Logging-While-Drilling (LWD) Results in Indonesia

Abstract

ABSTRACT 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 by way of mud pulse telemetry. Drilling related parameters such as down hole weight on bit and downhole torque are also transmitted in real time. This system has been 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 zones 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 useful for determining coring points and improving the casing point location, due to precise correlation in real time. Logging While Drilling measurements 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, Logging While Drilling (LWD) measurements have aided in the correct identification of gas zones, that wireline logs incorrectly identified as oil zones. This identification error was the result of significant invasion that occurred prior to wireline logging. EQUIPMENT The Logging While Drilling equipment was placed into operation in July of 1990. The LWD equipment included Compensated Dual Resistivity (CDR)* and Compensated Density Neutron (CDN)* tools, a computer surface unit, and a downhole measurement while drilling (MWD) system. Downhole weight on bit and torque along with directional data are transmitted to the surface through the mud column (Fig. 1). The CDR tool makes a borehole compensated electromagnetic propagation measurement at 2 MHz with two depths of investigation, 35 to 65 inches and 20 to 45 inches depending on true resistivity. The measurement behaves similar 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 in. and 8 in. drill collars which are used for 8-1 /2 in. and 12-1/4 in. hole sizes. The 8 in. collar can be used for 17-1/2 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.