As the market drives interest in deeper and hotter wells, logging-while-drilling (LWD) becomes more attractive technically and economically. Hotter holes are more expensive to drill and bring increased safety and economic risks as well. Borehole stability can be problematic, with higher temperature wells, increasing the risk of being unable to log the well with wireline tools or even losing the well entirely. LWD in unstable situations provides much better assurance than with a formation log that is obtained. LWD sensors measuring drilling-related parameters, such as downhole pressure and borehole diameter, can provide information critical to the successful drilling of the well in a hostile environment.
Oil well logging is the process of acquiring data for formation evaluation (FE), the purpose of which is to determine the location of hydrocarbons in the formation, as well as to estimate the quantity in place. The primary measurements on which FE relies are the natural gamma ray, resistivity, neutron porosity, and bulk density. The gamma ray and resistivity measurements assist in locating formations which are likely to contain hydrocarbons, and the neutron and density measurements are used to discriminate between oil and gas, as well as to determine the volume of hydrocarbons in the formation.
For most of well logging history, logs were obtained after the well was drilled. Tools were lowered into the well, and the measurements were obtained as the tools were pulled from the hole by a wireline, which provided both the power and the telemetry for the tools. During the 1980s and 1990s, tools for making these measurements were designed into drill collars. Sperry-Sun Drilling Services was a pioneering company in this field, introducing the first LWD neutron and density measurement tools, as well as the first electromagnetic wave resistivity (EWR) tool. The electromagnetic wave technology evolved to become the most popular type of LWD resistivity measurement in the industry.
While one goal of LWD is to replace wireline measurements, it also has become an essential technology in other areas, providing measurements which enable precision horizontal drilling and real-time information that can significantly enhance drilling efficiency and well safety.
LWD FE sensors, along with telemetry modules for communicating data to the surface in real time, are built into drill collars of various sizes to allow LWD throughout a well. Until recently, most LWD tools had a maximum operating temperature of around 150°C. This provided LWD capability in most interested reservoirs of the world. However, deeper wells where gas is frequently found can exceed this temperature.
To increase its presence in the high temperature (HT) market, Sperry-Sun developed the Solar 175 tool in the 1990s. This is a HT version of a Sperry-Sun system for economically making real-time directional and gamma ray measurements. Since borehole diameters in the deeper, higher temperature wells tend to be smaller, the Solar 175 tool was constructed in the smaller 4¾” diameter drill collar. This system provided the starting point for the HT-LWD system of this project.
This “High Temperature LWD Project” was awarded by the U.S. Department of Energy in September 1997 with Sperry Sun. After this project began, Dresser Industries, the parent company of Sperry Sun, merged with Halliburton Company. Previous to the merger, Halliburton also had a sub-contract under Maurer Engineering. Maurer was contracted with DOE to build a HT, real-time directional/gamma system, consisting of modules for communication, power, directional, and natural gamma ray measurements. The technology and development departments of Halliburton and Sperry Sun were integrated, and the Maurer/Halliburton DOE contract was completed, resulting in a system qualified to operate at 195°C. The knowledge gained in the Halliburton DOE contract contributed to the HT-LWD project.