Imaging innovations in logging-while-drilling (LWD) technology enable operators to place wells more accurately while anticipating drilling problems.

The trend toward horizontal and high-angle boreholes from vertical and slanted geometries has established the need for geosteering and formation evaluation during drilling. These more complex well designs increase the risk of wellbore instability problems during drilling operations and require greater control to obtain accurate wellbore placement. Adapting and developing formation evaluation techniques to the downhole drilling environment and incorporating it into measurement-while-drilling technology has led to advancements in LWD measurements that provide important information for geosteering and drilling optimization.
LWD tool advancements include azimuthal measurement capabilities. They record measurements in numerous sectors around the borehole. Detailed formation images are produced from this azimuthally acquired data. Borehole images enhance decision-making with respect to the drilling process. They also serve to reduce the risks inherent with drilling operations.
LWD images initially were available after drilling through a recorded data set. Now they can be transmitted by mud-pulse telemetry and interpreted in real time. Additionally, advances in secure, Internet-based communication technologies make timely data delivery possible to asset teams around the world.
While real-time images aid decisions at the time of drilling, stored data available following drilling is used for the reservoir characterization and geological evaluation required for a field's overall appraisal. "Logging for drilling" is a new phrase describing how timely information is provided and used to define the reservoir environment and refine the drilling process. In other words, logging for drilling provides the real-time data essential for confirming or updating the predicted mechanical earth models during drilling operations. Inconsistencies between prediction and reality may require preventative or remedial actions before reaching the targeted zones.
Most azimuthal data are obtained to increase understanding of a reservoir's geology and petrophysics. However, LWD images also are used to evaluate the geomechanics of the reservoir rock. The images often display features resulting from geomechanical phenomena, the analysis of which improves the geological and petrophysical interpretation of the reservoir. In addition, the geomechanical information provided by real-time images is used to optimize well planning and certain aspects of the drilling program, such as designing appropriate mud weights.
Within the Schlumberger Vision system are two types of LWD tools that have azimuthal measurement and image-generating capabilities. The GeoVision resistivity tool, equipped with laterolog measurements, enables thin bed resistivities to be detected and imaged in conductive mud environments. Detailed borehole images are produced from 56 resistivity measurements taken directionally around the borehole at three depths of investigation.
Available in real time or from the tool memory, this data can be used to image or visualize directly the borehole and the formation. Resistivity images can reveal bedding planes, stratigraphic features and structural dips. Additionally, the resistivity images can be used to identify natural faults and fractures, induced fractures and borehole breakouts.
Another LWD tool with imaging capabilities is the Vision Azimuthal Density Neutron tool, which provides compensated neutron and detailed litho-density measurements while drilling in conductive, nonconductive and oil-based muds. Its enhanced azimuthal capabilities compute density and photoelectric factor measurements in 16 individual sectors. Real-time density data allows geosteering decisions and formation evaluation. Quantitative imaging from the azimuthal density data provides a source of petrophysical and geological information regarding net pay, structural dip and more significant stratigraphic features. Density images will be available in real time within 2001.
Conventional LWD measurements are averaged circumferentially. This technique tends to smear the output at the bed boundaries, especially in horizontal wells when the relative angles of the beds to the wellbore are large. Directionally acquired quadrant information can help detect and evaluate bed boundaries while the bottomhole assembly rotates, enabling reliable measurements in horizontal and highly deviated boreholes. The amount of information and the ease of interpretation increases significantly from directional curves to full wellbore images.
Optimizing wellbore placement
Accurate wellbore placement involves reaching and precisely placing the wellbore within the desired target to maximize production. This requires geosteering, using geological and accurate survey information to steer and optimally position the wellbore in the target reservoir. Azimuthal data in the form of borehole images is used increasingly for decision-making in geosteering applications. Borehole images allow placement of the wellbore to geological features at a scale smaller than the wellbore diameter (inch scale).
The real-time images allow visualization of the borehole relative to the formations, enabling anisotropy around the borehole to be detected and quantified. Bed boundaries are defined clearly, determining when the top and bottom of the borehole cross them (Figure 1). This information reduces geosteering uncertainty, enabling drilling engineers to place boreholes parallel or at a known direction relative to bedding.
Formation heterogeneity, thinner beds and larger stratigraphic features can be identified using density images and higher-resolution resistivity images. High-resolution resistivity images also can reveal subtle stratigraphic features. Images can help differentiate between bed boundaries and other features such as fractures and faults.
Defining geologic structure during drilling is important for accurate geosteering. Images are used to confirm structural position and permit directional changes, if required. Dips for correlation and structural interpretation can be computed using real-time images, reducing uncertainty and improving interpretation of the structural model.
Dip calculations aid in directional well control, particularly in horizontal and high-angle wells. High apparent dips greater than 70° in horizontal or high-angle wells present an ideal situation for LWD imaging tools. In such scenarios, the dip computations that borehole imaging provides are critical to geosteering wells. They use a different technique compared to conventional wireline dipmeter calculations and improve structural and stratigraphic interpretation of a geologic feature's origin.
Dips can be computed automatically downhole in real time, or they can be hand-picked off images at the surface, either from images collected in real time or stored in memory during bit runs (Figure 2). Image-derived dips allow features such as bedding, faults, or natural or drilling-induced fractures to be categorized. This capability reduces uncertainty and makes the images a powerful interpretation tool.
High-resolution resistivity images identify the presence and orientation of fractures. Fracture identification helps optimize well direction to maximize production. Knowing fracture orientation indicates the optimal well trajectory for intersecting the maximum number of fractures. Knowing fracture frequency, size and location along the horizontal section aids in future completion design, remedial plans and reservoir engineering analysis. LWD images have been successful in detecting large fractures and dense groups of smaller fractures (Figure 3). This information helps confirm whether a well's trajectory is sufficiently perpendicular to the fracture trend.
Wellbore instability prevention
Drilling optimization avoids problems by assessing, managing and mitigating risks inherent in the drilling process. One such risk is wellbore instability. Wellbore instability problems encountered during drilling operations require excessive time to solve and may jeopardize future well procedures such as cement zonal isolations. Instability problems can quickly increase well construction costs and reduce completion success, putting a well's economic viability into question.
Downhole drilling mechanics are too complex to be characterized by just one measurement. Experience has shown that by combining downhole measurements, synergies result, allowing better understanding of how drilling operations affect the borehole. Drilling effects on the borehole influence the LWD measurements. Incorporating real-time images with conventional LWD and drilling data can improve interpretation dramatically and provide remedial strategies to optimize drilling operations.
The earth's far-field stresses are converted to wellbore stresses at the borehole wall. When the borehole pressures either exceed the formation strength or fall below the confining pressure during drilling, near-wellbore deformations occur. These can be irreversible and catastrophic. The optimum time to analyze how formations respond to stress is when a borehole is constructed, and images help diagnose most wellbore failure mechanisms.
Developing strength and stress profiles are the first step in understanding potential problems regarding a wellbore's stability. When combined with annular pressure while drilling (APWD) data, LWD images can be used to calibrate and refine the estimated strength and stress profiles, which in turn are used to generate a wellbore stability forecast. The forecast helps with identifying possible drilling hazards and designing corrective actions such as appropriate mud densities, thereby optimizing the drilling process.
Borehole effects or drilling-induced changes range from formation invasion to mechanical failures such as sloughing, fractures and breakouts. Real-time images help differentiate between features created by geological events and subsequent drilling activities, natural as opposed to induced fractures. Differentiating between the two fracture types permits modifications to the drilling program to minimize negative impact and ensure accurate formation evaluation.
For example, resistivity images generated for three depths of investigation reveal information about petrophysical measurements and drilling effects on the borehole. While drilling-induced fracture effects will fade with increasing investigation depths, natural fractures will not. Although these are memory images, similar effects are seen on real-time multipass images (time lapse).
Real-time resistivity and density images combined with APWD data also can identify formation breakouts as well as determine whether they are natural or induced. Recognizing formation breakdown is paramount in avoiding costly remedial operations. Integrating LWD images with APWD data not only distinguishes drilling-induced alterations but also determines the mechanisms of the borehole's failure.
Acknowledgments
The authors thank Schlumberger for permission to publish this article. The authors also thank Frank Hood, business development manager, Schlumberger Drilling & Measurements, for his contributions to this article.
References
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