Design and Field Application of Large-Size PDC Bit in Gravel Layer in Tarim Basin

ABSTRACT: It is one of the key difficulties that extremely thick gravel layer distributed in the shallow and middle layers in the Tarim Oilfield. The maximum thickness of the gravel layer is more than 5000m, which seriously restricts the efficient exploration and development of Piedmont tight sandstone oil and gas reservoir. According to the characteristics of gravel layer in Bozi block, this paper selects the appropriate outcrop, studies the parameters such as conglomerate composition, hardness and compressive strength through rock mechanics experiments, and summarizes the diagenetic characteristics of gravel layer in Bozi block. The experimental results show that there are great differences in mineral composition in the plane of rock samples and a wide range of spatial strength variation, so the impact load on the bit during drilling is large. According to the results of experiments, a new large-size PDC bit is designed from optimization of cutters, tooth arrangement and force balance design. The designed large-size PDC bit has obvious effect of speed-up, which achieves the footage of a single bit is 510m, and the ROP(rate of penetration) is 1.89m/h. Compared with adjacent wells with the same lithology in the same well section, the footage and average ROP of a single bit are doubled, which achieves a good speed-up effect, and provides technical reference and support for efficient exploration and development of oil and gas reservoir in the Tarim Basin. 1. INTRODUCTION Huge thick gravel layer is developed in the upper part of Kelasu structural belt in Kuqa depression, Tarim, especially in Bozi block. The average thickness of gravel layer is 3000m and the maximum thickness is more than 5000m(Teng et al., 2015; Zhong et al., 2012). Multiple sets of strata contain gravels with different compositions, which are poor sorting, strong heterogeneity and poor drillability. During the drilling process, cutting teeth of conventional PDC bits are easy damaged due to the positive impact of gravel. The damage of PDC teeth leads to slow ROP(rate of penetration), less bit footage and more bit consumption(Pei, Yan, & Hongbo, 2020; Zhang, Dang, Chen, & Zhang, 2017), that seriously restricts the efficient exploration and development of oil and gas reservoir.