Today, with increasing wellpath complexity and the need to make the best economic decision as quickly as possible, real-time logging-while-drilling (LWD) images can play a significant role whenever operators face critical wellbore stability and geological well placement decisions. In addition to their impact on time-critical decisions, LWD images are also the preferred option when traditional wireline techniques pose an unacceptable risk.

What is LWD imaging?
Conventional LWD measurements are designed to be omnidirectional, reducing the sensitivity to drilling dynamics and providing the best bulk average rock property. The bottomhole assembly (BHA), however, provides a rotating platform for sensors, ideal for producing a 360° scan of the borehole wall.

By using directional shielding for nuclear measurements, focusing for shallow electrical and

Figure 1. A high-resolution electrical image showing a fossilized soil with several distinct components: 1410.0 – 1412.3 ft: Laminated mudrocks with no sedimentary disturbance. 1412.3 – 1414.5 ft: Sub-vertical, branching resistive streaks. These are calcretized (fossilized) root traces from vegetation growing at the surface. 1414.5 – 1416.4 ft: Mostly structureless mudrock or marl with a minor calcrete development. The lower laminated interval is preserved background lamination. 1416.4 – 1419.1 ft: Mudrock or marl containing nodular (enterolithic) calcrete concretions. The vertical orientation to some of these features is caused by the vertical migration of water in the when lost by evaporation. The maturity of palaeosols is a useful guide to the proximity to fluvial sandbodies. Recognition of such features within mudrocks can provide useful information for advanced geosteering.
acoustic measurements and sophisticated sensor and processing techniques for deep resistivity, a variety of oriented readings can be produced. A magnetometer package, as used in the directional surveying tool, tracks this scanning motion around the borehole.

These readings are usually displayed as an image that can be thought of as similar to a digital photograph, with individual readings representing colored pixels. Depending on the tool’s resolution, the area these pixels represent may range in size from a few millimeters square to a rectangle 100 by 200 millimeters.

The images can be visualized two ways, either in 3-D space, wrapped around the inside of the well, readily showing the relationship between well bore and formation or unwrapped from the well bore on a flat log, designed for analysis of dips and geology.

In order to obtain high-resolution LWD images in real-time, increased telemetry bandwidth is typically required along with a software toolkit and expert geoscience interpreters to turn the data into valuable information.

Comparing wireline and LWD images
While the end results and method of interpretation of both LWD and wireline images are very similar, the acquisition platforms are distinctly different and each has its own benefits and limitations.

Wireline images are acquired on a depth basis, with the wireline operator controlling the cable speed, in a static fluid column. The borehole has often been open for some time and degradation will be at a maximum. And, while sensor resolution is typically greater than LWD, the borehole coverage is often less in the case of pad-based devices. In comparison, LWD images are acquired in time and converted to depth, in a dynamic environment, with a constantly changing mud column. While the LWD operator often has no control over logging speed, the borehole is typically in its most pristine state and the LWD device will often makes multiple passes over the same formation interval as part of the drilling and hole cleaning process.

Also, when it comes to employing data, wireline images can only impact the well’s completion program or the next well whereas real-time LWD images permit changes to the current well.

Drilling and evaluation answers
LWD images can provide timely answers for both drillers and geologists. Drilling questions often center around hole-stability issues. In contrast, the multiple geological questions image logs are used to answer tend to focus on hydrocarbon volumes and production strategies.

Some of the more specific drilling and evaluation questions operators are using real-time images to answer include:
• What is the orientation of the reservoir in 3-D?
• Are beds dips from seismic correct?
• Are there fractures in the well bore?
• Can I steer in the reservoir’s “sweet spot”?

Examining case studies offers a glimpse of how some of these real-time images have been used to refine pre-well seismic models, optimize wellbore placement, improve completion decisions, and enhance geological interpretation.

LWD images can provide great value in upper hole sections. The resolution of the pre-well geological model may vary, but some level of uncertainty is always present. Correlating
Figure 2. This image image, showing a series of complex thin beds, has been wrapped around a borehole in three dimensions allowing the relationship between fracture sets, the geological model and changes in well trajectory to be seen and shared more easily using real-time visualization technology.
position on a model by using offset geological information is useful, but if one can attribute thickness variations to dip changes, rather than to lateral geological changes, uncertainty is reduced significantly. In the North Sea’s Brent field, an operator was drilling
a series of wells through fault blocks. Based only on the seismic data, it was unclear how to properly steer the well to intersect the faults at the optimal angle for reservoir characterization and ultimate production. Using the real-time density images, the operator’s geologists were able to identify the bed dip within each fault block, quickly determine when each fault was crossed and adjust the wellpath as needed prior to intersecting the next fault. Geological correlations based on these images proved more reliable than curve data alone.

Questions regarding wellbore integrity, which is especially critical in extended horizontal sections, can often be answered using real-time images. In a recent well in Trinidad, real-time density images were used to identify wellbore breakout. The operator can use this data while drilling to optimize mud weights and drilling parameters to enhance wellbore stability. On a well in the North Sea, electrical LWD images helped the operator identify drilling-induced fractures in the reservoir. This data permits the operator to adjust drilling properties and refine their geomechanical model to minimize non-productive time on future wells within the field.

LWD images can also play a critical role in properly landing the well within the reservoir. On a series of wells in China, the operator used real-time gamma images to correlate well trajectories on the fly — optimizing well entry into, and placement in, an extremely thin reservoir (<1 m). Recently in the Middle East, an operator transmitted LWD images (density and electrical) to a data center to provide real-time geosteering guidance through a complex carbonate reservoir. The results were 100% net-to-gross and an improved understanding of the well’s fracture pattern.

In other wells, high-resolution LWD images have shown fracture sets, some sub-seismic faults with watering potential and few moderately cemented zones, allowing the reservoir engineering teams to optimize the completion program without waiting for wireline logs to be run.

Conclusion
LWD imaging is a rapidly developing technology that now offers 360° borehole images comparable to wireline quality. Still some challenges remain, including learning the subtleties of LWD image interpretation and effective integration of this information into the operating program.

However, because LWD images are available to operators in real time, they permit the optimization of well placement and wellbore stability, which improves recovery and enhances drilling efficiency.