In the process of oil drilling, drilling fluid (i.e. mud) is the blood during drilling. A large number of research data at home and abroad show that mud plays an important role in carrying and suspending cuttings, stabilizing wellbore, balancing formation pressure, cooling, lubricating bits, drilling tools and transferring hydrodynamic force during drilling, which directly affects the drilling cost, and even affects the success or failure of drilling. The performance parameters of mud mainly include density, viscosity, water loss, sand content and PH value.
01. Density of drilling fluid
The density of drilling fluid refers to the mass of drilling fluid per unit volume. The function of drilling fluid density is to generate pressure on the bottom hole and wellbore through the drilling fluid column to balance the oil, air pressure and rock lateral pressure in the formation, prevent blowout, protect the wellbore, and prevent high-pressure oil, gas and water from invading the drilling fluid, so as to avoid damaging the performance of the drilling fluid and causing downhole complications. In actual work, appropriate drilling fluid density should be selected according to the specific situation. If the drilling fluid density is too small, the formation fluid pressure cannot be balanced and the borehole wall cannot be stabilized, which may cause blowout, borehole collapse, pipe sticking and other accidents. If the density of drilling fluid is too high, the formation will be lost and the oil and gas reservoir will be easily damaged. The drilling fluid has a great impact on the drilling speed. The drilling fluid with large density has a large pressure on the fluid column, which slows down the drilling speed. The positive pressure difference between the drilling fluid column pressure and the formation pressure hinders the removal of rock cuttings, causing repeated crushing, reducing the efficiency of the bit to break rocks, and reducing the drilling speed. Generally, under the premise of ensuring normal downhole conditions, low-density drilling fluid should be used as much as possible to improve the drilling speed.
02. Viscosity of drilling fluid
The viscosity of drilling fluid refers to the total reflection of internal friction between solid particles, between solid particles and liquid molecules, and between liquid molecules when drilling fluid flows. The viscosity of drilling fluid has a great impact on the cuttings carrying capacity of drilling fluid. Generally speaking, the viscosity of drilling fluid is large and the cuttings carrying capacity is strong. However, during drilling, the viscosity of drilling fluid must be appropriate, or it will cause adverse consequences. If the viscosity of the drilling fluid is too low, it is not conducive to carrying rock cuttings. The sand setting in the well is fast and the well wall is washed away, which is easy to cause the wall peeling, collapse and lost circulation. If the viscosity of drilling fluid is too high, the following hazards may be caused:
● Large flow resistance, high pump pressure, poor bottom hole cleaning effect, seriously affecting the penetration rate;
● The bit is prone to mud inclusion, pumping or pressure excitation when tripping, which may cause lost circulation, blowout, collapse and other complex situations;
● Difficult sand setting, poor purification, and wear of drilling tools and accessories;
● It is difficult to degass and the density of drilling fluid decreases, which is easy to cause complex running in;
● Rock cuttings form false mud cake on the well wall, which is easy to cause jamming;
● Cement slurry is easy to cross grooves during cementing, affecting cementing quality.
03. Water loss of drilling fluid
Under the action of the pressure difference between the drilling fluid column pressure and the formation pressure, the water in the drilling fluid seeps into the formation from the pore fractures in the well wall. This phenomenon is called water loss (or filtration). The amount of water loss is called the water loss of the drilling fluid. At the same time of water loss, the clay particles in the drilling fluid are blocked and deposited on the well wall to form a layer of solid particles called filter cake. In the process of drilling, filter cake can be formed only when there is water loss. The formed filter cake can consolidate the well wall and prevent further water loss. Generally speaking, drilling fluid is required to have low water loss and thin, tough and dense mud cake. Excessive water loss and thick and loose filter cake are unfavorable to drilling. First, the formation is soaked, which is easy to cause hole shrinkage or shale peeling and collapse; Second, water seeps into the production layer, causing bentonite in the oil layer to expand, reducing the permeability of the oil layer and gas layer, and reducing the production capacity. The smaller the water loss is, the better. If the water loss is too small, the cost of drilling fluid will increase and the penetration rate will decrease. If the mud cake is required, the water loss is not necessarily controlled very small. It should be determined according to the characteristics of formation rock, well depth, wellbore structure, drilling fluid type, etc.
04. Sand content of drilling fluid
The sand content of drilling fluid refers to all solid substances in the drilling fluid, including clay drilling cuttings, chemical treatment agents, barite powder, etc. The percentage of these solid substances in the volume of drilling fluid is called the solid content of drilling fluid. Practice has proved that when drilling with clean water, the penetration rate is the highest, and once the water enters the bentonite particles, the penetration rate decreases. The increase of the solid content in the drilling fluid is an important reason for the decline of the penetration rate. The high solid content of the drilling fluid will also seriously affect the performance of the drilling fluid, and bring many hazards to drilling. The bit footage is reduced, and the drilling equipment is severely worn.
05. PH value of drilling fluid
PH value of drilling fluid is the acid base value of drilling fluid. PH value has a great influence on the performance of drilling fluid. The clay particles in drilling fluid are relatively stable in alkaline medium due to more negative charges and large cation exchange capacity. Secondly, there are many organic treatment agents, such as tannin and humic acid, which can only function under the action of alkalinity. In addition, the PH value is low, and the organic treatment agent is easy to ferment and deteriorate under high temperature, so the PH value of drilling fluid is gener
