Logging-while-drilling (LWD) services are today often provided by several separate tools that, when used in combination, result in some measurements being made far from the bit. Time to make up and break out LWD tools, memory dumping, as well as multiple tool reliability issues can significantly add to the well construction costs.
An innovative new LWD tool has been developed to simultaneously provide drilling-related measurements and an industry standard suite of formation evaluation measurements, equivalent to the classic "triple-combo" service, but co-located and closer to the bit, using only a single short collar. This means that less rathole is needed for logging, and fewer collar connections have to be made, resulting in a more efficient drilling operation. In addition, the co-location of the sensors reduces uncertainties in the interpretation of measurements that rely on multiple sensors.
A key focus during the development of the tool has been improvement in service delivery. This has been achieved through increased operational efficiency, higher rate of penetration (ROP) capability, significantly improved reliability throughout the system, and greater ease of maintenance. The use of a pulsed neutron source instead of a chemical source also dramatically reduces the environmental and operational risks normally encountered with the use of traditional LWD tools.
Pulsed neutron measurements delivered by this service include formation capture cross section (sigma) and elemental analysis from neutron capture spectroscopy. These new measurements are used to compute mineralogy while drilling. Azimuthal measurements are also available, allowing the formation and borehole to be imaged using a variety of properties. The result is a robust and complete interpretation with significantly reduced uncertainty, as well as a large choice of steering methods for effective well placement.
In this paper we describe the development of this new LWD service and show field-test examples of improved formation evaluation, increased drilling efficiency, and more effective well placement that the tool has provided.
Tool Overview
The new tool is a multifunction LWD tool that combines both drilling and formation evaluation measurements in a single collar (Fig. 1). These measurements include APWD* Annular Pressure While Drilling, calipers, gamma ray, resistivity, density, and porosity. At the heart of the new LWD tool is a pulsed neutron generator (PNG) that allows neutrons to be generated only when the tool is powered, eliminating the need for an americium beryllium (AmBe) chemical neutron source. The PNG provides several new measurements to LWD, such as Best-Phi* porosity, a hydrogen index equivalent, elemental capture spectroscopy providing detailed lithology, formation capture cross section (sigma) providing an alternative to resistivity for fluid saturation calculations, and NGD* Neutron Gamma Density, which is a density measurement obtained from the gamma rays generated from neutron interactions with the formation.1 Therefore, the use of a PNG in the new LWD tool gives the option of obtaining nuclear logs without the use of a chemical logging source.
