Wax deposition mechanism of Bozi ultra-deep gas condensate reservoir in Tarim basin

Technical Summary: Wax Deposition Mechanism of Bozi Ultra-Deep Gas Condensate Reservoir in Tarim Basin

Problem Statement
The Bozi gas field in the Tarim Basin represents a typical ultra-deep (burial depth >7,000 m), ultra-high-pressure (125 MPa), and high-temperature (127.53°C) gas condensate reservoir. A critical challenge in developing this field is wax deposition, which occurs readily in gas wells during autumn and winter due to low temperatures and high wax content. This phenomenon leads to production reductions, well closures, and blockages in both wellbores and surface facilities. While existing research has addressed wax deposition under medium-to-low pressure conditions, there is a significant gap in understanding the mechanisms under the extreme high-pressure and high-temperature conditions specific to ultra-deep reservoirs. Specifically, the dynamic influence of drastic fluid phase changes, the role of sand particles as crystal nuclei, and the quantitative critical daily production thresholds for wax prevention in such environments remain unclear.

Methodology
This study focuses on high-wax gas condensate wells in the Bozi field, utilizing a combination of laboratory experiments and dynamic modeling:

• Sampling and Characterization: High-pressure fluid samples were collected from four wells (104, 102, 104-1, 102-2). Samples were analyzed using Differential Scanning Calorimetry (DSC) and chromatographic analysis to determine wax content, wax deposition points, and density.
• Visual Observation: A high-temperature and high-pressure fluid visual phase-change polarizing microscope was used to observe gas-solid precipitation and fluid phase changes (gas-liquid-solid) under formation conditions.
• Dynamic Wax Deposition Testing: The authors developed a custom "slim tube dynamic wax deposition test device" capable of simulating ultra-deep conditions (up to 100 MPa). This apparatus allowed for the measurement of wax deposition rates under varying conditions:
  • Flow Rates: 0.1, 0.2, and 0.3 ml/min.
  • Gas-Oil Ratios (GOR): 20, 50, and 100 m³/m³.
  • Particle Content: 1%, 2%, and 3% mass fraction of solid particles.
• Modeling: A multiphase flow wellbore deposition dynamic model was employed to predict wax deposition mechanisms under different daily production rates. The model calculates wax deposition thickness and speed based on pressure differences, incorporating dynamic friction coefficients that account for the "dynamic roughness" of the wax layer.

Key Results

• Wax Characteristics: The Bozi gas condensate exhibits a maximum wax deposition point of 33.71°C. At -20°C, the wax accumulation was found to be lower than the total wax content, indicating that wax cannot be completely deposited during DSC cooling. Well 102-2 exhibited the highest wax content (8.08%) and density (0.8096 g/cm³), making it most prone to plugging.
• Flow Rate Impact: There is an inverse relationship between flow rate and wax deposition rate. Higher flow rates increase the shear stress on the pipe wall, physically stripping deposited wax layers and inhibiting the orderly nucleation and growth of wax molecules, thereby reducing the net deposition rate.
• Gas-Oil Ratio (GOR) Impact: Increasing the GOR significantly decreases the wax deposition rate. High GOR dilutes the mass concentration of wax in the liquid phase and alters the rheological properties of the fluid (reducing effective viscosity), making it harder for wax molecules to concentrate and precipitate near the pipe wall. Conversely, low GOR conditions (e.g., during shut-in) increase the risk of severe plugging.
• Particle Impact: Solid particles (sand/clay) act as crystal nuclei, promoting wax growth. The study found a direct proportionality between particle content and wax deposition rate; higher particle content leads to faster deposition.
• Critical Production Threshold: The dynamic prediction model determined that for Well Bozi 1-1, there is no wax deposition in the wellbore when the daily production exceeds 3.4 × 10⁵ m³. Below this threshold (e.g., at 2.7 × 10⁵ m³), the wellbore temperature intersects with the wax deposition temperature, leading to blockage at depths around 100m.

Engineering Application and Mitigation
The study applied these findings to Well Bozi 1-1, which had previously suffered from severe wax plugging (oil pressure dropping to 2 MPa). A remediation strategy was implemented involving:

1. Circulation of high-temperature crude oil (80°C) from the annulus to melt existing wax.
2. Injection of a paraffin remover (2‰ concentration) followed by the addition of anti-wax and pour-point depressants (0.1%).
3. Maintenance of production above the critical threshold.

Post-treatment results showed a recovery of oil pressure from 2 MPa to 89 MPa and a stabilization of daily production, increasing from 2.7 × 10⁵ m³ to 4.0 × 10⁵ m³.

Significance and Claims
The paper claims to provide theoretical data support for understanding wax deposition mechanisms in ultra-deep, high-pressure, and high-temperature gas condensate reservoirs. By systematically quantifying the coupling effects of flow velocity, GOR, and particle content, the study establishes a critical daily production threshold (3.4 × 10⁵ m³) to prevent wax deposition in the Bozi field. The research validates that maintaining production above this critical level, combined with effective sand control and chemical/thermal remediation, is essential for stable production. The authors state that these results offer key theoretical support for optimizing production and controlling wax in similar ultra-deep reservoirs, while noting that future work will focus on expanding these models to multi-well network optimization and intelligent early warning platforms.