In oilfield drilling operations, “logging” is the process that enables the operators to collect data about the formation through which they are drilling. The graphical representation of the geological data acquired in the process is called a log.
Before drilling begins, operators must decide which type of data to acquire (drilling mechanics, resistivity, density, porosity, permeability, Gamma Ray, etc.), and determine how the geological data will be acquired: on wireline or while drilling.
In wireline logging, an electrical cabling system is used to lower tools or measuring devices into a borehole, that then collect and transmit wellbore properties data. The wireline logging operations are carried out after the drilling operation has been completed.
Alternatively, “logging while drilling (LWD)” technologies enable data collection in real-time or in a recorded mode as the drilling operations are in process. The LWD tools are made up to the Bottom Hole Assembly (BHA), along with MWD (measurement while drilling), RSS (rotary steerable system), and other “dumb” iron (non-smart equipment).
Drilling operators generally agree that there are numerous benefits of running LWD technologies over wireline:
LWD tools are heavier, tougher and more robust
The tools acquire data continuously during the drilling operations without interruption (they are transparent to drilling)
Most LWD equipment has no directional logging limitations: the tools can be run in high dog-leg and high inclination angles
Acquired LWD data serves real-time well placement and other decision-making purposes (imagine that you want to remain within, above, and below a specific reservoir while drilling; the only optimal solution is one that will enable your team to steer the well according to the position of the reservoir’s bed boundary)
LWD measurement is as accurate and repeatable as wireline logging
LWD eliminates the need for post-drilling logging, hence saving rig time and cost, and avoiding safety risks associated with operations and well integrity
There is no well trajectory or inclination restriction on running LWD tools: you can acquire data up to your end depth, and at times ahead of the bit in vertical, S-shape, and horizontal wells.
With the emergence of LWD technologies, many people anticipated the death of wireline formation evaluation in drilling operations. However, multiple renowned oilfield service companies continue to invest in wireline technologies to offer both formation logging approaches.
Why is wireline logging still viable?
Despite the many benefits of LWD, the system still has drawbacks that must be weighed in any decision about which logging platform to use. Significantly, the data density in real-time for LWD is lower than wireline due to MWD data transmission rate (telemetry), which might be a deal-breaker for certain operators. Power could also be a problem for some, as LWD tools are either powered with batteries, by an MWD tool, or by a power section of the tool that generates its own power from a mud turbine. Finally, downlinking capabilities are somewhat limited with LWD. LWD services are often also priced higher than wireline.
Nevertheless, complex well trajectories and deeper reservoir targets increase well integrity constraints and require a controlled rate of penetration (ROP). The controlled ROP favors the acquisition of good data density during the drilling process. Recent developments with wired drill pipes now enable very high telemetry rates, extend downlink capabilities and offer a whole new suite of bottom hole sensorial while drilling measurements.
LWD technologies may not yet offer (or may be experimenting with) some wireline services such as sidewall coring, formation imaging, cement-bound logging and formation sampling. Yet, whenever the measurements are available on wireline and while drilling, I will strongly recommend using LWD services to maximize operational safety, improve the real-time drilling decision-making process, identify and mitigate well integrity challenges efficiently, optimize the well placement, and minimize the overall cost of well construction. In my experience, when I have a choice, I choose LWD.
Schlumberger equipment catalog at www.slb.com
About the author: Anye Ndefru is an Employee Development and Engagement Manager within the Drilling and Measurement (D&M) Technology Lifecycle Management sub-business unit of Schlumberger; the world’s largest oilfield services company. Mr. Ndefru, in his current role, manages engagement initiatives as well as the training and development planning of engineers and technicians of the Sub-Saharan Africa region, an area that spans across 37 countries. Prior to this role, he worked in field drilling operations, then in operations support, and finally, at Houston Formation Evaluation technology center; Schlumberger’s biggest worldwide technology center. During his assignment at the technology center, he was responsible for knowledge and process management, internal technical communications and global technical support to D&M business unit operations. While in this role, he also received the SETC recognition for his exceptional technical expertise and leadership with respect to matters relevant to business systems and processes development, organizational change management, quality and compliance management, continuous improvement and technology lifecycle management.
Data output shows comparable reliability between wireline logging and Logging While Drilling.
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