With the advancement of the social economy, the demand for oil resources is rapidly increasing. As exploration and development progress, the industry has expanded from shallow wells to deep and even ultra-deep wells. Additionally, the focus has shifted from conventional oil and gas to unconventional resources such as shale gas, shale oil, and tight gas. With the rising costs of exploration, it is essential to control investment and improve drilling efficiency, and accelerating drilling is one of the most effective ways to reduce costs.
To meet this need, PDC (polycrystalline diamond compact) drill bits, combined with torsion impactors and other specialized tools, have been widely adopted, significantly increasing drilling speed. However, because the rock-breaking technique differs from traditional methods, particularly due to the unique structure of PDC drill bits, the crushed rock particles are very fine. This makes it challenging to identify rock chips and can complicate the quality of rock chip logging. Strengthening research on rock chip logging technology for rapid drilling conditions and implementing technical measures is essential for maintaining accurate rock chip logging quality in oilfield exploration.
Impact of Drilling Speed on Rock Chip Logging
1. Characteristics of PDC Drill Bits in Rock Breaking
PDC drill bits have high abrasion resistance and impact strength, making them a reliable replacement for traditional carbide drill bits. They are widely used in hard formations, with rock-breaking primarily reliant on the cutting action of the teeth. The performance of PDC drill bits mainly depends on the following characteristics:
- High Hardness and Abrasion Resistance:
The composite material in PDC bits directly interacts with the formation, enabling excellent abrasion resistance due to its high hardness. This allows the cutting teeth to endure prolonged rock crushing while maintaining sharpness.
- High Strength:
Although diamond itself is brittle, its backing of tungsten carbide provides high pressure-bearing capacity and impact resistance, making the cutting teeth extremely strong.
- Self-Sharpening Ability:
One of the most valuable attributes of PDC bits is their self-sharpening feature. As the tungsten carbide matrix wears, the diamond layer remains effective, maintaining sharpness and enabling continuous, high-speed drilling.
2. Impact of Drilling Speed on Chip Logging
PDC drill bits typically require low drilling pressure, high rotation speed, and high displacement. Their unique structure produces extremely fine rock chips, making traditional visual rock chip logging methods inadequate. This can result in challenges, such as difficulty in identifying the type, purity, and mineral distribution of rock chips, which is particularly problematic in hydrocarbon-bearing reservoirs, where delicate reservoir structures may be missed.
Technical Countermeasures
1. Effective Collection and Cleaning of Rock Chips
Sampling and cleaning fine rock chips should strictly follow rock chip logging protocols:
- Ensure the sample basin is positioned appropriately to catch fresh rock chips from the vibrating screen.
- Use fine-mesh sieve cloths (80 mesh or finer) to minimize loss of fine chips.
- After filling the basin, use a small water stream to clean the chips, gently stirring to prevent loss of fine particles.
- Avoid excessive handling of chips before drying to maintain their clarity and integrity.
For high-speed drilling conditions, additional personnel may be needed for dual-post operation to ensure accurate collection and documentation of rock chips.
2. Proper Rock Chip Description Techniques
Describing fine rock chips requires both dry and wet methods. Newly washed, undried chips reveal lithological characteristics clearly. After drying, loose sandstone particles may further fragment, making description challenging. For accuracy, initial observations should be made when chips are wet, followed by detailed description post-drying.
Oil and gas testing on fine rock chips should occur immediately after washing, as prolonged exposure can lead to the evaporation of hydrocarbon indicators.
3. Reservoir Rock Chip Sampling
When drilling through reservoirs, particular attention should be paid to selecting oil-bearing rock chips. The tracing disk method can help isolate real sandstone chips from mudstone. Applying fluorescent light further helps identify oil-bearing particles, aiding geochemical analysis and fluorescence testing.
4. Enhanced Use of Rock Chip Analysis Technology
The fine nature of PDC-drilled rock chips sometimes makes it difficult to observe lithological changes and rock properties by eye. Using a microscope allows detailed analysis of cementation types, essential for accurate lithology identification. Generally, siliceous and basal cementations indicate hardness, while muddy cementation points to improved drillability.
Through these methods, maintaining high-quality rock chip logging under high-speed drilling conditions becomes feasible, providing essential support for accurate geological exploration and maximizing exploration benefits.
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