TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractReservoir navigation with LWD resistivity has traditionally relied on matching real time measurements with ideal logs. Reservoir navigation engineers initially build one or more resistivity models including all expected resistivity boundaries such as oil-water contact, reservoir to cap rock interface, faults and unconformities. Then, during drilling, they direct the well and update the earth model by matching actual measurements with forward response model data.Because common LWD resistivity sensors cannot differentiate between an oil-water contact approaching from below and a shale lens approaching from above or from the side, the reservoir navigation engineer fills in the missing information through expertise and local knowledge. In case of complex geology however, such as reservoirs with tilted or rotated fault blocks, multiple fluid contact levels, cross-stratification and shale intrusions, navigation becomes much more challenging and the risk of getting geologically lost is high. In recent years imaging LWD tools were introduced to help reduce the azimuthal uncertainty but they were limited to a few inches in lateral investigation.A new azimuthally sensitive propagation resistivity tool was recently tested for reservoir navigation and formation imaging in some of the more complex reservoirs of the North Sea. In cases where standard omni directional tool responses would lead to ambiguous interpretations, the azimuthally sensitive tool provided the basis for clear geosteering advice. A new imaging algorithm helped visualize approaching beds much like modern imaging devices, but with a depth of investigation reaching several feet into the formation. At fault crossings, the azimuthally sensitive signal helped recognize the relative movement of the formations on either side of the fault. In other instances where the well was run immediately below the cap rock, deep looking azimuthal propagation anticipated the intersection by several hundred feet. Also, analysis of the detailed deep electrical images brought a more complete understanding of the subsurface.
Summary Optimal field development often entails placing the wells in prescribed locations within the reservoir. An error of a few meters in height above the oil/water contact or with respect to the roof may result in leaving behind a significant portion of the producible reserves. Driven by this key requirement, new technologies continue to emerge to help geologists, drillers, and reservoir engineers geosteer the wells. In recent years, two types of logging-whiledrilling (LWD) information have been used. On one hand, wellbore imaging can determine when a well path has left the reservoir and the angle of exit. On the other hand, traditional axisymmetrical resistivity logs help to quantify the distance to an approaching boundary through inversion, but fail to tell its azimuth. A newly deployed azimuthal deep resistivity instrument recognizes an approaching geological event before it intersects the well while continually imaging it at multiple depths of investigation. Of particular interest is the azimuth of approach with respect to the well path, advising in real time the most favorable change of direction. In addition, a series of transverse electromagnetic measurements specific to azimuthal resistivity, called geosignals, are presented. Geosignals help to quantify the distance and the rate of approach with great accuracy, before the actual intersection could occur. With this real-time information, geosteering engineers can remain at prescribed distances from important boundaries, including oil/water contacts and overlying shale roofs. Modeling and actual logs demonstrate that the new LWD instrument performs at its best when the reservoir is overlaid by shale. Modeling suggests, however, that suboptimal performance occurs in reservoirs with very resistive caprock, such as anhydrite.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractReservoir navigation with LWD resistivity has traditionally relied on matching real time measurements with ideal logs. Reservoir navigation engineers initially build one or more resistivity models including all expected resistivity boundaries such as oil-water contact, reservoir to cap rock interface, faults and unconformities. Then, during drilling, they direct the well and update the earth model by matching actual measurements with forward response model data.Because common LWD resistivity sensors cannot differentiate between an oil-water contact approaching from below and a shale lens approaching from above or from the side, the reservoir navigation engineer fills in the missing information through expertise and local knowledge. In case of complex geology however, such as reservoirs with tilted or rotated fault blocks, multiple fluid contact levels, cross-stratification and shale intrusions, navigation becomes much more challenging and the risk of getting geologically lost is high. In recent years imaging LWD tools were introduced to help reduce the azimuthal uncertainty but they were limited to a few inches in lateral investigation.A new azimuthally sensitive propagation resistivity tool was recently tested for reservoir navigation and formation imaging in some of the more complex reservoirs of the North Sea. In cases where standard omni directional tool responses would lead to ambiguous interpretations, the azimuthally sensitive tool provided the basis for clear geosteering advice. A new imaging algorithm helped visualize approaching beds much like modern imaging devices, but with a depth of investigation reaching several feet into the formation. At fault crossings, the azimuthally sensitive signal helped recognize the relative movement of the formations on either side of the fault. In other instances where the well was run immediately below the cap rock, deep looking azimuthal propagation anticipated the intersection by several hundred feet. Also, analysis of the detailed deep electrical images brought a more complete understanding of the subsurface.
A recently introduced azimuthal resistivity LWD imaging tool has been upgraded with advanced high resolution sensors that are capable of differentiating reservoir and borehole features down to a size of 0.4 in. when drilling in well consolidated formations. The high vertical and azimuthal resolution, along with 100% borehole coverage, yield an image quality comparable to that of wireline service for applications that include fracture characterization and formation evaluation. This paper describes a field test of the high resolution tool in 5 7/8" and 8 3/8" holes in Saudi Arabia and shows the application of LWD images for estimating carbonate reservoir producibility involving the characterization of secondary porosity. The LWD imager provides significant economic and logistic benefits, especially in slim horizontal sections; in addition, it can identify fractured zones with mud loss potential shortly after penetration. The real-time resistivity provides a good basis for accurate dip calculation and geosteering in general. In its default configuration, the high resolution tool is equipped with six high resolution sensors arranged in two rows. One of the benefits of the multi-sensor configuration, demonstrated by the field test, is the ability to validate the image quality by comparing data from various sensors. Another benefit is the depth correction achieved by correlating images from identical sensors located at various depths. The paper also discusses the fundamental principles behind high resolution resistivity imaging in conductive mud and makes extensive use of modeling techniques to characterize the sensor performance in various practical situations.
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