PAC fluid loss control agent

Polyanionic cellulose (PAC) is a commonly used thickener, filtration reducer, and rheology modifier in drilling fluids. The process of preparing drilling fluids with PAC requires adjusting the formula according to drilling conditions (such as fresh water, brine, and high-temperature wells). The core is to meet the functional requirements of drilling fluids such as "suspending rock cuttings, stabilizing the wellbore, cooling the drill bit, and controlling filtration loss" through reasonable dissolution, compounding, and performance regulation.
 

1. Core Preparations Before Drilling Fluid Preparation

a. Raw Material List (Taking Conventional Freshwater Drilling Fluid as an Example)
The base solvent can be freshwater, brine, or seawater, serving as the continuous phase of the drilling fluid. The choice depends on the formation water salinity; freshwater is used for shallow, low-salinity formations, while brine is used for high-salinity formations. The core processing agent is PAC for oil drilling fluids (PAC), which primarily thickens and reduces filtration loss, increasing the drilling fluid viscosity and shear strength.

To reduce drilling fluid loss to the formation, auxiliary fluid loss reducers commonly include carboxymethyl cellulose (CMC) and starch, which can synergistically enhance the fluid loss reduction effect with PAC and reduce costs. Clay stabilizers such as potassium chloride (KC) and polyquaternium salts are used to prevent formation clay from swelling and dispersing upon contact with water, thus stabilizing the wellbore. Lubricants such as vegetable oils and polyether lubricants can reduce the frictional resistance between the drilling fluid and the drill pipe and wellbore, reducing torque.

Weighting agents, such as barite (BaSO4) and iron ore powder, are selected as needed for high-pressure formations to increase the drilling fluid density and balance formation pressure (e.g., deep wells, 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 has better stability in a weakly alkaline environment and can inhibit clay hydration and swelling.

b. Equipment Preparation A high-speed shear mixer is required for mixing, with a speed controlled between 800-1500 rpm to ensure complete dissolution of PAC and prevent clumping. Testing instruments include a viscometer (for measuring the apparent viscosity and plastic viscosity of the drilling fluid), a filtration loss meter (for measuring API filtration loss), and a densitometer. The mixing container should be a storage tank equipped with a stirring device, with a capacity typically ranging from 1 to 10 m³, depending on drilling requirements.

2. Step-by-Step Preparation Process of Polyanionic Cellulose (PAC) 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: Pretreatment of the base solvent (aqueous phase adjustment)
Add the calculated amount of freshwater (or brine) to the mixing tank, start stirring, and control the speed at 500-800 r/min; slowly add NaOH, usually at 0.1%-0.3% of the total mass of the drilling fluid, stir for 5-10 minutes, and adjust the pH to 8-10; if clay stabilization is required, add 1%-3% KCl, continue stirring for 10 minutes to ensure complete dissolution. K+ can replace Na+ on the clay surface, inhibiting swelling.

Step 2: Dissolving Polyanionic Cellulose (PAC)
Pass PAC powder through an 80-mesh sieve (to reduce the probability of clumping). Slowly and evenly sprinkle the PAC powder into the stirred aqueous phase according to the formula amount (usually 0.2%-0.8%, adjusted according to viscosity requirements). (Do not pour it all in at once to avoid PAC agglomeration 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 should 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 in alkaline water to form carboxylate salts (COO-). The molecular chains fully extend, forming a network structure, thus exerting a thickening and filtration loss reduction effect.

Step 3: Compounding of Auxiliary Treatment Agents
Keep stirring at 800 rpm. First, add the auxiliary filtration reducer (e.g., CMC, 0.1%-0.3%) and stir for 15 minutes. If improved lubricity is required, add 1%-2% lubricant and stir for 10 minutes. For high-pressure wells, slowly add the weighting agent (e.g., barite, calculated according to density requirements, typically requiring a density of 1.2-2.0 g/cm²), and continue stirring for 30 minutes to ensure uniform dispersion of the weighting agent (avoiding sedimentation).

Step 4: Performance Adjustment and Testing Stop stirring and let stand for 5-10 minutes. Measure key drilling fluid properties: For viscosity, apparent viscosity (AV) should be controlled at 20-40 mPa·s, and plastic viscosity (PV) at 15-30 mPa·s (adjust according to well depth; deeper wells require higher viscosity to suspend cuttings); for filtration loss, API filtration loss (at normal temperature and pressure) S ≤ 10 mL/30 min (for high-pressure wells, control should be within 5 mL/30 min); for density, designed according to formation pressure, typically 1.0-1.1 g/cm² for shallow wells and 1.5-2.0 g/cm² for deep wells.

During performance adjustment, if the viscosity is insufficient, add 0.1%-0.2% PAC and continue stirring for 20 minutes; if the filtration loss exceeds the standard, add 0.1%-0.3% CMC or PAC, or adjust the pH to 9-10; if the density is insufficient, add barite while stirring until the density reaches the standard.

Step 5: On-site Circulation and Adjustment The prepared drilling fluid needs to be circulated to the bottom of the well using a drilling pump. During this process, the performance should be monitored in real time (e.g., if the viscosity decreases after circulation, add Polyanionic cellulose) to ensure it meets the requirements of drilling operations.