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Neutron porosity measurement could provide basic and important parameters of the formation while drilling and it is indispensable for the real-time comprehensive formation evaluation and geosteering. As deeper and more complex reservoirs are being drilled, the needs of slim-diameter tools are increasing. This paper presents a new neutron porosity LWD tool using a generator source. The tool can operate in environments with pressures up to 172 MPa and temperatures to 175 degC, and can be combined with the 4.75-in geologic steering drilling system.Most neutron porosity tools, including wireline and LWD devices, use Americium-Beryllium (AmBe) and Cf-252 radiological sources which have great impact on the environment and have problems of handling, shipment and storage. This tool in hardware includes a pulsed neutron generator (PNG) and three Helium-3 (He-3) filled detectors. With two different source distance combinations, the tool could be working in multi-detection or high-sensitivity measurement mode which would adapt the tool to different kinds of formation. Standard well tests show that the data of this tool is in good agreement with the wireline logging tools. It also get good results in temperature and pressure test. All these preliminary tests and data analysis show that the tool could be very helpful in formation evaluation and reservoir monitoring in HT-HP conditions. This paper also presents compatibility design of the tool and possible applications as a part of the 4.75-in geosteering drilling system.

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.