Completion fluids are critical working fluids used in well completion operations, directly impacting reservoir protection and productivity optimization. Depending on reservoir characteristics and engineering requirements, various chemical products are used as additives in completion fluids. Below are some commonly used chemicals and their functions:
1. Base Fluids
- Seawater/Freshwater: Commonly used in water-based completion fluids, such as the ultra-high-temperature water-based system in the Bozhong buried hill reservoirs.
- Oil-based fluids (mineral oil, diesel, synthetic hydrocarbons): Suitable for water-sensitive or high-temperature/high-pressure (HTHP) reservoirs.
- Brine (NaCl, KCl, CaCl₂, NaBr, CaBr₂, etc.): Used to adjust density and inhibit clay swelling.
2. Weighting Agents
- Sodium chloride (NaCl): Cost-effective and highly soluble, often used in low-density completion fluids.
- Calcium bromide (CaBr₂), Zinc bromide (ZnBr₂): Used for high-density requirements (e.g., high-pressure reservoirs).
- Calcium carbonate (CaCO₃): Acid-soluble temporary bridging agent for reservoir protection.
3. Viscosifiers & Fluid Loss Control Agents
- Hydroxypropyl starch (HPS): Acid-soluble fluid loss reducer, suitable for acid-soluble completion fluids.
- Xanthan gum, Hydroxyethyl cellulose (HEC): Provide rheological stability, used in polymer brine completion fluids.
- Cross-linked polymer gels: Used in high-viscosity displacement fluids, such as screen completion fluids in horizontal wells.
4. Corrosion Inhibitors
- COROHIB-1: High-temperature corrosion inhibitor, used in Bozhong’s 210°C ultra-high-temperature completion fluid.
- XN-1: Acid-soluble corrosion inhibitor for post-acidizing completion fluids.
- Amine-based inhibitors: Used in high-salinity completion fluids to reduce tubular corrosion.
5. Surfactants & Anti-Water Blocking Agents
- STIMFC-1: Cleanup surfactant that reduces interfacial tension and mitigates water-blocking effects.
- Oil-soluble temporary shielding agent (TR-5): Used in horizontal well screen completion fluids to enhance productivity.
- Alcohols (methanol, ethanol): Suitable for tight gas reservoirs to lower surface tension.
6. Biocides & Eco-Friendly Additives
- Hypochlorous acid (HClO): Used for antibacterial and anticorrosive purposes in oil and gas completion fluids (e.g., sulfate-reducing bacteria (SRB) inhibition).
- Nitrate-based completion fluids: Environmentally friendly and degradable (e.g., patented products from Shengli Oilfield).
- Biodegradable polymers: Used in seawater-based solids-free eco-friendly drilling/completion fluids.
7. Acidizing Agents
- Mud acid (HF + HCl): Used to remove mud cake and improve permeability.
- Retarded acid: Suitable for carbonate reservoirs, extending acid reaction time.
- Polymer breakers: Degrade residual polymers in completion fluids to reduce formation damage.
8. pH Adjusters & Defoamers
- NaOH, NaHCO₃: Adjust pH to prevent corrosion.
- Silicone defoamers: Control foam stability in foam completion fluids.
Summary
Completion fluids involve a wide range of chemicals, selected based on reservoir temperature, pressure, lithology, and environmental requirements. Examples include:
- High-temperature reservoirs: Require high-temperature-resistant corrosion inhibitors (e.g., COROHIB-1) and thermally stable polymers.
- Environmental regulations: Nitrate-based or seawater-based solids-free systems may be preferred.
- Horizontal well screen completions: Require multi-stage fluid systems, including high-viscosity displacement fluids and acidizing agents.
For detailed technical specifications or case studies, refer to relevant patents and research literature.
