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.
Nuclear magnetic resonance (NMR) data acquisition and interpretation in carbonate reservoirs is much more challenging than in sandstones, where it is a well-established technology. Heterogeneous porosity distribution, a broad range of pore sizes, a wide variety of complex textures, and low surface relaxivity combine to complicate the picture considerably. The successful practical application of NMR in these reservoirs requires the development of acquisition and interpretation techniques specifically suited to the task. In carbonate reservoirs dominated by intercrystalline or intergranular porosity, NMR can deliver accurate estimates of porosity, permeability, bound-fluid volume, and residual oil saturation. In vuggy, heterogeneous carbonates more complex interpretation models, based on the integration of whole-core and log data, are required for reliable answers. NMR answer products, based on these new techniques, are presented and validated with core data and by comparison to other logs. Introduction In many clastic reservoirs the CMR Combinable Magnetic Resonance tool has proven its ability to easily and accurately provide a number of answers not possible with conventional logging tools. From a single measurement of signal amplitude and transverse relaxation time (T2), it is possible to determine porosity, permeability, and bound- and free-fluid volumes and to estimate residual oil volumes. Extending this success to the carbonate reservoirs of West Texas is the focus of this study. CMR interpretation in these formations is not always straightforward and normal acquisition parameters are not necessarily sufficient to produce data relevant to the task at hand. For instance, under normal reservoir conditions, the oil signal and the water signal cannot be differentiated in most carbonates. Also, permeability estimated using the same simple treatment given to sandstones does not always match up well with core permeability. Despite these hurdles, quality answers are still attainable. The CMR* tools' accurate lithology-independent porosity is often critical in these complex carbonate reservoirs. Correct bound-fluid volumes are easily obtained using the right cutoff. Good permeability estimates are possible in carbonates, although this may initially require calibration versus core data and other logs in each field. And finally, a simple mud- doping procedure will allow the correct determination of residual oil saturation (ROS). NMR Petrophysics of Carbonates Petrophysically speaking, the most obvious difference between carbonates and sands lies in the heterogeneity of porosity distribution. In general, carbonates can be said to possess a wider range of pore sizes and geometries than sandstones, which are homogeneous and predictable by comparison. This gives rise to a number of physical properties in carbonates that directly affect NMR measurements. First, there are the properties that affect the T1 and T2 distributions of the formation. Because a wider range of pore sizes occurs in carbonates, the T2 distribution will generally be more dispersed than in sands. The largest of these pores will result in very long relaxation times; we show that this directly impacts logging speed and interferes with residual oil measurement. Additionally, an inherent matrix property of carbonates, low surface relaxivity, makes for longer relaxation times (Timur, 1972). Sandstone reservoirs consistently contain about 1% iron by weight. This results in a surface relaxivity of about 15 microns/s. By contrast, a typical carbonate matrix contains less impurities and has surface relaxivity in the range of 5 microns/s (Chang et al, 1994). P. 217^
Following the Gulf of Mexico Deepwater drilling moratorium in 2010, the industry focus towards well integrity assurance has significantly increased. Several new and updated regulations and best practices have been published in the last two years, including API Standard 65 -Part 2 and API RP 96. These two industry accepted standards highlighted that determining the hole volume to confirm the cement slurry volume, to fill-up the annulus to the designed top of cement, is one of the many factors impacting cement placement success during well construction process.A new solution for riserless section was developed based on existing Logging While Drilling (LWD) electromagnetic propagation resistivity measurements, which meets the requirement to understand hole volume drilled with water based mud in deepwater environment.Historically, deriving an accurate caliper from LWD electromagnetic propagation resistivity measurements has never been easy due to big uncertainty of mud resistivity (Li et al, 2003). The implementation of a novel simultaneous inversion model and forward modeling database from standard 2-Mhz propagation resistivity, for water-based mud (WBM) and large boreholes, provided the solution to overcome that uncertainty (Whyte et al, 2012).This novel solution was specifically developed to address the needs for the riserless top-hole sections of high cost deepwater wells: from cement volume calculations, identification of borehole degradation through time, and new opportunities for identification of shallow water flows using the full capacity from the inversion process.The extensive validation of this innovative approach with wireline mechanical calipers in numerous hole sections resulted in far better results than initially anticipated (Whyte et al, 2012). The information obtained provided significant insights into the reliability and limitations of the current algorithm. The ability to monitor the borehole size while drilling, as well as analyzing the reaming and trip out passes from recorded data, makes this measurement a valuable source of time-lapse information. The next validation process consisted in the comparison between cement volumes computed using these measurements against identification of cement returns during riserless cementing operations in deepwater wells.The LWD caliper derived from propagation resistivity measurement was analyzed in more than a dozen of wells in Gulf of Mexico. Spefic case studies covering the drilling and cementing operations are presented in this paper.
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