Magnetic resonance is best known for differentiating between producible and non-producible fluids. This unique capability came originally with significant operational requirements and drawbacks. In wireline, early magnetic resonance tools ran at an extremely slow speed and required extensive pre-planning to get around some of the hardware limitations. In LWD, vibrations of the bottom hole assembly prevented in many instances a direct measurement of T2 during drilling, often requiring a separate sliding pass for data acquisition. This operational complexity accounts in part for the slow acceptance of magnetic resonance by the industry.
By returning to basics and by optimizing the fundamental sensor design and measurement method, it was possible to design a new LWD magnetic resonance device tolerant to vibration and capable of acquiring T2 under normal drilling conditions. At the heart of the new design is a static field that is nearly flat over a relatively broad zone of investigation. The absence of a high gradient field may be seen as a drawback for those who wish to study diffusion, but on the positive side, enables T2 measurement while drilling and simplifies interpretation of the data for the purpose of measuring total porosity, clay bound water and capillary trapped water. Besides, alternate methods such as a dual wait time measurement can be successfully applied to recognize the presence of light hydrocarbon. One additional inherent advantage of the sensor design is a sharp nominal bed resolution.
The new device was applied to a multilayer reservoir in the Adriatic Sea. The availability of NMR-LWD in this area integrates the while-drilling suite, further reducing the number of wireline runs required to assess reservoir quality, with consequent rig cost savings. As well as measuring capillary bound water and total porosity, NMR is sometimes the only source of a clay indicator in the radioactive sands of the Adriatic Sea. The job ran while drilling with default acquisition parameters. There was no noticeable interference with the drilling operation. From a quick look interpretation, the magnetic resonance log identified immediately the layers with movable fluid and quantified total and effective porosity in many zones where the combination neutron-density would have required considerable petrophysical analysis and local knowledge input to reach the same result. The vertical resolution was seen as an improvement over the wireline tool run for comparison, resulting in better characterisation of the laminated sand-shale sequences.