Changbei is a marginal tight-gas field in Yulin, west China. Because of high drilling costs associated with the Changbei gas field, operators focus on improving drilling performance by means of reducing drilling and tripping times. One effective way to maximize drilling efficiency is proper bit selection. This paper presents an improved bit design and successful field results from two representative sections: CB 17–2 and CB18–2, both 12.25-in. sections. Both applications were characterized by a 2500-m long directional section, usually drilled through varying formations ranging from soft abrasive to very hard, with compressive strength ranging from 7 to 20 kpsi. Three main challenges were encountered during this operation. First, when drilling the upper portion of the sections, a series of transitional hard/soft formations, such as abrasive sandstone/siltstone and claystone, were encountered, which generated extensive vibrations and decreased the rate of penetration (ROP). Secondly, the long soft claystone intervals and frequent thin but very abrasive sandstone stringers in the lower part of the section not only increased bit balling potential, but also created premature cutter wear and loss of aggressiveness. The third challenge using a polycrystalline diamond compact (PDC) bit in this area related to drilling the whole section in fewer trips and maximizing the ROP to reduce drilling costs. To overcome these challenges, extensive geological research was performed within the 12.25-in. section of the Changbei gas field. Then, an improved bit design proposal was implemented, which consisted of three aspects:
apply new cutter technology to the bits,
optimize the bit design based on directional and formation challenges, and
develop optimum drilling parameters. Based on field results, this approach set both ROP and interval records compared to offsets and helped the operator save 3.26 drilling days_one trip and one bottomhole assembly (BHA) in Section CB17–2. Drilling efficiency for both sections was enhanced by more than 20% compared to offset wells. Furthermore, for both sections, the bit used exhibited improved dull conditions compared to offsets.