Downhole nuclear magnetic resonance (NMR) measurements are evolving into a powerful formation evaluation tool, providing unique and critical information including formation porosity, pore-size distributions, bound-fluid volume (BFV), free-fluid volume (FFV), permeability, and fluid properties. Obtaining this information while drilling can have a significant impact on drilling and completion decisions. In addition, low rates of penetration common in many drilling environments can be advantageous in improving the NMR measurement statistics.
This paper describes results gained during field tests of a new NMR logging-while-drilling (LWD) tool that has been designed to run in any standard measurement-while-drilling (MWD) bottomhole assembly. The new tool presents no special operational complications in terms of mechanical specifications or wellsite hardware and software, and it has been tested successfully in both real-time and recorded modes in a wide range of formations and drilling conditions. A key consideration in the design of this tool has been to deliver an NMR measurement of wireline quality with a minimum of interference to the drilling process. To this end, the tool is usable in various modes of operation (stabilized, unstabilized, while drilling, while reaming, etc.).
The detrimental effects of tool motion on the NMR measurement are minimized through the hardware design. Motion-effects modeling and log examples address these issues. Having the capability of acquiring data in the conventional T2 mode, this tool offers a familiar interpretation strategy for those users accustomed to evaluating wireline NMR data along with a significant advantage in statistical precision over a T1 acquisition mode.
Introduction
Many oil exploration and production companies have been awaiting NMR technology capability in the LWD environment. With the inclusion of an LWD NMR tool in the bottomhole assembly (BHA) transmitting in real time, well-placement decisions that impact the overall economics and productivity of a well can be readily addressed. In highly deviated wells or in difficult logging conditions, an LWD NMR tool is the obvious replacement for wireline NMR data acquisition. Therefore, it is extremely important to design an LWD NMR tool that can produce industry-accepted NMR measurements and that can be added to the BHA with a minimum of additional time, cost, and disruption to the normal drilling process. Desirable features include precise and repeatable measurements, operation at high rates of penetration (ROP), measurements familiar to and accepted by the industry (similar to those made by wireline tools), high vertical resolution, flexibility of placement in the BHA, and real-time calculation and transmission of petrophysical and tool-motion information in the while-drilling, bit-on-bottom environment to reduce the need for additional passes after drilling.
In the past few years, the LWD NMR tool described in this paper has been run in a large number of offshore wells on the shelf and in the deepwater of the Gulf of Mexico, offshore eastern Canada, the North Sea and Nigeria, and in a number of onshore wells in North America. During these field tests, two generations of the tool were run. The first-generation tool was powered only by batteries and a significant improvement came with the second-generation tool that was designed to address the large power consumption of T2-mode acquisition with the inclusion of a dedicated mud turbine. In conjunction with increasing on-board memory, the mud turbine power supply provided the second-generation tool with essentially unlimited downhole run time, which is a key benefit to any LWD tool when dealing with the highly variable nature of the overall drilling process and unscheduled rig operations. In a number of field tests with the battery-powered tools, data acquisition over target objectives could not be accomplished because the tool ran out of power before the objectives had been reached.
