The drilling construction process together with cementing process will be introduced in the next chapter
Drill fluid also known as drilling mud and commonly know as mud is a circulate fluid which have many functions and can satisfy the drilling work need.
1 Drilling fluid classification
According to liquid the drilling fluid can be divided into three categories: Water-based drilling fluid, oil-base drilling fluid and gas drilling fluid
2 Drilling fluid composition
- Clay (bentonite): sodium soil, calcium soil, organic soil ect.
- Water: fresh water, salt water, saturated water, sea water.
- All kinds additives: dispersants, fluid loss additives, flocculants, diluting agents ect. Weighting additive: barite, Super fine calcium carbonate powder.
- Oil: crude oil and diesel oil.
- Gas: air and nitrogen.
3 Drilling fluid effection
(1) Suspension and carrying debris, cleaning the bottom of well, keeping the well clean. The most basic functions of drilling fluid are suspension and carrying debris. Drilling fluid jet from the drill bit nozzle, produce injection effect, make the rock debris suspending in the drilling fluid at the bottom of the well to keep well clean. And because the drilling fluid has density structure and viscosity, drilling cuttings can't sink soon, and will be brought to the ground along with the cycle of drilling fluid.
(2) Because the drilling fluid have density, when drilling different kinds of complex formation, the formation pressure and wall lateral pressure can balanced by adjusting drilling fluid density. That makes the formation pressure and wall lateral pressure not easing to release to the borehole and the sidewall is stable, at the same time, can effectively prevent leakage, well blowout and well collapse accident.
(3) Transfer water power, help breaking rock. Drilling fluid form a high-speed jet in the bit, that produce a strong impact force at the well bottom. The force can break rack and make rock debris suspended in drilling fluid. That can keep bit always contact new stratigraphic, and not cause the broken bits cuttings scraps again. At the same time, the strong flow of water make rock crack expanding, and help drill bit to break rock, so as to improve the mechanical penetration rate.
(4) Cooling and lubrication drill bits and tools. In the drilling process, drill bit rotating under high temperature and broken rock will produce a lot of heat, at the same time, the friction of drilling tools with well wall will produce a lot of heat. The heat is hard to be transferred by rock, so it must be transferred through the circulation of drilling fluid to cool and lubricate the drill bit.
(5) Partly supporting the weight of the drill string and the casing. The drilling fluid inside the well give drilling column and casing in the well the upward buoyancy, and make the load the ground equipment and devices supported reduce greatly. In the deep wells, the buoyancy the drill string and casing understood, increases along with the increase of the drilling fluid density.
(6) Provide motivation for underground drilling tools. Drilling fluid within the flow drive the turbine rotating to break rock, and provide turbo drill motivation when they flow through the drill pipe with high speed.
(7)Get geological date of the formation drilled from drilling fluid. According to the debris brought out by drilling fluid from the bottom, can do geological logging (cutting logging) and fluorescence logging. According to the drilling fluid composition return from the bottom, can do gas logging, and obtain relevant geological strata drilled data.