Abstract
To improve the rate of penetration (ROP) in drilling deep and hard formations, this paper proposes a new drilling method called coiled tubing partial underbalanced drilling (CT-PUBD). As a preliminary investigation into the new drilling method, this paper presents predictions of hole cleaning efficiency, drilling speed, cuttings migration and pressure loss in the drilling process with CT-PUBD. Based on numerical simulation and full-scale experimental studies, we conclude that using CT-PUBD, an underbalanced drilling condition can be achieved near the bit while maintaining wellbore safety at the same time. This condition can be achieved using a cuttings discharge device, a rotary packer and a backflow controller. According to the numerical simulations performed in this study, CT-PUBD can achieve high efficiency of hole cleaning. Along the cuttings migration process, the fluid velocities can reach the maximum values in the backflow holes. A full-scale laboratory experimental system was used to test the hydraulic characteristics and obtain the drilling performance of the new technology. The result shows that CT-PUBD significantly improves the ROP compared to the conventional drilling method.
Highlights
With the increasing demand for energy, exploration and extraction of oil and gas have stepped into deep formations which normally consist of hard rocks (Chen et al 2016a; Rui et al 2017)
To improve the rate of penetration (ROP) in drilling deep and hard formations, this paper proposes a new drilling method called coiled tubing partial underbalanced drilling (CT-PUBD)
As a preliminary investigation into the new drilling method, this paper presents predictions of hole cleaning efficiency, drilling speed, cuttings migration and pressure loss in the drilling process with CT-PUBD
Summary
With the increasing demand for energy, exploration and extraction of oil and gas have stepped into deep formations which normally consist of hard rocks (Chen et al 2016a; Rui et al 2017). The backflow controller is located between the drill bit and the drill collar, in which there are holes These holes are backflow channels for the low-density drill fluid to the micro-annulus between the drill pipe and the coiled tubing, from the bottom annulus. Due to the presence of the sealing rotary packer, the low-density drilling fluid flows back into the micro-annulus between the drill pipe and the coiled tubing through the backflow holes, carrying cuttings from the bottom annulus. Cuttings discharge device (Fig. 2a): Since the main backflow channel is changed to the micro-annulus between the drill pipe and the coiled tubing, a cuttings discharge device is designed especially for the backflow of the fluid circulation of the low-density drilling. We applied the discrete phase model (DPM) to study the hydraulic characteristics of the flow fields of the annulus and the micro-annulus, i.e., the fluid velocity and the cuttings removal efficiency
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