The Development and Field Applications of Ultra-Short Radial Radius Drilling Technology in Low Permeable Reservoir of Western South China Sea
Authors: Guan Shen , Liu Shujie , Yang Jin , Liu Zhiqin , Xu Dongsheng , Zhao Yuhang , Ma Kuo , Zhang Xun , Ling Tong 
Published: March 2023

Abstract
Weizhou Oilfield, located in Beibu Gulf Basin, is a complex fault-block structure oilfield formed at the late periods of the Eocene epoch. The reservoir is in the second member of Liushagang formation, of which lithology is mainly argillaceous siltstone, with high shale content, and is characterized by strong heterogeneity with rapid transition of horizontal pay zone, low porosity and permeability, poor oil recovery due to early-stage extraction through conventional well completion and perforation. In order to expand the drainage area of reservoir and improve the connectivity of the reservoir for improvement of control reserves and enhance oil recovery, the research of "Ultra-short Radius Well Drilling Technology was carried out. The Multi-lateral Ultra-short Radius Well Drilling Technology refers to a novel multi-lateral horizontal well with a radius of curvature of 1~3m or even less, which is completed by special flexible drill pipe composed of mechanical flexible units. Ultra-short Radial Radius Drilling Technology is used in land oilfields in the early stage, which is expected to not only improve recovery and production, but also reduce drilling operation cost, so it is considered as an important measure for enhance oil recovery (EOR), further exploration of residual oil, and stable production in oilfields, and can be suitable for the development of low permeable reservoirs, thin pay zone, and other reservoirs. However, the key challenges include: 1) build-up with dogleg severity of greater than 10°/m; 2) conventional drill pipes cannot meet the demand of high dogleg severity and horizontal drilling. Since there is no case available for reference in the early stage of applying "Ultra-short Radial Radius Drilling Technology" in offshore oilfields, besides the aforementioned challenges are for onshore oilfields, other challenges for application of this technology offshore include: (1) Staggering high offshore operation cost, and strict requirement for work schedule. During offshore drilling operation, the footage cost is USD $1850/m, the novel technology is required to be used economically; (2) as for offshore operation, most development wells are deviated, and the trajectory is complex. Since offshore oilfields are mostly developed by cluster wells, and the sidetrack process is subject to well trajectory and dogleg severity.
In light of build-up with dogleg severity of greater than 10°/m, conventional drillpipes cannot go through the sidetracking point with dogleg greater than 10°/m. As a result, novel "Ultra-short Radial Radius Drilling Technology" was introduced by developing a series of novel downhole tools, including highly deviated (23°) Ø209mm whipstock, Ø127mm flexible build-up drillpipe, and Ø144mm build-up bit, combined with selection of sidetracking point, well trajectory design optimizing, and solving difficulties related to operation. Field application results showed that the "Ultra-short Radial radius Drilling Technology" can lead to enhance oil recovery, and high efficient development of oilfields.