1. Performance of drilling fluid.
The plastic viscosity of the drilling fluid decreases with increasing temperature [2], and the density decreases with increasing temperature [3]. The higher the temperature, the worse the rock-carrying performance and the smaller the transmission power. In order to maintain the high-temperature performance of the drilling fluid, various additives need to be added, which increases the cost of the drilling fluid. In addition, high temperature will also make drilling fluid control more difficult, the reason is: some treatment agents are easy to decompose at high temperature; High temperatures can exacerbate chemical reactions that affect the efficacy of the treatment agent.
2. Mechanical penetration rate.
In the process of drilling, the circulating drilling fluid exchanges heat with the rock at the bottom of the well, which changes the temperature of the rock at the bottom of the well. Especially during deep well drilling, this heat transfer state will be more intense due to the higher temperature of the rock. When the lower temperature of the drilling fluid comes into contact with the rock at the bottom of the well, the rock surface in contact with the drilling fluid will be cooled immediately, the temperature will drop, and cold shrinkage will occur, while the internal temperature of the rock will still be the original temperature, resulting in the thermal stress characteristic of tensile stress on the rock surface. Because the rock has low tensile strength and high compressive strength, the rock becomes easy to break when tensile stress occurs, which is conducive to increasing the ROP. The greater the temperature difference, the greater the effect on the strength of the rock.
3. Downhole drilling tools and downhole instruments.
Downhole power drilling tools mostly use screw drilling tools, and the key part of the stator is rubber material, and the overheated temperature will seriously shorten the service life of the rubber stator. In addition, the higher downhole ambient temperature will seriously affect the measurement accuracy and service life of the electronic components in the geosteering and measurement-while-drilling tools, and even some wells can not be used because the downhole environment temperature is too high, resulting in the current downhole drilling tools and instruments can not meet the temperature requirements. If the temperature of the circulating drilling fluid is reduced, it is equivalent to reducing the ambient temperature of downhole drilling tools and downhole instruments, which is equivalent to expanding the temperature range of downhole drilling tools and downhole instruments, improving their reliability and extending their service life.
4. Downhole stability.
The circulating temperature of drilling fluid is one of the main factors causing the instability of the borehole. In deep well drilling operations, the temperature of the lower formation is higher and the temperature of the upper formation is lower. During the circulation of drilling fluid, the temperature of the surrounding rock of the lower borehole decreases, and the temperature of the surrounding rock of the upper borehole increases. Due to the thermal expansion, the tangential stress of the upper surrounding rock increases, and when the sum of the thermal stress and the in-situ stress exceeds the rock strength, it will lead to the instability and failure of the upper borehole.
Therefore, in the drilling process of timely cooling of the drilling fluid, so that it reaches the appropriate temperature is very necessary, Xi'an Kexun mud cooling system design process to meet the higher temperature of the mud cooling, mud cooling temperature difference in 20-40, effectively prolong the service life of the drill bit and drill pipe, improve the efficiency of the drilling rig, while greatly reducing the operation risk caused by the mud temperature is too high and other factors.