Magnetic Resonance While Drilling: a Quantum Leap in Everyday Petrophysics

Abstract

Abstract Magnetic resonance (MR) logging was originally intended for measuring fluid filled porosity, and for differentiating between producible and non producible fluids. Early tools however came with special operational requirements. Wireline logs were slow and sensitive to wash-outs; early logging while drilling (LWD) measurements did not tolerate tool vibrations. Both required a good deal of preplanning to accommodate limited capability hardware. The paper shows how problems of the past could be solved by the innovative technology of Magnetic Resonance Logging While Drilling (MR-LWD). By returning to basics a new LWD magnetic resonance tool tolerant to vibration and easy to apply could be designed. The novel device acquires valuable data which can be used as well for formation evaluation as for geo-steering under normal drilling conditions and with standard operating practices. This was made possible by a short inter-echo spacing, by a special stabilization, and by a low magnetic field gradient. The new sensor operates with a minimum of preplanning and requires little to no interference with the drilling process. We present an application of the new system in a European onshore well. In the case described we show how reliable deliverables such as porosity and pore-size distribution can be determined from MR-LWD. Formation boundaries and fluid contact can easily be determined. The clear logs and the intuitive presentation of data makes MR while drilling ready for everyday petrophysics. Introduction Magnetic resonance logging in its current design was constantly improved, since it started to establish in the early 1980s. Still MR is not considered a "conventional" measurement. Over the years many of the operational limitations were overcome and in parallel sophisticated acquisition methods have been developed. MR-LWD has only been introduced several years ago[1] and is now getting ready to fill a gap in Formation Evaluation (FE), by providing lithology independent porosity, pore-size distribution and permeability index while drilling. LWD-specific limitations such as the motion-sensitivity of the MR measurement have been realized and are overcome by motion insensitive design of the measurement system and means to reduce motion effects on the measurement. On the other hand the implementation of a MR measurement in a drillstring offers possibilities like the improvement of vertical resolution by exploiting the smaller axial tool velocity compared to wireline; and the measurement of MR before invasion of drilling fluid and borehole breakouts severyly impact the signal. By taking into account a few basic design considerations a new, easy-to-operate MR-LWD tool that can tolerate even harsh drilling conditions has been built and applied in the field. Insensitivity to motion was achieved by a short inter-echo spacing TE, by a special stabilization, and by a low magnetic field gradient G. The new device acquires valuable data which can be used for standard FE as well as for real-time geosteering. A minimum of preplanning is required and the sensor causes little to no interference with the drilling process. Preprogrammed measurement modes can be used for real-time data acquisition. The measurement mode can be switched by a bi-directional data communication directly during the drilling process. MR Deliverables One of the outstanding strengthes of MR logging lies in its ability to measure fluid-filled porosity and to differentiate between producible and non producible fluids. The porosity determined by MR-LWD is independent of lithology and is acquired without radioactive sources. Thus, it can be used as an alternative or complementary porosity to the commonly used Density and Neutron nuclear tools. Additionally MR measurements can be carried out in a well defined zone of investigation which lies well within the formation and does not contain any contribution from borehole fluid. The application becomes particularly important where nuclear sources are not desired or allowed due to safety, environmental, or import regulations.