1. Drilling fluid rheological properties adjustment
Thickening effect :
In offshore drilling fluid systems, polyanionic cellulose (PAC) can effectively increase the viscosity of drilling fluid. The offshore drilling environment is complex. For example, in deep sea areas or when there is strong current interference, sufficient drilling fluid viscosity is required to ensure the smooth progress of drilling operations. The anionic groups on the PAC molecular chain can form strong hydrogen bonds with water molecules, allowing the molecular chain to fully stretch in water, thereby increasing the friction inside the fluid and increasing the viscosity of the drilling fluid. For example, in the application of some deep-sea drilling platforms, adding an appropriate amount of PAC can make the viscosity of the drilling fluid meet the requirements of carrying cuttings and prevent the cuttings from falling back to the bottom of the well during the ascent.
Improving shear thinning properties :
PAC can also improve the shear thinning properties of drilling fluid. During the drilling process, the shear force on the drilling fluid is different in different areas. At the drill bit, the shear force is high, and good shear dilution performance can reduce the viscosity of the drilling fluid, which is conducive to quickly carrying cuttings from the bottom of the well; in low shear areas such as the annulus, the viscosity increases, which helps to suspend the cuttings. PAC achieves this characteristic through the orientation change of the molecular chain under different shear forces. Under high shear force, the molecular chain will be oriented along the shear direction, the fluid resistance will be reduced, and the viscosity will be reduced; under low shear force, the molecular chain will return to the entangled state, the viscosity will increase, and the performance of the drilling fluid will be effectively improved.
2. Reducing filtration loss
Forming high-quality filter cake :
The offshore formation conditions are complex, and PAC can form high-quality filter cakes on the well wall. When the drilling fluid contacts the well wall, the PAC molecules will be adsorbed on the surface of the well wall rock and form a dense filter cake through the interaction between molecules. This layer of filter cake can effectively prevent the liquid in the drilling fluid from penetrating into the formation and reduce the filtration loss. Especially in some offshore formations with high permeability, the filter cake formed by PAC can prevent a large amount of drilling fluid filtrate from entering the formation, avoid problems such as formation clay expansion and pore blockage, and protect the original permeability of the formation.
Increase filtrate viscosity :
PAC can also increase the viscosity of drilling fluid filtrate. In offshore drilling, seawater, as one of the components of drilling fluid or as an external environmental factor, will affect the filtration performance of drilling fluid. PAC can form a network structure in the filtrate, hinder the flow of liquid molecules, and increase the viscosity of the filtrate. In this way, when the drilling fluid contacts the formation, the high-viscosity filtrate can better resist penetration into the formation, further reduce the filtration loss, and ensure the stability of the drilling process.
3. Stabilization of wellbore wall
Inhibit clay hydration expansion :
Offshore formations may contain a large amount of clay components, and PAC can effectively inhibit the hydration expansion of clay. Its molecules can be adsorbed on the surface of clay particles, and the surface properties of clay particles can be changed by ion exchange or physical adsorption, reducing the contact opportunities between water molecules and clay particles. For example, in water-sensitive formations, the anionic groups of PAC can exchange and adsorb with the cations on the surface of clay particles, forming a protective film around the clay particles to prevent the entry of a large number of water molecules, thereby reducing the degree of clay expansion due to water absorption, preventing the well wall from collapsing, and ensuring drilling safety.
Enhancing the cementation of well wall rocks :
PAC can react with certain components in the well wall rocks to enhance the cementation of the rocks. There are various types of rocks encountered in offshore drilling, such as sandstone, shale, etc. PAC can combine with mineral ions in these rocks to form a reinforcement layer on the rock surface. Taking sandstone formations as an example, PAC may interact with components such as silicate in sandstone to make the rock particles more tightly bonded, improve the overall strength of the well wall, enhance the stability of the well wall, and reduce drilling accidents caused by well wall instability.
