ABSTRACT: Logging While Drilling (LWD) Results in Indonesia
NORBY, PHIL, Unocal, Houston, TX, and KEN HENRY,* Schlumberger, Houston, TX
The engineering of well log sensors into drill collars enables the recording of Formation Resistivity, Gamma Ray, Neutron Porosity, Bulk Density, and Photoelectric Factor (Pe). Data are recorded downhole and selected data are transmitted to the surface in real time via mud pulse telemetry. Drilling related parameters such as downhole weight on bit and downhole torque are also transmitted in real time.
This system is being utilized in Offshore Kalimantan in both exploration and development wells drilled with oil base mud (OBM). Exploration wells drilled with OBM are difficult to evaluate because of invasion which can cause wet zoners to appear potentially oil bearing. The real time Logging While Drilling (LWD) system successfully identified hydrocarbon zones soon after drill bit penetration. Another benefit of the real time measurement was to provide accurate
measurements in zones which soon after drilling became thief zones and caused loss of circulation. Real time transmission was very useful for determining coring points and improving the casing point location, due to precise correlation in real time.
Logging While Drilling measurement are comparable to wireline measurements, and in some instances superior to wireline logs when invasion effects are great or when borehole deterioration has affected the quality of wireline logs. On several occasions, the LWD measurements have aided in the correct identification of gas zones, that wireline logs would have been incorrectly identified as oil zones. This is due to significant invasion which had occurred prior to wireline logging.
The LWD equipment was placed into operation in late July of 1990. The LWD equipment included Compensated Dual Resistivity (CDR)*, Compensated Density Neutron (CDN)*. and a computer surface unit along with a downhole measurement while drilling system (MWD). Downhole weight on bit and torque along with directional data are transmitted to the surface via the mud column (Figure 1).
The CDR tool makes a borehole compensated electromagnetic propagation measurement at 2MHz with two depths of investigation, 35 to 65 inches and 20 to 45 inches depending on true resistivity. The measurement behaves similarly to induction measurements with similar limitations. The CDR tool also contains a spectral gamma ray measurement yielding relative proportions of thorium, uranium, and potassium. It operates in all drilling fluids, including salt muds, oil base muds, and air filled holes. The transmitter and receivers are placed inside 6.5 and 8-in. drill collars which are used for 8.5 and 12.25-in. hole sizes. The 8-in. collar can be used for 17.5-in. hole size or larger with acceptable results.
The CDN tool gives epithermal neutron porosity, formation bulk density, and photoelectric factor, Pe. The density measurement is borehole compensated and employs two gain stabilized photomultiplier/crystal scintillation detectors. This type of detector used in conjunction with low density "windows" in the drill collar, enables spectroscopy methods to be employed. The photoelectric absorption factor of the formation is also measured for use in lithology identification. The borehole compensated, epithermal neutron measurement provides a high quality porosity log that is free of the many interferences that degrade a capture gamma ray measurement (Figure 2).
The radioactive sources, 7.5-curi americaum-beryllium for the neutron measurement and 1.7-curi cesium 137 for the density measurement are safely within the collar and are connected to each other by a solid link so they can be easily fished by slickline if the drill collars get stuck. The sources have been successfully fished twice during operation in East Kalimantan, once at a depth of 11,800 ft with 15.4 lb/gal OBM.
The CDN tool must be positioned above the CDR and MWD to permit fishing of the nuclear sources. The makeup of the bottomhole assembly dictates the lag between CDN and CDR data.
AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)