Development And Case Histories Of Short-Radius Measurement While Drilling
Authors: Angel J.F. , Massey L.S. 
Published: October 2008

ABSTRACT
Case history results show that, in short radius horizontal drilling applications it is possible to run a measurement while-drilling (MWD) tool in place of conventional surveying devices. With proper supervision and planning, Short Radius MWD can be conducted economically, reliably, and effectively. Modifications to existing MWD probe tools can allow them to maneuver the curve sections of short radius wellbores; that is, wellbores with a build rate on the order of 1.5 degrees per foot (4.9 degrees per meter). The objective for the development of this tool was to complement the short radius bottom-hole-assembly (BHA) with drilling efficiency, and provide a safer overall drilling operation.
This paper discusses the criteria required to effectively plan for the use of MWD in short radius well applications. In addition it will take an overview to the development of this system, rig site operations, as well as examine actual case histories.

INTRODUCTION
This patented MWD system was introduced in February 1992, as a replacement for wireline steering instruments. Efficiency and safety concerns warranted a new look at MWD systems and their capability to assist the overall drilling project economics. The tool is a modified version of an MWD service currently available within the drilling industry. Through the adaptation of existing technology, a probe style, real-time, directional only, measurement-while-drilling instrument, was designed to economically assist in wellbore positional surveying and directional assembly control in short radius holes.
The system is structured to accommodate the addition of other parameters, such as natural gamma-ray detection; thus, enhancing tool usage by providing a geosteering capability in short radius applications.
In the initial development of the system, the ability to draw on design engineering from short radius mud motors, and conventional surveying technology, provided a means to complete the engineering design specification for a 1.5°/ft (4.9°/m) bending ratio in an accelerated time frame. This bending ratio translates to a radius of approximately 40 feet (12 meters) from vertical to horizontal.
Extended lateral sections and deep hole short radius projects prove to be additional drivers for the development of this system, as safety concerns were raised over the use of a side-entry sub, and the associated wireline, for a steering tool when encountering a pressure situation.

DEVELOPMENT
Critical to the success of the system was the ability to remove bending stress from the probe modules which house the sensor, electronic, power, and pulser components. This was done by placing patented flexible tandems between modules and selecting the proper collar material to house the probe. Tool fit charts were developed, using the principle illustrated in Figure 1, to determine module lengths and their associated minimum curve radius. In addition to the concern over module length was the fatigue associated with the drillstring tubulars, and the frequency of passes through the actual wellbore radius. Through the use of Gerber Fatigue Criteria, a Von Mises stress limit was established for the complete downhole assembly (Figure 2). From this, guidelines were specified to minimize the possibility of stress cracks in the drillstring components.