Combined with other measurements, neutron porosity tool provide basic and important parameters for real-time formation evaluation and geosteering. In earlier neutron porosity LWD systems, the porosity instruments containe an isotopic chemical source which emit fast neutrons.
Using the chemical neutron source, it should be meeting the problem of handing, shipment, and storage. For radiation safety consideration, non-chemical source based neutron instruments will be the best choice for measurement while-drilling. A new type of non-chemical source based neutron porosity instrument has been developed and prototyped.
The instrument was designed for porosity measurement while-drilling. it consists basically of an accelerator neutron source and several neutron detectors. The accelerator based neutron generator source can be switched on and off by electronic circuit and software control.
The development of new porosity instrument includes two parts - method study and tool design. In method studies, computer modeling was use to tool structure designs and detector parameter choices. Both computer modeling and experimental measurements made in standard formation were used to determine the tool responses. The field test has been made with two prototype tools.
Comparing the measured porosity data with wire-line neutron logging data, they are in good agreement. The preliminary test and data analysis show that the designed tools have many new features over chemical source based neutron porosity tools. This paper will highlight the technical development and present helpful case histories and recommendations.