Circulating fluid is indispensable in modern drilling. Except for a small amount of circulating fluid using gas and foam, most circulating fluids for drilling are liquids, so the circulating fluid used in drilling is called drilling fluid. Because the early drilling fluid was only a mixed liquid of clay and water and had almost no other chemical components, it was customarily called "mud" on the field. Drilling fluids play an important role in the drilling process. Its main functions are:
- Cleaning the bottom of the well
During the drilling process, the drill bit breaks the rock and generates a large amount of cuttings. These cuttings are removed by the 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.
- Cooling and lubricating drill bits and drill strings
During the drilling process, due to the severe weight-on-bit load on the drill bit, the drill bit bearing and the working face of the drill bit became hot. At the same time, because the drill string constantly rubs against the borehole wall during the rotation process, not only heat is generated, but also the drill string is worn away and power consumption is increased, while the drilling fluid can take away the heat in time through the circulation to cool the drilling tool. It also acts as a lubricant, thereby reducing the side effects of friction.
- Form mud cakes to protect the well wall
Rocks deep in the formation are in a three-dimensional compression state due to pressure from the top and surrounding rocks. When the wellbore is drilled, the rocks around the wellbore must undergo compression from various directions to produce a stress increment 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 containing a weighting agent and low filtration loss is applied to the surrounding borehole wall, the stress on the rock will be reduced, thereby reducing the chance of "compressive collapse". However, the density of the drilling fluid must not be too large, otherwise it will cause "fracture fracture" of the rock, and then cause the leakage of the drilling fluid. In addition, the drilling fluid added with various additives will form a layer of mud cake on the well wall during the circulation process. The high-quality mud cake can not only protect the well wall from being washed away by the drilling fluid, but also reduce the leakage of drilling fluid to the formation.
- Controlling and balancing formation pressure
When drilling into a high-pressure oil (gas) layer, if the formation's oil (gas) layer pressure is higher than the pressure of the drilling fluid column, a “blowout” will occur and a drilling accident will occur. To this end, a drilling fluid of 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 must not be too large, otherwise the oil (gas) layer may be blocked.
- Suspended rock cuttings and weighting agents
During the drilling process, the velocity of the drilling fluid in the annulus of the wellbore cannot be too high, and the clear water has a poor suspension capacity due to its low density. To circulate the cuttings and weighting agents to the ground, various other additives must be added to the drilling fluid To increase its suspension capacity.
- Provide relevant geological data for the formation being drilled
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.
- Pass water power to the drill
When drilling fluid flows from the water hole of the bit to the bottom of the well, it not only cleans the bottom cuttings, but also reduces the energy to the bottom of the auxiliary mechanical rock breaking. Jet drilling is a drilling process that uses the water power of the bit. Drilling practices at home and abroad have proven that jet drilling processes improve drilling speed and reduce drilling costs.
- Separation of drill cuttings on the ground
The solid phase particles dispersed in the drilling fluid are called solid phases in the drilling fluid. The solid phase in the drilling fluid is derived from drilling cuttings and artificially added to meet the drilling process requirements.
According to the role of solid phase in drilling fluid, it can be divided into two categories: one is useful solid phase, such as bentonite, chemical treatment agent, barite, etc .; the other is harmful solid phase, such as drill cuttings, inferior bentonite , Grit, etc. The solid control system of drilling fluid is to remove harmful solid phases, retain useful solid phases, or control the total solid phase content and particle size level in the drilling fluid within the range required by the drilling process to meet the drilling fluid performance. Requirements. It is also customarily referred to as mud purification on the field.Read More »