The function of drilling fluid and the importance of drilling fluid solids phase control
Circulating fluids are indispensable in modern drilling. Except for a small amount of circulating fluids that use gas and foam, most of the circulating fluids used in drilling are liquids. Therefore, the circulating fluids used in the drilling process are called drilling fluids. Since the early drilling fluid was just a mixture of clay and water, with almost no other chemical components, it is customary to call it “mud” on site.
Drilling fluid plays an important role in the drilling process. Its main functions are:
1) Cleaning the bottom of the well
The drill bit breaks the rock during the drilling process to produce cuttings, which are removed by the continuously circulating drilling fluid. If the cuttings cannot be removed in time, it will be repeatedly broken, which will affect the drill bit to continue drilling into the rock.
2) Cooling and lubricating drill bit and drill string
During the drilling process, the drill bit bears a very serious weight-on-bit load, which causes the bit bearing and the bit working surface to heat up. At the same time, because the drill string is constantly rubbing against the borehole wall during rotation, it not only generates heat, but also accelerates the wear of the drill string and increases power consumption. The drilling fluid can take the heat away in time through circulation to cool the drill tool, and at the same time It can also play a role in lubrication, thereby reducing the side effects of friction.
3) Form mud cake to protect the well wall
The rock deep in the formation is in a three-dimensional pressure state due to the pressure of the top and surrounding rocks. After the borehole is drilled, the rocks around the borehole must be squeezed from all directions to produce a stress increase value, that is, stress concentration. In this case, if the strength of the rock at the wellbore is not strong enough, it may cause the well wall to collapse. However, if a drilling fluid with a weighting agent and low fluid loss is used on the surrounding well wall, the stress on the rock will be reduced, thereby reducing the chance of “compressive collapse”. However, the density of the drilling fluid should not be too high, otherwise it will cause the “fracture fracture” of the rock, and then cause the leakage of the drilling fluid. In addition, the drilling fluid with various additives will form a layer of mud cake on the well wall during the circulation process. The mud cake can not only protect the well wall from being washed by the drilling fluid, but also reduce the leakage of the drilling fluid to the formation.
4) Control and balance formation pressure
When drilling into a high-pressure oil (gas) layer, if the pressure of the oil (gas) layer in the formation is higher than the pressure of the drilling fluid column, a “blowout” will occur and cause a drilling accident. To this end, a drilling fluid with a suitable density should be prepared so that the pressure of the drilling fluid column is equal to (usually slightly greater than) the formation pressure to ensure safe and rapid drilling. But the density should not be too high, otherwise the oil (gas) layer may be blocked.
5) Suspended cuttings and weighting agent
During the drilling process, the flow rate of the drilling fluid in the annular space of the borehole cannot be too high, and the low density of the clean water makes its suspension capacity poor. In order to circulate cuttings and weighting agents out of the ground, various additives must be added to the drilling fluid. To increase its suspension capacity.
6) Provide relevant geological data of the drilled formation
By sampling and analyzing the drilling fluid flowing out of the ground, the lithology and oil and gas content of the drilled formation can be obtained.
7) Transfer water power to the drill bit
When the drilling fluid flows from the drill bit water hole to the bottom of the well, it not only cleans the bottom hole cuttings, but also reduces the energy and transmits it to the bottom hole auxiliary machinery to break the rock. Jet drilling is a drilling technology that uses the water power of the drill bit. Drilling practices at home and abroad have proved that the jet drilling technology increases the drilling speed and reduces the drilling cost.