Low-viscosity polyanionic cellulose (PAC LV) is a special type of polyanionic cellulose with a low molecular weight and low aqueous viscosity. Due to its unique rheological properties and interfacial activity, it is primarily used in oilfield drilling, completion, and production processes where enhanced fluid loss control or wellbore protection is required despite minimal impact on drilling fluid viscosity.
I. How to Use Low-Viscosity Polyanionic Cellulose in Oilfield Drilling Operations
Low-viscosity PAC-PAC LV utilize their low viscosity and strong adsorption properties. They are typically used in oilfield operations where drilling fluid viscosity is sensitive. The specific methods are as follows:
1. Preparation and Addition Method
Direct Dissolution:
Low-viscosity PACs have superior solubility compared to high-viscosity versions and can be directly dispersed in the drilling fluid base (water, brine, or water-in-oil emulsion). The stirring speed should be controlled between 600-1000 rpm (high speed is not necessary to avoid excessive shear). The dissolution time is approximately 5-15 minutes. The addition rate is generally 0.05%-0.5% of the total drilling fluid weight (much lower than high-viscosity PACs). Staged Addition:
In the early stages of drilling (wellbore stabilization), a small amount of PAC LV (0.05%-0.1%) can be added to form a preliminary filter cake on the wellbore wall and reduce filtrate seepage.
During the drilling process: When encountering high-permeability formations or shale formations, the PAC LV addition rate can be increased to 0.2%-0.5% to enhance fluid loss reduction and wellbore protection.
During the completion phase: Continue adding 0.05%-0.1% PAC LV with clean water or low-viscosity drilling fluid to utilize its adsorption properties to assist in wellbore cleaning while preventing clogging of the oil and gas reservoir.
2. Synergistic Use with Other Treatment Agents
Due to its low viscosity, PAC LV is particularly suitable for use with additives that control overall drilling fluid viscosity:
Combined with viscosity reducers (such as ferrochrome lignin sulfonate): In high-viscosity drilling fluid systems, PAC LV can enhance fluid loss control without increasing viscosity, preventing viscosity reducers from excessively degrading drilling fluid performance.
Combined with shale inhibitors (such as polyamines and potassium chloride): In shale formations, the adsorption properties of PAC LV can help the inhibitors form a composite protective film on the wellbore surface, enhancing the anti-collapse effect.
Combined with bridging agents (such as calcium carbonate and silica fume): When drilling in high-permeability reservoirs, low-viscosity PAC acts as a "binder," helping bridging particles quickly form a dense filter cake and reducing drilling fluid losses.
3. Typical Application Scenarios
During horizontal and directional well drilling, drilling fluid viscosity must be controlled to reduce annular friction. PAC LV stabilizes the wellbore without increasing viscosity.
In oilfield operations in high-permeability sandstone formations, PACLV reduces fluid loss into the reservoir, protecting the oil and gas formations (avoiding pore blockage).
In oilfield operations in salt-gypsum formations or highly salinized formations, PACLV's salt tolerance helps maintain stable fluid loss performance without increasing viscosity.
In completion and workover fluid applications, PAC LV can be used as an additive in low-viscosity systems, achieving both wellbore cleaning and reservoir protection.
II. Precautions for Using Low-Viscosity Polyanionic Cellulose
The use of low-viscosity polyanionic cellulose (PAC) requires consideration of its properties to avoid potential problems and maximize its effectiveness:
1. Strictly control the dosage to avoid "counter-effects"
Although low-viscosity PAC has low viscosity, excessive addition (over 0.5%) can still cause a slow increase in drilling fluid viscosity, especially in freshwater drilling fluids. Field testing is required to determine the optimal dosage.
Excessive addition can also lead to excessively thick filter cakes, increasing friction between the drill pipe and the wellbore wall, and even causing pipe sticking.
2. Pay attention to uniform dissolution to prevent localized agglomeration.
Low-viscosity PAC has good solubility, but it should still be added slowly with stirring to prevent powder agglomeration and the formation of "fish eyes" (undissolved particles). Otherwise, localized fluid loss control failure and even reservoir blockage can occur.
It is recommended to use a mixing device with a shear pump to ensure complete dispersion in the drilling fluid.
3. Avoid direct contact with strong cationic substances.
Low-viscosity PAC molecules are negatively charged. Direct mixing with strong cationic treatment agents (such as certain high-valent metal ion crosslinkers and cationic polymers) may cause charge neutralization, leading to precipitation and loss of activity.
If combined, completely dissolve the low-viscosity PAC first before slowly adding the cationic additive with continuous stirring.
4. Adjust the system pH according to formation characteristics.
Low-viscosity PAC is most stable in a neutral to slightly alkaline environment (pH 7-9). If the drilling fluid is strongly acidic (pH < 5), its carboxyl groups may protonate (-COO⁻ → -COOH), reducing its solubility and weakening its fluid loss reduction effectiveness.
When drilling into acidic formations, add soda ash (Na₂CO₃) or caustic soda (NaOH) in advance to adjust the pH.
5. Use with Solids Control Equipment
Low-viscosity PAC forms a thin filter cake, requiring prompt removal of cuttings and solids from the drilling fluid using equipment such as vibrating screens and desanders to prevent excessive solids content from affecting its adsorption performance.
Before completion, drilling fluid containing low-viscosity PAC must be fully displaced with displacement fluid to prevent residual chemicals from clogging the reservoir.
Summary
Low-viscosity polyanionic cellulose, with its core advantages of "low viscosity and strong fluid loss control," is primarily used in oilfield applications requiring controlled drilling fluid viscosity (such as horizontal wells and reservoir drilling). It effectively reduces fluid loss, stabilizes the wellbore, and protects the reservoir. During use, strict control of dosage, ensuring uniform dissolution, and adjusting system parameters according to the formation environment are crucial to maximize its performance. Compared to high-viscosity PAC, low-viscosity PAC is more suitable for complex well conditions with demanding drilling fluid rheological requirements, making it a crucial additive for refined drilling operations.
