Polyanionic cellulose (PAC) is a commonly used thickener, fluid loss control agent, and rheology modifier in drilling fluids. The formulation of drilling fluids must be adjusted based on drilling conditions (e.g., freshwater, saltwater, high-temperature wells, etc.). The key is to achieve the desired functional requirements of drilling fluids, such as suspending cuttings, stabilizing the wellbore, cooling the drill bit, and controlling fluid loss, through appropriate dissolution, compounding, and performance control.
1. Core Preparation Before Drilling Fluid Preparation
a. Raw Materials List (Using conventional freshwater drilling fluid as an example)
The base solvent, which serves as the continuous phase of the drilling fluid, can be freshwater, brine, or seawater. The choice depends on the salinity of the formation water: freshwater is used in shallow, low-salinity formations, while brine is used in high-salinity formations. The core treatment agent is polyanionic cellulose (PAC), which primarily thickens and reduces fluid loss, increasing drilling fluid viscosity and shear strength, thereby reducing fluid loss into the formation. Auxiliary fluid loss agents commonly used include carboxymethyl cellulose (CMC) and starch, which synergize with PAC to enhance fluid loss reduction and reduce costs. Potassium chloride can be used as a clay stabilizer. (KCl) and polyquaternium salts are used to prevent formation clay from swelling and dispersing upon contact with water, thereby stabilizing the wellbore. Lubricants such as vegetable oils and polyether lubricants can reduce friction between the drilling fluid and the drill pipe and wellbore, thereby reducing torque. Weighting agents such as barite (BaSO₄) and iron ore powder can be used as needed in high-pressure formations to increase drilling fluid density and balance formation pressure (such as in deep wells and oil and gas wells). Sodium hydroxide (NaOH) is commonly used as a pH adjuster to adjust the drilling fluid pH to 8-10, as PAC is more stable in a weakly alkaline environment and can inhibit clay hydration and expansion.
b. Equipment Preparation
A high-speed shear mixer should be used for mixing, with a speed controlled between 800-1500 rpm to ensure thorough dissolution of the PAC and prevent agglomeration. Testing instruments include a viscometer (for measuring apparent viscosity and plastic viscosity of the drilling fluid), a fluid loss meter (for measuring API fluid loss), and a densitometer. Liquid mixing vessels should be tanks equipped with agitators, with a capacity typically ranging from 1 to 10 m³, depending on drilling requirements.
2. Step-by-Step Preparation Process for Polyanionic Cellulose Drilling Fluid
Taking freshwater-based PAC drilling fluid (suitable for shallow, low-salinity formations) as an example, the preparation steps are as follows:
Step 1: Pre-treating the Base Solvent (Aqueous Phase Adjustment)
Add the calculated amount of fresh water (or brine) to the mixing tank and start stirring at a speed of 500-800 rpm. Slowly add sodium hydroxide (usually 0.1%-0.3% of the total drilling fluid mass), stir for 5-10 minutes, and adjust the pH to 8-10. If clay stabilization is required, add 1%-3% potassium chloride and continue stirring for 10 minutes to ensure complete dissolution. The potassium chloride replaces the sodium chloride on the clay surface, inhibiting swelling.
Step 2: Dissolve the polyanionic cellulose (PAC)
Pass the PAC powder through an 80-mesh sieve (to reduce clumping). Slowly and evenly sprinkle the formulated amount (typically 0.2%-0.8%, adjusted based on viscosity requirements) into the stirring aqueous phase. (Do not pour all at once to prevent PAC from agglomerating and forming "fish eyes" that prevent dissolution.) Increase the stirring speed to 1000-1500 rpm and continue stirring for 20-30 minutes until the PAC is completely dissolved (the solution will be transparent or slightly transparent, with no visible particles). The key principle is that PAC molecules contain a large number of carboxyl groups (-COOH), which ionize into carboxylates (-COO⁻) in alkaline water. The molecular chains fully extend, forming a network structure, which exerts its thickening and fluid loss reduction effects.
Step 3: Prepare auxiliary treatment agents.
Maintain stirring at 800 rpm. First, add an auxiliary fluid loss agent (such as CMC, 0.1%-0.3%) and stir for 15 minutes. To improve lubricity, add 1%-2% lubricant and stir for 10 minutes. For high-pressure wells, slowly add a weighting agent (such as barite, calculated based on required density, typically 1.2-2.0 g/cm³). Continue stirring for 30 minutes to ensure even dispersion of the weighting agent (to prevent settling).
Step 4: Performance Control and Testing
Stop stirring, let the fluid sit for 5-10 minutes, and measure key drilling fluid properties: For viscosity, control the apparent viscosity (AV) to 20-40 mPa·s and the plastic viscosity (PV) to 15-30 mPa·s (adjust according to well depth; deeper wells require higher viscosity to suspend cuttings). For fluid loss, maintain API fluid loss (normal temperature and pressure) ≤ 10 mL/30 min (high-pressure wells should be controlled within 5 mL/30 min). Density is designed based on formation pressure, typically 1.0-1.1 g/cm³ for shallow formations and 1.5-2.0 g/cm³ for deep wells. During performance adjustments, if viscosity is insufficient, add 0.1%-0.2% PAC and continue stirring for 20 minutes. If fluid loss exceeds the specified value, add 0.1%-0.3% CMC or PAC, or adjust the pH to 9-10. If density is insufficient, add barite, stirring continuously until the density meets the specified value.
Step 5: On-site Circulation and Adjustment
The prepared drilling fluid must be circulated to the bottomhole via a drilling pump. During this process, performance must be monitored in real time (if viscosity decreases after circulation, PAC must be added) to ensure it meets drilling requirements.
3. PAC Drilling Fluid Formula Adjustment for Different Operating Conditions
PAC drilling fluid must be optimized based on formation characteristics (salinity, temperature, and clay content). For shallow freshwater wells, the PAC dosage is 0.2%-0.5%. Using the basic formula, no additional salt tolerance agent is required; a small amount of starch can be added to reduce fluid loss. For saltwater/seawater wells, the PAC dosage is 0.5%-0.8%. Increased PAC dosage is required (salinity tolerance: PAC > CMC), and 2%-5% NaCl can be added to simulate the effect. Formation water should be treated to prevent swelling of salt-sensitive clays. In high-temperature deep wells (>120°C), the PAC dosage should be 0.6%-1.0%. High-temperature-resistant PAC (such as PAC-LV, which has low viscosity and high temperature resistance) should be selected, and a sulfonated fluid loss additive (such as SMP) should be added to improve temperature resistance. In shale wells prone to collapse, the PAC dosage should be 0.4%-0.7%, and the KCl dosage should be increased to 3%-5%. A 0.2%-0.5% polyquaternium salt should be added to enhance clay stability.
4. Key Considerations During Preparation
PAC dissolution has its contraindications. It is strictly forbidden to pour PAC directly into stagnant water (causing lumps to form). It must be added slowly with stirring. Avoid mixing with strong acids (pH<6). Acidic environments hinder the ionization of the PAC carboxyl groups, resulting in a loss of thickening and fluid loss reduction properties. Regarding equipment, the mixing equipment must have sufficient shear force (speed ≥ 1000 r/min) to ensure full extension of the PAC molecular chains. The mixing tank must be clean to avoid residual oil and salts (e.g., Ca²⁺ reacts with PAC carboxyl groups, degrading performance). Regarding safe operation, PAC powder can easily fly, so operators must wear dust masks. Chemicals such as NaOH and KCl should be avoided from direct skin contact, and gloves must be worn during handling. Regarding performance stability, prepared drilling fluid must be used within 24 hours. Long-term storage requires regular stirring (to prevent settling of the weighting agent). During field use, performance should be retested and treatment agents added every 100-200 meters of drilling progress.
5. The Core Function and Principle of PAC in Drilling Fluids
PAC's fluid loss reduction effect is reflected in its molecular chains forming a "thin, dense filter cake" on the wellbore wall, preventing water loss from the drilling fluid into the formation and protecting reservoirs (especially oil and gas formations) from contamination. Its thickening and rheological properties are due to its network structure, which increases drilling fluid viscosity and shear force, ensuring cuttings suspension (preventing sand settling and sticking the drill bit) while also ensuring the fluid's rock-carrying capacity in the annulus. Its salt and pollution resistance stems from the carboxyl groups in PAC molecules' strong tolerance to metal ions such as Na⁺ and Ca⁺, making it more suitable for saline environments than CMC, thereby reducing the deterioration of drilling fluid performance due to salt contamination.
Through the above process and adjustment strategy, polyanionic cellulose drilling fluids with stable performance and adaptability to various drilling conditions can be prepared to meet the core requirements of drilling operations.
