Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. "Logging While Drilling" is one form of "measurement while drilling" - to differentiate it from mud logging, wireline logging, etc. See Logging While Drilling.
Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. Some downhole tools can be controlled by mud pulses and some data is transmitted from bottom hole to the surface via pressure pulses.
Directional surveys obtain the measurements needed to calculate and plot the 3D well path. Instruments for conducting directional surveys can be set up in several different variations, depending on the intended use of the instrument and the methods used to store or transmit survey information. Depending on the method used to store the data, there are film and electronic systems. Survey systems can also be categorized by the methods used to transmit the data to the surface, such as wireline or measurement while drilling (MWD). Magnetic sensors must be run within a nonmagnetic environment [i.e., in uncased hole either in a nonmagnetic drill collar(s) or on a wireline].
Telemetry methods had difficulty in coping with the large volumes of downhole data, so the definition of MWD was broadened to include data that were stored in tool memory and recovered when the tool was returned to the surface. Power systems in MWD generally may be classified as one of two types: battery or turbine. Both types of power systems have inherent advantages and liabilities.
When downhole tools that collect data were created, they stored the data in memory on the tool itself. The data were downloaded when the tool was next pulled from the hole. Communication with downhole tools while drilling is currently achieved with either mud-pulse telemetry or electromagnetic-based systems. The maximum data transmission rate (correlated with bandwidth) of these systems is about 10 bits per second. As a result, much of the information from measurement while drilling (MWD) and logging while drilling (LWD) must be processed and stored in computer memory associated with the downhole instrumentation near the drill bit.
MWD is now standard practice in offshore directional wells, where the tool cost is offset by rig time and wellbore stability considerations if other tools are used. The measurements are made downhole, stored in solid-state memory for some time and later transmitted to the surface. Data transmission methods vary from company to company, but usually involve digitally encoding data and transmitting to the surface as pressure pulses in the mud system. These pressures may be positive, negative or continuous sine waves. MWD tools that measure formation parameters (resistivity, porosity, sonic velocity, gamma ray) are referred to as logging-while-drilling (LWD) tools.
Successful underbalanced drilling (UBD) requires downhole equipment to provide real-time information to the surface for monitoring conditions during drilling operations. Pressure while drilling (PWD) sensors have proved invaluable in every UBD operation to date, when they have been included in the drillstring and operated without downtime. However, quite a number of these sensors have proved problematic, because of the vibration problems and fast drilling rates encountered with UBD. The most common technique for transmitting measurement while drilling (MWD) data uses the drilling fluid pumped down through the drillstring as a transmission medium for acoustic waves. Mud-pulse telemetry transmits data to the surface by modifying the flow of mud in the drillpipe in such a way that there are changes in fluid pressure at surface.
No other technology used in petroleum-well construction has evolved more rapidly than measurement while drilling (MWD) and logging while drilling (LWD). Early in the history of the oil field, drillers and geologists often debated conditions at the drillbit. With advances in electronic components, materials science, and battery technology, it became technically feasible to make measurements at the bit, and transmit them to the surface so that the questions could be answered. Directional measurements were the first measurements to have commercial application, with almost all use in offshore, directionally drilled wells. As long as MWD achieved certain minimum-reliability targets, it was less costly than single shots, and it gained popularity accordingly.
Results to date are compared with previous performance in the Gulf of Thailand (GoT). The purpose of this paper is to demonstrate how inaccuracy in standard directional-surveying methods affects wellbore position and to recommend practices to improve surveying accuracy for greater confidence in lateral spacing. Wellbore position is computed from survey measurements taken by a measurement-while-drilling (MWD) tool in the bottomhole assembly (BHA).