Exploration offshore Australia requires the operator to drill an 8½” vertical borehole through the difficult Plover sandstone. The highly abrasive formation, with unconfined compressive strength between 15–30kpsi, has been problematic causing extremely short PDC bit runs and low rates of penetration (ROP). Erratic torque response was causing a reduction in bottom hole assembly (BHA) performance and system reliability. The operator required a PDC bit solution to increase run lengths and ROP.
To solve the problems, a new system was devised that enables a PDC shearing element to fully rotate while drilling to increase cutting efficiency/bit durability while reducing frictional heat. To strategically position the rolling cutters (RC) to obtain maximum benefit, a forensic analysis was performed on PDCs used in offsets. After investigation, engineers identified accelerated wear flats in the shoulder area and a new PDC design (RC813) was implemented by positioning two 13mm rolling cutters in the shoulder area on each of the eight-blades. The target objectives would be to drill the 8½” hole section through the Plover with one PDC bit and improve ROP.
The first well required one conventional PDC and one RC813 to drill through the Plover to TD. The standard PDC (10blades/16mm) drilled 90m (3739–3829m) at an average ROP of 5.4m/hr. The bit was pulled for low ROP and graded 8–3-CR-C-X-IN-BT-PR. The rolling cutter equipped RC813 drilled 126m (3829–3955m) at a ROP of 4.3m/hr. The RC813 was pulled in good condition and dull graded 1-2-CT-S-X-IN-PN-TD.
Parameter/bit adjustments in the second well enabled an RC813 to drill the entire Plover interval (3283m–3425m) in one run at an ROP of 9.26m/hr. The rolling cutter PDC bit run was 319% faster than a direct offset that required four standard bits to TD the section at an average ROP of 3.1m/hr. The RC813 torque response was smooth compared to erratic torque response of standard PDC in Plover. The increased meterage and ROP saved the operator over $1million USD.