Drilling in complex rock formations: different formations require different drilling methods

In drilling operations, complex geological formations are typically characterized by low drilling efficiency, high tool wear, and a high incidence of downhole incidents. Formations such as pebble layers, broken rock/gravel layers, interbedded soft and hard strata, and high-hardness bedrock each impose distinct requirements regarding the bottom-hole assembly (BHA), drilling parameters, and operational techniques. Applying parameters suitable for soft soil to these complex formations can easily lead to issues such as stuck pipe, borehole collapse, hole deviation, and tooth breakage on the drill bit. Therefore, operations in complex formations must be based on specific geological conditions, incorporating targeted designs and dynamic parameter adjustments.

I. Loose Pebble and Gravel Formations: Prioritizing Borehole Wall Stability

Formation Characteristics: Loose structure, non-uniform particle gradation, low degree of cementation, and the presence of large-diameter pebbles in localized areas.

Key Risks: Borehole wall collapse, rockfall, stuck pipe, and abnormal bit wear.

Construction Recommendations: Employ low rotational speeds, low weight-on-bit (WOB), and steady feed rates to avoid disturbing the borehole wall through rapid pressurization; select wear-resistant and impact-resistant drill bits; optimize drilling fluid properties by appropriately increasing specific gravity and viscosity to enhance wall support and cuttings removal; consider casing-advancement drilling or casing-supported methods for thick, loose formations.

II. Interbedded Soft and Hard Formations: Dynamic Parameter Adjustment and Layer-Specific Tool Matching

Formation Characteristics: Alternating soft and hard rock layers with significant strength disparities and poor formation homogeneity.

Key Risks: Hole deviation, drill string vibration, localized bit damage (chipping), and fluctuating drilling efficiency.

Construction Recommendations: Monitor the rate of penetration (ROP), cuttings size, and drill string vibration in real-time; increase rotational speed and WOB when entering soft layers, but reduce them before entering hard layers to ensure a smooth transition; utilize combined tool assemblies and stabilizers to improve directional control; select PDC or composite-cutter bits to accommodate the interbedded nature of the formation.

III. High-Hardness Bedrock: Controlling Drilling Pace to Minimize Tool Damage

Formation Characteristics: High rock compressive strength, high abrasiveness, and high drilling resistance. Key risks: bit slippage, tooth breakage, deformation of the drill rod due to stress, and low drilling efficiency.

Operational recommendations: Begin with low rotational speed and low weight-on-bit (WOB) to allow for a break-in period and ensure stable contact between the bit and the rock face; subsequently, gradually increase WOB while maintaining a steady drilling speed. Avoid frequent starting/stopping and intermittent "pecking" drilling to minimize impact loads. Periodically retract the drill string to clear cuttings and inspect the condition of carbide teeth, PDC cutters, bearings, and threads, replacing worn components promptly.

IV. General principles for complex rock formations: Survey before drilling; strengthen process control.

Before commencing formal operations, determine rock layer thickness, hardness, and distribution patterns through test holes or geological surveys; select the drill string assembly and initial drilling parameters accordingly. During operations, shorten inspection intervals; inspect the condition of the bit, drill rods, joints, and stabilizers daily, addressing even minor wear immediately. Additionally, maintain detailed records of drill string configurations, rotational speeds, WOB, and penetration rates for each formation layer to serve as a reference for subsequent borehole operations.

Conclusion

There is no "one-size-fits-all" template for drilling in complex rock formations; success relies on selecting equipment and optimizing parameters tailored to specific formation characteristics. Through appropriate drill string assembly, stable drilling parameters, and standardized process management, operators can effectively reduce the incidence of issues such as stuck pipe, borehole collapse, deviation, and bit tooth breakage, thereby enhancing efficiency and controlling overall costs.

Contact Information：

Emily Peng (Product Manager)

E-mail: [email protected] 

Tel: +86 191 7333 7543