Drilling fluid and solids control function and importance
Circulating fluids are indispensable in modern drilling. Except for a small amount of circulating fluids that use gas and foam, most 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 is just a mixed liquid of clay and water, with almost no other chemical components, it is customary to call it “mud” at the oilfield site.
Drilling fluid plays an important role in the drilling process. Its main functions are:
1) Clean the bottom of the well
The drill bit breaks the rock as it drills and creates cuttings, which are removed by the constantly circulating drilling fluid. If the cuttings cannot be removed in time, it will be broken repeatedly, which will prevent the drill bit from continuing to drill into the rock.
2) Cool and lubricate the drill bit and drill string
During the drilling process, the bit bearing and the working face of the bit are heated due to the severe WOB load on the bit. At the same time, due to the continuous friction between the drill string and the well wall during the rotation process, not only heat is generated, but also the wear of the drill string is accelerated and the power consumption is increased. It also has a lubricating effect, thereby reducing the side effects of friction.
3) Form a mud cake to protect the well wall
Rocks deep in the formation are in three-dimensional compression due to pressure from the top and surrounding rocks. When the wellbore is drilled, the rock around the wellbore must be squeezed from all directions to generate a stress increment value, that is, stress concentration. In this case, if the rock at the wellbore is not strong enough, it can cause the wellbore to collapse. However, if a drilling fluid containing a weighting agent and low filtration loss acts 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 large, otherwise it will lead to the “fracture fracture” of the rock, which will lead to the loss of drilling fluid. In addition, the drilling fluid with various additives will form a layer of mud cake on the wellbore during the circulation process, which can not only protect the wellbore 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, resulting in a drilling accident. For this purpose, a drilling fluid with 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 large, otherwise it may block the oil (gas) layer.
5) Suspended cuttings and weighting agent
During the drilling process, the flow rate of the drilling fluid in the annular space of the wellbore cannot be too high, and the low density of clean water makes its suspension ability poor. In order to circulate the cuttings and weighting agents out of the ground, other additives must be added to the drilling fluid. to increase its suspension capacity.
6) Provide relevant geological data of the drilled stratum
By sampling and analyzing the drilling fluid flowing out of the surface, the lithology and oil and gas content of the drilled formation can be obtained.
7) Pass the water power to the drill bit
When the drilling fluid flows from the water hole of the drill bit to the bottom of the well, it not only cleans the cuttings at the bottom of the well, but also reduces energy and transmits it to the bottom of the well to assist in mechanical rock breaking. Jet drilling is a drilling process that utilizes the water power of the drill bit. Drilling practice at home and abroad has proved that the jet drilling process improves the drilling speed and reduces the drilling cost.