The modern drilling indispensable to use the circulating fluid, in addition to a small amount of circulating fluid is the use of gas and foam, the vast majority ofcirculating fluid drilling are used in liquid, so called circulating fluid used in the process of drilling for drilling fluid. Because the drilling fluid early just mixed liquidclay and water, almost no other chemical composition, oil field used to be called “mud”.
Drilling fluid plays an important role in the process of drilling. Its main function has:
1) cleaning the bottom
Cutting rock resulting in a large number of cuttings in the drilling process, the cutting is by drilling fluid to remove the continuous cycle. If the debris can not be promptly removed, it will be repeated fragmentation, will affect the further drilling rock drill.
2) cooling, lubricating the drill bit and drill string
In the process of drilling, drill bit for drilling pressure load to bear on the very serious, resulting in bit bearing and bit working surface heating. At the same time,because of the drill string friction constantly in the process of rotating with the wall, not only to generate heat, but also accelerates the drill string wear, power consumption is increased, and the drilling fluid can pass through the loop carries heat away in time, cooling of drilling, but also to the lubrication, so as to reduce the side effects produced by friction.
3) the formation of mud cake, protect wall
The formation of deep rock in three-dimensional compressive state due to the top and the surrounding rock pressure. When after drilling, borehole surrounding rock must bear the extruded from all directions and generates a stress incrementvalue, i.e., stress concentration. In this case, if the wellbore rock strength is not big enough, it may lead to the wall collapse. But if the containing increasing agent, filter loss of drilling fluid action low around the borehole wall, will make up the rock stress decreases, so that the ”compression collapse” to reduce the chances. But the density of drilling fluid can not be too large, otherwise it will lead to the rock’s “fractured broken”, causing the leakage of drilling fluid. And by adding various additives in drilling fluid has formed a layer of mud cake on the borehole wall will in the cycle process, mud cake quality can not only protect thewall is not drilling fluid erosion, but also can reduce the leakage of drilling fluidinto formation.
4) control and balance formation pressure
When drilling into the high pressure oil (gas) layer, if the formation of oil (gas)reservoir pressure higher than that of drilling fluid column pressure, will happen “blowout” led to drilling accident. This should be the preparation of drilling fluid density appropriate, make the drilling fluid column pressure is equal to the(usually slightly greater than) the formation pressure, so as to ensure the safeand fast drilling. But the density is not too large, otherwise it may put the oil (gas)layer blocked.
5) floating debris and weighting agent
In the process of drilling, drilling fluid, flow in the annular space of the wellboremay not be too high, and the water because of low density makes the suspensionability is poor, as the debris and weighting agent in drilling fluid circulatingdischarged from the ground, but also to join the other various additives to increase the suspension ability.
6) the geological data to provide the drilled strata
Through drilling fluid sampling on out of the ground of the analysis, can obtain the drill data of lithological and stratigraphic oil gas content.
7) the water power is transmitted to the drill bit
Drilling fluid from the bit nozzle flow to the bottom, not only clean cuttings, but also lower the energy to bottom auxiliary rock breaking, jet drilling is a drilling technique using bit hydraulic power. Proof of drilling practice at home and abroad, jet drilling technology to improve drilling speed, and reduce the drilling cost.
8) removal of drill cuttings on the ground
Solid particles are dispersed in the drilling fluid called solid from drilling fluid.Solid phase in drilling fluid, one is from cuttings, two is to meet the requirement of the drilling technology and adding the man-made
According to the solid phase in drilling fluid action can be divided into two categories:
A class is a useful solids such as bentonite, chemical treatment agent and bariteetc.;
Another kind is the harmful solid, such as cuttings, inferior bentonite, sand etc..The so-called solid drilling fluid control is to remove harmful solid, retain theuseful solids or solid content and total level of granularity in the drilling fluidcontrol scope required in drilling process inside, to meet the requirements ofdrilling fluid properties of drilling process. Usually, the drilling fluid solid control process referred to as solid control, oil field is used to be called mud purification.
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