Through Bore Drilling Systems: a New Drilling Option

Abstract

This paper describes a Through Bore Drilling and Logging System, which has been developed and tested over the past four years with increasing levels of success. The key components of the system, as it stands today, are:A PDC bit with a removable and re-insertable 2 1/2" ID center insert and latching mechanism.A string of memory logging tools (gamma, resistivity, density, neutron and sonic measurements) with 2 1/4" OD.A 3D rotary steering device, with 2 1/2" full through bore.A removable and re-insertable MWD probe type tool, which leaves a 2 1/2" full through bore after retrieval. The trials and jobs carried out to date are also briefly described, together with the currently perceived strengths and weaknesses of the current system. Introduction Rotary drilling hardware, in its most basic form, consists of a jointed steel drillstring with a drill bit at the downhole end and a drive mechanism at the surface (kelly or top drive). Boreholes are drilled and secured to provide a conduit for fluids to move from downhole to surface, or vice-versa. For various reasons, the E&P industry is being driven to deliver increasingly complex well trajectories, penetrating more tightly controlled geological targets. One result of this is that the bottom hole assemblies (BHA's) and formation evaluation tools and techniques used on today's wells have become highly sophisticated. It is not uncommon in the current environment to see million dollar BHA's being run into the hole, to ensure and demonstrate acceptable placement of a well. (Fig. 1) While this equipment allows a wide range of functionality for well construction, it is complex and expensive to manufacture and maintain, with exposure to large costs if lost in hole. Well Engineers are routinely faced with the requirement to balance the risked cost of increased BHA complexity (fig. 2) (increased probability of tripping due to a tool failure, plus, probability of losing the BHA in the hole) versus the rig time cost of wireline or pipe conveyed formation evaluation tools (fig 3). As a solution to this dilemma, a modular system has been developed that uses the drillstring as a conduit for the delivery of payloads to and through the drillbit. The objective in developing this system is to improve the accuracy and reliability of well placement and evaluation, while reducing direct and risked cost to both the Operator and the Service Company (see advantages below). The following key components have been developed so far (fig. 4):A PDC bit with a removable and re-insertable 2 1/2" ID center insert and latching mechanism.A string of memory logging tools (gamma, resistivity, density, neutron and sonic measurements) with 2 1/4" OD.A 3D rotary steering device, with 2 1/2" full through bore.A removable and re-insertable MWD probe type tool, which leaves a 2 1/2" full through bore after retrieval. With these components it is now possible to directionally drill a hole section with a rotary steering device including MWD, take open hole logs and continue drilling without tripping out of the hole.