Hole Cleaning and Mechanical Friction in Non-Circular Wellbore Geometry

Abstract

Drilling is one of the most important cost drivers in the finding and development of oil and gas fields. More efficient drilling reduces the overall costs and enables drilling of longer wells to reach new reservoir targets. Mechanical friction and poor hole cleaning are the main limiting factors for drilling and well completion of longer wells. Poor hole cleaning can lead to costly drilling problems such as stuck pipe, reduced ROP and higher annular pressure drop leading to lost circulation. Excessive torque and drag caused by mechanical friction, often leads to the inability of reaching the target in high-angle extended reach drilling wells (ERD).

Over the years, the oil industry has attempted to stretch the hydraulics and mechanical limitations for drilling longer wells. However, currently available solutions used by the oil industry are not sufficient to overcome these challenges for drilling longer wells that will be required in the future. In addition, stretching these mechanical and hydraulic limitations is essential for improving the drilling efficiency and reducing the overall well construction costs.

Drilling boreholes with non-circular geometry is a new concept with step-change potential to stretch operational limits allowing drilling longer wells and more efficient well construction. This concept is based on creating helical grooves in the borehole sidewalls to improve hole cleaning and reduce the mechanical friction. Rifle shaped spiral grooves can be made in the borehole wall with the use of a specialized tool installed as a part of the bottom hole assembly (BHA).

The spiral grooves can increase the wellbore cross sectional area and reduce the drill string and formation contact area. The spiral grooves can introduce swirling flow regime in the annulus which may improve the cutting transport. Reduced contact area and improved hole cleaning may reduce the overall mechanical friction. Initial flow simulations indicated differences in the flow regime of circular and non-circular wellbore geometry with integrated helical grooves inside the annulus. More uniform fluid velocity distribution and swirling flow in the non-circular geometry indicated the potential for improving hole cleaning and cutting transport in the new geometry.

The objective of this PhD study was to investigate key features and further understanding of the non-circular wellbore concept. Cutting transport and wellbore cleaning as well as mechanical friction has been investigated through flow loop experiments and in the tribology laboratory. A large scale flow loop rig with concrete borehole with rotational drill string was designed and built as the tool for the main part of the PhD work. The modern flow loop includes a 12 meter long test section with a 2" OD freely rotating drill string inside a 4" ID wellbore made of concrete. Solid particles were injected while circulating the drilling fluid through the test section at two inclinations; horizontal and 30o inclined from horizontal.

Test results of the big scale flow loop demonstrated the difference in the hydraulics and mechanical performance of circular and non-circular wellbores geometries. For fluid and flow conditions which are relevant for oilfield applications, the results indicated lower pressure gradients and better cuttings transport in the non-circular wellbore than in a circular wellbore. In addition, the torque measurements showed lower mechanical friction in the non-circular wellbore geometry.

To support the finding of reduced friction in the non-circular geometry in the flow loop setup, the mechanical friction was also studied in a tribology lab. Test results in the pin-on-disc were consistent with the results obtained from the flow loop experiments. The experiments in the tribology lab confirmed the hypothesis that reducing contact area in the non-circular wellbore geometry can decrease the mechanical friction.

Such a comparative, experimental study of hole cleaning in different wellbore geometries has to our knowledge previously never been performed. The results of the present PhD work and the experimental approach could therefore be of value for any one working in the drilling industry.