Drilling Optimization in Deep Horizontal Wells 

1. Introduction 

Drilling long horizontal sections in complex lithologies presents multiple operational hurdles, particularly in formations characterized by variable mechanical strength and reactive shales. The Northern Oman field, known for its interbedded formation layers, typifies these challenges. This case study outlines the optimization efforts undertaken to drill high-quality horizontal wells efficiently while managing formation-related risks and minimizing sidetracking costs. 

2. Operational Background 

Location: Northern Oman 
Reservoirs: Confidential 
Target Depth: ~3175 meters subsea 
Reservoir Thickness: Confidential 

Drilling Challenges: 

  • Abrasive and heterogeneous lithology 

  • High Unconfined Compressive Strength (UCS) zones (up to 60,000 psi) 

  • Reactive shales causing borehole instability 

  • Slow ROP and frequent sidetracks with conventional assemblies 

    3. Problem Identification 

Drilling horizontal wells in this geologically complex field posed several technical and operational hurdles: 

  • Interbedded Shale and Abrasive Formations: Suboptimal bit performance and wear and tear on drilling equipment. 

  • High Unconfined Compressive Strength (UCS): With values up to 60,000 psi, formations presented difficulties in maintaining consistent ROP. 

  • Borehole Instability: Reactive shales contributing to wellbore collapse, requiring careful mud and BHA management. 

  • Low ROP: Slow penetration rates, particularly in the vertical and build sections. 

  • Cost Overruns: Inefficient sidetracking attempts and frequent BHA trips increase non-productive time and the well cost. 

4. Optimization Strategy 

4.1. Trajectory Optimization 

  • To enhance sidetracking success and minimize torque and drag, the original vertical well trajectory was redesigned into a modified S-curve profile. This approach involved deviating the pilot hole to an inclination of approximately 15°. It allowed gravitational assistance during sidetracking from the tangent section rather than from the vertical hole. 

  • Furthermore, the build-up rate was optimized, beginning at 6–7°/30m and tapering down to 3°/30m in deeper intervals. This adjustment accounted for tool wear and less aggressive directional work at deeper intervals, enhancing directional control, prolonging tool life, and reducing the number of BHA runs. 

4.2. Technology Deployment 

The introduction of fit-for-purpose technologies was central to the performance improvement in both the vertical and buildup sections. 

  • Point-the-Bit Rotary Steerable System (RSS): 
    A closed-loop RSS was used for precise directional control and efficient sidetracking. Unlike traditional steerable motors, this system enabled continuous rotation of the drill string, resulting in smoother boreholes and better hole cleaning. In deviated sections, it helped avoid the need for sliding, enhancing ROP, and reducing vibration-related tool damage. 

  • Custom Polycrystalline Diamond Compact (PDC) Bits: 
    The bit design was tailored for the interbedded formations. Bits with 5- or 6-blade configurations and 16–19 mm cutters were selected to provide a balance between durability and steerability. These bits included features like engineered back rakes, gauge reinforcement, and torque-reduction elements to improve stability and reduce lateral shocks and stick/slip behavior. 

  • Tungsten Carbide Sleeves: 
    To protect high-value rotary steerable tools from abrasion and wear in laminated formations, tungsten carbide sleeves were used around critical components. These sleeves significantly extended tool life and minimized the need for sleeve replacements during the section. 

4.3. Drilling Practices 

Several operational enhancements were introduced to improve drilling efficiency and reduce tool failures: 

  • Mechanical Specific Energy (MSE) Monitoring: 
    MSE quantifies the energy required to remove a unit volume of rock. Real time analysis of MSE, was used to optimize Weight on Bit (WOB) and RPM. By comparing actual MSE to calculated rock strength, the team could identify inefficiencies and mitigate issues such as stick/slip, whirl, and BHA stalling. The optimization of drilling parameters through MSE analysis led to more stable drilling and better tool performance. 

  • Oil-Based Mud (OBM): 
    OBM was selected due to its superior shale inhibition and lubricity, which is critical when drilling through reactive formations. This choice minimized borehole collapse, reduced torque and drag, and ensured cleaner hole conditions for casing and liner runs. 

  • Single-Run Sidetracking and Build-Up Section Drilling: 
    Utilizing a combination of RSS, high-power mud motors, and optimized bit design, the entire sidetrack and build-up section was drilled in a single BHA run. This eliminated the need for multiple trips and avoided the use of whipstocks or turbine assemblies, resulting in substantial time savings and reduced risk of tool failure. 

4.4. Cementing Strategy 

To ensure sidetracking success, particularly when kicking off from deviated pilot holes, the cementing approach was significantly improved: 

A tailored cement slurry was engineered with a density of 18.6 kPa/m, designed specifically for use as a base for sidetracking over cement plugs. The formulation was optimized for: 

  • Rapid compressive strength development: reaching 1,200 psi within 12 hours and 3,000 psi by 24 hours. 

  • Controlled fluid loss: maintained below 100 cc to prevent slurry dehydration across permeable formations. 

  • Zero free water content: ensuring plug integrity. 

  • Enhanced rheology: providing turbulence during displacement and better placement efficiency. 

This cement design supported effective plug placement even in deviated pilot holes, enabling RSS-based sidetracking without requiring whipstocks. As a result, the operation benefited from both improved reliability and significant cost savings. 

5. Results and Performance Benefits 

Time and Cost Savings 

Total 7.5 days saved per well, reducing overall well AFE: 

  • 1 day in the vertical section due to faster, smoother drilling 

  • 6 days saved through RSS efficiency and improved sidetracking technique 

  • 0.5 day saved by eliminating the need for wiper trips and improving casing runs 

Performance Improvements 

  • Record ROP of 19.2 m/hr in the 12¼" section 

  • 1000 meters drilled in a single run through hard formations at 15 m/hr 

Operational Efficiencies 

  • 100% sidetracking success rate using deviated pilot holes and point-the-bit RSS 

  • Avoided use of whipstocks, turbine BHAs, and multiple correction trips 

  • Achieved smooth borehole quality, reducing the risk of tool sticking and back reaming requirements 

6. Lessons Learned 

  • Customized Technology Matters: RSS tools and PDC bits designed for formation-specific challenges vastly improve drilling performance. 

  • Well Path Planning is Key: A carefully engineered trajectory eases sidetracking and reduces mechanical loads. 

  • Real-Time Data Enables Smart Drilling: MSE-based optimization allows timely responses to downhole dynamics, improving efficiency. 

  • Effective Cementing Supports Drilling Goals: Cement properties should match operational needs, especially when reliable sidetracking is a requirement. 

7. Conclusion 

This case study demonstrates a comprehensive and successful application of drilling optimization techniques in a geologically complex field. The integration of advanced trajectory planning, fit-for-purpose technology, enhanced drilling practices, and improved cementing strategies resulted in measurable performance gains and cost savings. The project achieved its goal of delivering high-quality horizontal wells with enhanced efficiency, providing a benchmark for future development in similar challenging reservoirs worldwide.