Reducing Uncertainty and Costs in Drilling and Completion: Innovative Underreamer Verifies Wellbore Diameter in Real-Time

Abstract

Applications such as close tolerance casing design and expandable liners, often necessary for deepwater and subsalt well construction, require underreaming to ensure adequate reservoir diameters. However, underreaming generates uncertainty sometimes, particularly in hard formations. Instead of direct real-time verification, reliance is placed on indirect indicators such as increased standpipe pressure or drilling torque. The uncertainty as to whether the desired wellbore-diameter is actually delivered exists until a calliper is run. Subsequently, a correction run may still be needed, hence additional cost. This paper explains a design process to solve these problems. An explanation of a novel technology illustrates how it verifies and detects variations in underreaming diameter in real-time. A telemetry system alerts the user if there is a significant difference between planned and actual diameters and prompts a check of operational parameters such as WOB, drilling fluid pump rates or RPM, if needed, repeat underreaming in the uncertain interval. A comparative evaluation was made of the drilling dynamics of underreamers and the root cause analysis of NPT. For example, bit only RoPs are higher than underreaming RoPs and in a combination BHA the bit tends to out-drill the underreamer. Not only does the underreaming have limited cutter contact with the wellbore and limited hydraulics, BHA modelling shows these limitations are worsened by wall-side forces and bending moments which are concentrated at the underreamer. The design process sought to improve traditional technology by considering RoP, improved underreaming BHA stability and generate a more balanced cutting action. CFD (Computational fluid dynamics) show how a novel configuration of nozzle distances and orientations improves cuttings evacuation and reduces particle residence times. In conclusion, a drilling engineering risk table presents 20 underreaming applications and is used as a benchmark for a comparative evaluation of underreaming risk types. Introduction Oil and gas companies are exploring and developing reserves in more challenging basins such as those in water-depths exceeding 6,000 ft (1830m) or below massive salt sections. These wells have highly complex directional trajectories with casing designs including 6 or more well sections. Known as 'designer' or 'close tolerance casing' wells, such wells have narrow casing diameters with tight tolerances and have created a need to enlarge the wellbore to avoid very small diameter reservoir sections and lower production rates (Figure 1 PPFG). In other applications such as cemented solid expandable liners, underreamers are required to provide the tolerance for tubular expansion to occur or for increased cement sheath. The tolerances between the enlarged well-bore and the expanded tubular are very close. Therefore, the bottom-hole assemblies that are needed to drill or complete these wells routinely include devices to underream the well-bore. In this way, underreamed hole size has become an integral part of well construction and there is now an increased dependence on underreaming to meet planned wellbore diameters across the industry (Ref 2).

Design of bottom hole assemblies with two rock cutting tools for drilling wells of large diameter

“…If it is necessary to expand to large diameters, it is possible to use several eccentric reamers at the same time or combinations of such reamers with conventional and hard-alloy teeth [3][4]. However, for the construction of a wellbore with a diameter of more than 393.7 mm with the ability to manage the well trajectory, it is rational to use an option with two or more rock cutting tools included in the BHAa pilot bit and a pinand-roller type reamer [5]- [7]. In the existing methodology for calculating this type of BHA, developed by L.A. Raikhert and I.M.…”