Detailed analysis of downhole weight-on-bit, torque, bending moment and vibration signals measured between bit and hole opener have provided clear insight into the dynamic response of hole opening BHAs in interbedded formations. Based on transmitted real-time information from an advanced downhole multi-sensor data acquisition and processing system, application-specific drilling procedures and guidelines have been developed to avoid or to identify and overcome critical dynamic situations downhole.
Applying the new procedures, several hole sections in the GOM were drilled for the first time in one run from casing shoe to casing shoe. Here previous attempts had resulted in failures of the drilling equipment or even in twist-offs.
The paper starts with a list of observations made with the downhole multi-sensor tool placed between bit and hole opening device. Log examples with downhole and surface drilling data demonstrate the dynamic response of hole opening BHAs. For the first time, the paper presents downhole measurements showing the changes in weight on bit versus weight on reamer when drilling interbedded formations. Again, log examples are used to document the effect of the drilling procedures that were applied to successfully resolve critical downhole situations. The paper concludes with a summary of the lessons learned in planning and executing hole opening applications with shoe to shoe performance.
Introduction
Driven by a variety of technical and economic reasons, more and more drilling programs include hole enlargement operations while drilling. In deepwater applications, the ability to enlarge the hole below the last casing shoe allows for tighter tolerance intermediate casings to be run. Figure 1 shows a typically deepwater casing and drilling program, where the intermediate casing strings are instrumental to reach the reservoir with an economically viable production string size. In other applications, hole opening or underreaming techniques are applied to address ECD limitations or to deal with swelling shale or salt formations.
Earlier, hole enlargement operations were carried out by first drilling a pilot hole through the interval of interest and then opening it up with a second pass. Today, several enlargement options while drilling are available to eliminate the second pass: Bicenter PDC bits, eccentric reaming-whiledrilling (RWD) or concentric reaming devices1–10. In particular references 3, 9 and 10 discuss at some length the benefits and limitations of the three options for various applications with respect to dynamic stability, reliability, steerability and hole quality. Over the past years, the concentric reaming devices have gained significant market share, in particular in rotary steerable (RSS) applications. Concentric reamers include expandable arms, ribs or blocks, which are mechanically or hydraulically activated, after passing e.g. the last casing shoe8,9. All examples except one presented in this paper feature concentric reaming devices.
In general, drilling with a reaming device in the bottom hole assembly (BHA) with the associated lack of stabilization in the BHA in the enlarged borehole above the reamer significantly increases the risk of the drilling operation. In particular, dangerous backward whirl situations with high lateral vibrations can develop and result in failures of BHA components, and subsequently in costly trips or even more costly fishing operations. Several deepwater projects in the GOM suffered significant cost overruns as a result of vibration problems during hole enlargement operations while drilling. In some cases in the GOM and other areas, operators decided to go back to a second run for the hole enlargement operation.
