The use of radioactive source porosity measurements is strictly prohibited in Bohai Bay, China. As an alternative, logging-while-drilling nuclear magnetic resonance (LWD-NMR) has been applied.
In one block of the Bohai Bay field, it is a big challenge to identify and quantify hydrocarbon reservoir sections due to little resistivity contrast between reservoirs and water zones. LWD-NMR technology and the azimuthal resistivity tool is able to solve this challenge. The low-gradient LWD-NMR tool not only measured porosity also provide T2 distributions, which can be used for fluid type characterization. The azimuthal resistivity tool has a deep azimuthal resistivity measurement that can identify nearby bed boundaries, up to 5 m from the borehole, while drilling. This information is available in real-time to the operator for decisions while drilling. The combination of the two measurements is capable of differentiating physical properties of the reservoir and providing information on fluid types. Thus, the operator can easily make decisions to optimize the wellbore placement and perform an accurate petrophysical evaluation of the reservoir.
A pilot hole and a horizontal hole are presented in this paper to show the advantages of LWD-NMR and the azimuthal resistivity respectively. In the pilot hole, the LWD-NMR has been used for fluid typing which can give a clear petrophysical interpretation of the reservoir fluids. Because in this low-resistivity contrast reservoir, it is difficult to differentiate oil zones (10+ Ohmm) and water zones (9- Ohmm) from resistivity data. The light oil, however, shows a much slower NMR T2 relaxation time than the water. Based on the identification of oil versus water by NMR in the pilot hole, three horizontal wells were successfully drilled in the oil zone. In one of them, LWD-NMR and azimuthal resistivity have been employed simultaneously for optimal wellbore placement by enabling to drill in the best part of the reservoir. The LWD-NMR measurements indicated reliably the presence of hydrocarbons and provided important reservoir properties. The azimuthal resistivity measurements identified the reservoir boundaries and the distance to them.
Under complex reservoir conditions as encountered in Bohai Bay, azimuthal resistivity and LWD NMR prove to be an accurate and robust source-less measurement combination. LWD NMR can effectively replace the traditional density-neutron source measurements for porosity. More importantly, the combination with azimuthal resistivity provides the operator with additional critical information for optimal wellbore placement and strengthening the confidence in petrophysical interpretation results.