Through bit logging is a method for data acquisition using the drill string and bit as a conduit to the well bore. Two case studies are presented demonstrating the unique capabilities of this technology to obtain data in difficult and troublesome well situations. Both cases were recent (early 2004) wells, offshore North Sea. Case study 1 concerns an exploration well with a large open salt interval open above the reservoir. Conventional logging tools could not pass the squeezing salt interval, and TBL was used to get past the troublesome interval and successfully obtain quad combo and pressure data. Subsequently, the cement stinger could not pass the same interval, and the TBL bit was again used, this time to cement through. Case study 2 describes a well drilled into a nearby block, suspected of being depleted. After loosing the BHA, the operator wanted to reduce the risk of getting stuck with an RFT and decided to use TBL to successfully obtain wireline pressure data in the subsequent side track. Recent developments include a new type of Side Entry Sub System to enable easy surface deployment as well as providing a rapid easy fishing method in case the logging tools were to get stuck during TBL operations. Compatibility with rotary steering and MWD is under development, delivering the concept of ‘Through Bore Drilling’, the ultimate low cost drilling system. This will allow directional drilling in combination with open hole logging and other operations through the bit, in a single trip. Conclusion: TBL opens up new low cost/low risk options for data acquisition (and other through bore operations) during well construction. It gives both drilling and petroleum engineers new and low cost options to choose from to optimise well construction and data acquisition. TBL is a first significant step on the way to Through Bore Drilling and has already proven its versatility in a number of ‘difficult well’ cases. Introduction Data acquisition in oil and gas wells has evolved over the last century since the Schlumberger brothers performed their first well bore profiling in 1927. Wireline logging is now fully mature, and logging-while-drilling (LWD) is being developed rapidly, giving the end user options to perform data acquisition either during the drilling phase, post drilling in the open hole, or subsequently in cased hole. All options have advantages and disadvantages, and a balance needs to be struck between cost, quality, risk and timing of data acquisition. As a major operator, Shell has from time to time experienced difficulties obtaining data with existing methods, and was motivated to seek alternatives. Through Bit Logging (TBL) has been developed over the last few years by Shell and Reeves Logging Services (now part of Precision Energy Services) together with other industry partners, as a cost-effective and operationally efficient alternative to standard open hole wireline logging and LWD. In the new system the logging tools are conveyed via the drill string and pass into open hole through a specially designed bit, with formation evaluation data being acquired with either wireline or memory tools; in the latter case while tripping drill pipe out of the well, thus improving rig efficiency. Acquiring log data with through bit logging - how does it work? The logging tools are lowered down the drill string and pass through the bit into the open hole (figure 1) below. The TBL bit is a conventional PDC design modified with a removable central section (figure 2) leaving a 2.5" passage way.
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.
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