G. Cavanough

Challenges of cuttings transport in micro–borehole coiled tubing drilling for mineral exploration

Coil tubing (CT) technology has been in use in the oil and gas industry since the 1990s. Since then, the applications of CT have expanded rapidly. Coiled tube drilling can offer more efficient and faster drilling operations resulting in lower operational costs. Micro-borehole CT drilling (CTD) has been used in oil and gas applications and is very attractive as a method for minerals exploration drilling due to the faster drilling i.e. high rate of penetration (ROP) that can be achieved resulting in reduced drilling costs. Due to the narrow annulus space there is a certain degree of uncertainty with regards to cuttings transport. In this paper we review the fluid flow and cutting transport models available for conventional drilling. Different aspects of fluid flow in micro-borehole CT will be addressed and discussed. The discussion illustrates the important parameters, including fluid properties, cuttings properties, fluid hydraulics and annular geometry affecting cutting transports in micro-borehole CT drilling in oil and gas as well as mineral exploration.

Rheological properties and slurry behaviour in hard rock drilling

Abstract Coiled tubing (CT) drilling offers more efficient drilling operations compared to normal drilling practises resulting in lower operational costs and drilling time in oil and gas operations. This technology is recommended to be used for mineral exploration drilling in hard rock. This requires drilling micro-boreholes at very high speed with small size cuttings. A large volume of such small size cuttings can influence the rheological properties of the drilling fluids significantly. The aim of this study is to understand the effect of small size inert cuttings on the rheological properties of drilling fluids and its consequences on pressure drop changes in the annulus space. Experimental tests were performed on cuttings collected from a mining site, and the results indicated that the fluid rheology is sensitive to the cuttings size. This conclusion suggests that careful measurement of rheological properties of the mud is necessary when it is designed for drilling hard rocks.

Coiled Tube Turbodrilling: a proposed technology to optimise drilling deep hard rocks for mineral exploration

The need to drill deep boreholes more efficiently for mineral exploration has raised the attention to investigate the feasibility of recent drilling technologies for such applications. The two principal methods of Reverse Circulation (RC) and diamond core drilling are usually used in combination by mine operators are subjected to certain limitations and inefficiencies. Considering that delivering large volume of reliable samples from deep zones to the surface in shortest possible time is of paramount importance in mineral exploration, then drilling small size holes as fast as possible and delivering the small chip samples to the surface would be a good alternative with several advantages over conventional drilling methods. As a result, the Coiled Tube (CT) turbodrilling technology is proposed here followed by presenting detailed calculations for the system required power and hydraulics and also Bottom Hole Assembly (BHA) selection suitable for hard rocks mineral exploration applications.

Turbodrills design and performance analysis for efficient drilling in hard rocks

The authors have recently proposed Coiled Tube Turbodrilling technology for drilling deep hard rocks mineral exploration. Coiled Tube (CT) is a continuous length of ductile steel or composite tube that itself cannot rotate and therefore a down hole motor is needed to provide mechanical power to the bit. Amongst the down hole motors, turbodrills (turbine down hole motors) are an excellent fit with CT operations for hard rocks, providing a smooth borehole with little vibrational effects during drilling with high output rotational speed. The turbine motor section has multistage of rotors and stators which convert the hydraulic power to mechanical power. This paper presents a methodology for designing turbodrills with asymmetric rotor or stator blades configurations. Here, the turbodrill is designed specifically for small size CT system providing suitable output power and rotation speed with applicable input flow properties. Also, the results of a few numerical simulations carried out using computational fluid dynamics (CFD) code are presented. The results help in choosing the best turbine motor configurations to obtain optimum rotational speed and torque during drilling hard rocks for small hole size exploration applications. Similar methodology can be used to design and choose the best turbodrill for other hard rocks drilling conditions.