What role does cellulose play in drilling fluid and what properties can it improve? ​

In drilling engineering, drilling fluid (commonly known as "mud") undertakes key tasks such as carrying rock debris, cooling drill bits, stabilizing wellbore walls, and balancing formation pressurecelluloseSubstances such as hydroxypropyl methylcellulose (HPMC) and carboxymethyl cellulose sodium (CMC) are commonly used as important drilling fluid treatment agents. With their unique molecular structure and physicochemical properties, they can optimize drilling fluid performance from multiple dimensions, ensuring drilling operation safety. The specific functions and performance improvement effects are as follows:

1、 Core role: "Regulator" and "Stabilizer" of drilling fluid performance

Cellulose molecular chains contain a large number of polar groups such as hydroxyl and ether bonds, which can form hydrogen bonds with water molecules and adsorb on the surface of solid particles through intermolecular forces. Their core functions can be summarized as thickening, reducing filtration loss, and stabilizing the system, thereby extending to improve multiple key properties of drilling fluids and solving common problems such as "wellbore collapse, rock debris settling, and excessive filtration loss" during the drilling process.

2、 Specific performance improvement effects and application scenarios

1. Improve viscosity and fluidity: achieve a balance between "rock carrying" and "pumping"

The drilling fluid needs to have a suitable viscosity - too low viscosity can cause rock debris to be unable to suspend, which can easily lead to sand settling and stuck drilling at the bottom of the well; If the viscosity is too high, it will increase the pumping resistance and consume more power. Cellulose can optimize its viscosity performance through the following methods:

Thickening effect: Cellulose molecules form a three-dimensional network structure in drilling fluid, restricting the movement of water molecules and significantly increasing the apparent viscosity and plastic viscosity of the drilling fluid. For example, when drilling in loose formations, adding 0.2% -0.5% CMC can increase the viscosity of drilling fluid by 20% -50%, ensuring that rock debris is stably carried to the surface and avoiding settling of rock debris in the wellbore;

Adjusting rheological properties: Cellulose has the characteristic of "shear thinning" - at high shear rates (such as when the drill bit rotates or the mud pump transports), the molecular chain network structure is temporarily disrupted, viscosity decreases, and pumping resistance is reduced; At low shear rates (such as when drilling fluid is stationary or flowing in the wellbore annulus), the mesh structure recovers, viscosity increases, and rock debris suspension capacity is enhanced. This feature is suitable for the dual requirements of "dynamic conveying" and "static suspension" in drilling operations, especially for complex well types such as large displacement wells and horizontal wells.

2. Reduce filtration loss: protect the wellbore and formation

Drilling fluid will infiltrate into the formation under pressure difference (i.e. "filtration"), and excessive filtration can lead to: ① rock cracking and collapse of the wellbore due to water loss; ② The formation pores are blocked by solid particles of drilling fluid, which damages the oil and gas reservoir; ③ Deterioration of drilling fluid performance (such as abnormal increase in viscosity). Cellulose can reduce filtration loss through a dual mechanism of "physical blockage+chemical adsorption":

Formation of dense filter cake: Cellulose molecules adsorb onto the surface of clay particles in drilling fluid, making the particles more evenly dispersed. When the drilling fluid forms a filter cake in the wellbore, cellulose molecules fill the pores of the filter cake, reducing its permeability. Experimental data shows that adding cellulose to drilling fluid can reduce the filtration loss from 20-30mL (API filtration loss, 30 min/690kPa) to 5-10mL, decrease the filter cake thickness from 2-3mm to below 1mm, and enhance the toughness of the filter cake, making it less prone to detachment;

Inhibiting clay hydration expansion:celluloseThe polar groups of molecules can bind with calcium and sodium ions on the surface of clay particles to form a stable adsorption layer, preventing water molecules from further entering the clay lattice and reducing clay hydration expansion. This is particularly important in drilling in water sensitive formations such as mudstone and shale formations, as it can effectively prevent wellbore collapse and reduce wellbore enlargement rate (usually controlled within 10%).

3. Stable drilling fluid system: resistant to harsh environmental interference

During the drilling process, drilling fluid will face challenges such as "temperature rise, salinity change, and invasion of formation pollutants", which can easily lead to problems such as stratification, flocculation, and sudden viscosity drop. Cellulose can enhance the anti-interference ability of drilling fluid:

Temperature stability: The molecular chains of modified cellulose (such as hydroxyethyl cellulose HEC) have good thermal stability and can maintain structural stability at high temperatures of 120-150 ℃, without significant decrease in drilling fluid viscosity due to thermal degradation. Compared to traditional starch processing agents (which are prone to gelatinization failure at temperatures above 80 ℃), cellulose is more suitable for deep and ultra deep well drilling (well depths exceeding 3000m, bottom hole temperatures often exceeding 100 ℃);

Salt pollution resistance: Salts in formation water (such as NaCl, CaCl ₂) can disrupt the colloidal stability of drilling fluid, leading to clay particle flocculation. The ether bonds in cellulose molecules have salt resistance and can remain dispersed in high salt environments (such as formations with a salinity exceeding 10 × 10 ⁴ mg/L), maintaining stable viscosity and filtration performance of drilling fluids. For example, in drilling in salt paste layers, adding cellulose can prevent the drilling fluid from becoming "salt sensitive" and ineffective due to salt contamination, ensuring continuous drilling;

Shear stability: Drilling fluid undergoes strong shear during high-speed rotation of the drill bit and repeated transportation by the mud pump. Traditional treatment agents are prone to failure due to molecular chain breakage, while cellulose molecular chains have a certain degree of toughness and stronger shear resistance. After long-term shear, the viscosity retention rate of drilling fluid can still reach over 80%, reducing the workload of frequent adjustment of drilling fluid performance.

4. Auxiliary lubrication and anti jamming: reduce underground accidents

During the drilling process, excessive frictional resistance between the drill string, wellbore, and casing can lead to "stuck drilling" (the drill string cannot move up and down or rotate), causing serious economic losses. Although cellulose is not the main lubricant, it can improve lubrication performance through indirect effects:

Optimizing the lubricity of the filter cake: After improving the quality of the filter cake with cellulose, the surface of the filter cake becomes smoother and the friction coefficient is lower (usually the friction coefficient of the filter cake can be reduced from 0.3-0.4 to 0.15-0.25), reducing the frictional resistance between the drill string and the wellbore wall;

Enhance lubricant compatibility: Cellulose has good compatibility with mineral oil and vegetable oil lubricants in drilling fluids, which can promote the uniform dispersion of lubricants in drilling fluids, avoid lubricant aggregation and failure, and further improve the overall lubrication effect. In directional drilling, this synergistic effect can reduce torque by 15% -20% and lower the risk of drilling jamming.

3、 Application differences of different types of cellulose

Cellulose with different degrees of modification has different properties and needs to be selected according to drilling requirements:

Carboxymethyl cellulose sodium (CMC): High cost-effectiveness, outstanding filtration reduction effect, suitable for conventional drilling fluids in shallow and medium deep wells, especially for water sensitive formations;

Hydroxypropyl methylcellulose (HPMC): It has better temperature and salt resistance, good viscosity stability, and is suitable for drilling in deep wells, ultra deep wells, salt paste layers, and oil and gas reservoirs;

Hydroxyethyl cellulose (HEC): With good water solubility and fast dissolution rate, it is suitable for emergency scenarios that require rapid adjustment of drilling fluid performance (such as well leakage and collapse treatment).

In conclusion,celluloseIn drilling fluid, it is a "multifunctional treatment agent" that provides "safe wellbore environment+rock carrying capacity+stability performance guarantee" for drilling operations by improving viscosity, reducing filtration loss, stabilizing the system, and assisting lubrication. It is an indispensable key material in complex formation drilling. The dosage is usually controlled at 0.1% -1.0% (mass fraction), which needs to be adjusted according to the formation characteristics, drilling fluid type (water-based, oil-based), and well type parameters.