vertical and horizontal drilling liquid tank(with a certain slop) under the function of gravity. The larger harmful solid particles will be sieved and discharged after the screening of shale shaker.
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When the gas invasion generated in the drilling fluid, the drilling fluid which is purificated by shale shaker get into the degasser under the high pressure for gas separation, drilling fluid after degassing row into the second warehouse of the mud purifying tank. In the case of no gas in the drilling fluid, the gas tank can be used as the high power drilling fluid mixer, and ensure the drilling fluid in the purification tank is not deposited.
The drilling fluid that has been purified by shale shaker enter into the sand tank of mud purification tank. And then using the sand pump to join the drilling fluid to the second level purifying equipment—the desander of combined mud cleaner and the drilling fluid will be separated from solids again.
After excepting sand, drilling fluid discharged into the third storehouse of drilling fluid purifying tank through overflow line of desilter and will be pressured into the third grade purification equipment—desilter of the combined mud cleaner by desilter sand pump, and be separated again of cyclone principle.
The drilling fluid still contains a lot of harmful solid phase after passing a three-stage purifying. When the drilling fluid is a non weighted state, we use two centrifuge in parallel to remove the harmful solid phase that is larger than 5 μm. When the drilling fluid is weighted, the use of two sets of centrifuge is needed with series connection. After separation the drilling fluid contained barites return drilling fluid purifying tank, and the harmful solids discharged outside the system and achieve the purpose of purifying and maintaining the stability of the drilling fluid performance.
Platform equipped with five levels of drilling fluid purification system equipment to ensure that the platform can meet the requirements of drilling fluid treatment in a variety of circumstances.
Many factors will affect the performance of the centrifugal pump, which must be considered when chosing the pump. In the design of drilling platform, people often think that centrifugal pump is a product of low consumption, and without too much engineering consideration.
Many times the selection and order of centrifugal pump is based on complete sets of equipment which has already existed on the drilling platform. And this will lead to very serious problems, because each drilling platform has different working conditions and the design of pipeline, the centrifugal pump is often used to feed for the sophisticated equipment. If centrifugal pump selection is not suitable, the work of them and other equipments will all be affected. The reasonable selection, design and installation of centrifugal pump can directly improve the efficiency and reduce the cost of the operation of the drilling platform.
According to the difference of material, structure, size and design parameters, centrifugal pump is divided into many types. In general, the choice of the centrifugal pump shoule meet the specific needs of the maximum, only to grasp the overall characteristics of the combined system can we carry out a precise selection of centrifugal pump.
It is necessary to obtain parameters such as working temperature, pipeline size, assembly height, fluid property, head pressure and so on. If there is no trade-off between these parameters, the result may be the failure of the pump and its working system, or cause excessive energy consumption.
The separation ability of centrifuge depending on density difference of solid and liquid phase and length of subsidence area. When the density difference of solid and liquid phase is similar, which means the viscosity of feeding slurry is large, the separation is different to carry out. In actual production, there are three main factors of technological conditions can influence the centrifuge separation effects, which are feeding temperature, feeding rate and abnormal technological conditions.
(1) The temperature of the feeding slurry can directly affect the viscisity of mothor liquid. The higher temperature of the solution is, the lower of will the viscosity is and the thinner is the liquid film on solid phase, and the more easier of fine particles to subside.
(2) Feeding rate. Sometimes too large feed quantity can lead to bad separation effect. That’s mainly because the settling time of particles on the wall of rotating cylinder is not enough. The permise of reaching the separation conditions of centrifuge is that time of solid phase settling on the drum wall must be less than the time od particles stay in the drum. That is to say, you must ensure the effective residence time of the separating slurry in he drum to make the solid phase particles have enough time to settle out.
(3) The abnormal technological condition mainly refers to insufficient crystal content in the feeding slurry or crystal does not crystallize and is flocculent, and it is very difficult for the centrifuge to get the ideal separation effect.
The screen paste and blocking phenomenon will reduce the handling capacity of shale shaker due to the effective flow of the mesh is reduced, which will easily lead to the loss of drilling fluid.
When dealing with the high viscosity drilling fluid, the steel wire of the screen is easy to sticked with drilling fluid which will make the shale shaker mesh size decrease gradually and then more small particles can be separted. For example, the 40*40 mesh of screen in China can separate the particle that more than 440 microns from drilling fluid when the screen is not pasted and can isolate the particles that above 220 microns after screen pasting.
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When the shale shaker is working, the critical particles will maybe wedge in the mesh which can cause blocking, and this phenomenon is called screen blocking.
Except for the drilling cuttings and clay cause screen paste and blocking, the soluble mineralls (such as salt, anhydrite and some carbonate, etc.) in drilling fluid will precipitate out and stick on the steel wire when the drilling fluid residual on the screen is dry, this will lead to the diminution of mesh or even being blocked. In addition, the grease in drilling fluid can aslo stick on the screen.
When shale shaker screen paste or blocking occur, we should spray water to rinse it timely or replace the screen and keep the mesh smooth. To do this we can remove the drilling fluid be pasting on the screen and aslo reduce the load of screen.
The correct choice of centrifugal pump and piping system is very important because most system use the low pressure drilling mud gun. It should be emphasized that, for a low pressure system, to make it work, must consider a few points followed:
(1) The size of nozzle;
(2) The number of the elbow, valve and tapered pipe;
(3) The length of pipe;
(4) The size of the pipes that are used.
These parameters are very important, before chosing a pump, the influence of each parameter must be calculated. In most system, when the fluid flow through the nozzle, it will lose most total head pressure transmitted by the pump, and discharged pipeline length, pipe diameter and pipe fitting all need the suitable pump and motor. In order to achieve a given flow velocity through the nozzle, a specific total pressure head is required to meet the firction force of the system.
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Low pressure mud gun requires about 23 meters of pressure head in normal work, the nozzle size ranges from 12.7 millimeters to 25.4 millimeters. Centrifugal pump pressurizes to nozzle through standard wall pipeline, the low pressure system does not need special thick wall pipe connection. But the variable flow velocity and the pipe with large diameter can be used to prevent excessive friction loss, the nozzle of low pressure mud gun is aslo larger than the high pressure mud gun. When a large number of liquid enter into the drilling mud tank through the nozzle of the low pressure drilling mud gun, the drilling fluid is stirred then. The liquid has a certain speed after being through the nozzle. Thereby providing the shear force which is known as the high –flow and low-pressure system.
No matter what kind of system, it shlould avoid the feeding pipeline of nozzle from being excessive bending or too long whether in economics or functions , this point is aslo very crucial for the effective operation of the equipments especially for the low pressure system.
From the bernoulli principle we can know that nozzle size and feed pressure determine that at what speed will the fluid flow through the nozzle. And at the same time, the more fluid flow through the nozzle, the larger flow rate is and greater the shear force generated. Some multi nozzle structure drilling mud gun used in the compression system can get four times liquid flow of the conventional nozzle that it can be very good in completing the mixing and diffusion of drilling fluid.
The nozzle of mud gun generate low pressure area around the area that the nozzle released, and the low pressure area is quickly sucked around the nozzle and is wrapped in liquid, which is similar to the effect of the emixing hopper. The two fluids are mixed in the funnel portion to flow into the storage tank at the slightly higher rate than te nozzle. The high speed fluid that comes out of the ejector can both come from the storage tank or other places (such as the mixed slurry tank).
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When the agitator is placed on a symmetrical circular or square mud tank(look from top to down), the effect is the best. Meanwhile, compared to the vessel design of square or rectangular , the container vessel design of circular has the following advantages:
(1) Circular mud tank body is installed together with the central drainage pipe or cleaning device. And the tank body is more easy to clean, and the need of the washing liquid is less than the squareness or the rectangular mud tank.
(2) It has the extra space to place the conduit and the pump.
(3) It needs no further cantonization because each container has its own compartment.
(4) Due to the symmetrical design of the circular mud tank, the drilling fluid mixture is more throughly, which can ensure the mixing is well. The blind angle of precipitable drilling fluid in the circular mud tank is less than that in the square mud tank.
But the circular mud tanks aslo have some shortcomings:
(1) They can not use the space more effective, compared with a retangular or square storage tank , the same volume occupies more space.
(2) Circular mud tank needs a dashboard to prevent the body from rotating and promote liquid suspension, which increases the cost of manufacturing.
Due to the restriction of the square or rectangular container to the mechanical agitator, it must use the mud gun. In most cases, the appropriate mechanical agitator should be selected for the full mixing in the container, and the mud gun should be properly installed to eliminate the blind area.