The viscosity of the drilling mud refers to the total reflection of internal friction between solid particles, between solid particles and liquid molecules, and between liquid molecules as the drilling mud flows, and it can be measured by funnel viscometer and rotary viscometer. Due to different measurement methods and different viscosity values, funnel viscometer is often used on site to measure the viscosity of drilling mud with seconds as the unit.
The viscosity of drilling mud has a great impact on the capacity of drilling mud to carry cuttings. In general, drilling mud with high viscosity has strong cuttings carrying capacity. But the viscosity of drilling mud should be appropriate dring the process of drilling, otherwise it will cause adverse consequences. It is not conducive to carrying cuttings if the mud viscosity is too low, whcih will cause sand sinking in the well fast and shaft wall scoured, and will make spalling, collapse, leakage and others easy to happen. However, high drilling mud viscosity may cause the following hazards:
I. The drilling rate is seriously affected by high flow resistance, high pump pressure and poor bottom hole cleaning effect.
II. It's easy to get bit balling and produce suction or pressure excitation when tripping, which will cause well leakage, blowout, well collapse and other complicated situations.
III. It makes sand sinking difficult and causes poor purification, which will wear and tear drilling tools and accessories.
IV. It will cause difficult degassing and decreased drilling mud density, which will easily make complex situation in drilling.
V. The cuttings form false mud cake on the shaft wall, which will be easy to cause jamming.
VI. During cementing, the cement slurry is easy to be channeled and affect the cementing quality. Therefore, the viscosity of drilling mud shall be decided according to the specific situation. Generally, under the premise of carrying cuttings, the viscosity should be low. The pump pressure should be high when the well is deep, and the pump displacement should be limited. In case of borehole collapse, more sand sinking or slight leakage, the viscosity should increase appropriately to eliminate the downhole complications. And lower viscosity helps the drill bit to break the cuttings when the drilling mud is ejected from the water hole of the drill bit. It is conducive to carrying cuttings to clean the bottom of the well and return in the annular space with high viscosity, which helps to improve drilling rate.