Modern drilling decision-makers are drowning in data; now, technology providers are pushing the logging-while-drilling (LWD) envelope to deliver, not more data, but answers.

Today's LWD services face a variety of challenges. The pursuit of complex geologic targets in increasingly challenging drilling environments has driven higher rig costs - translating into the need for enhanced data in a timely and efficient manner. These data provide operators the critical answers they need for geosteering, wireline replacement, and wireline enhancement (uncertainty reduction). This drive towards real-time answers has accelerated the migration of wireline technology into the drilling environment.

LWD design challenges

The drilling environment poses numerous challenges for design engineers attempting to adapt wireline logging technology to perform under more demanding conditions. Unlike early measurement-while-drilling (MWD) systems, which focused on basic data acquisition and transmission, it is now essential that real-time logging tools be engineered with an equal emphasis on measurement accuracy, system reliability and drilling performance. For example, dipole acoustic measurements have long been the wireline standard for obtaining shear data in slow and very slow formations. However, in the drilling environment, dipole measurements are negatively impacted by drilling noise, tool vibration and fundamental changes in waveform propagation physics. Because of this, Inteq LWD design engineers developed a unique acoustic transmitter capable of measuring shear reliably across all formation types.
In addition to the drilling environment itself, today's advanced drilling technologies present their own set of challenges and have had a significant impact on LWD tool design. Because advanced rotary steerable systems increase rates of penetration (ROP), today's LWD subs must offer rapid data acquisition to ensure neither log accuracy nor data density is compromised at higher ROP. Also, in systems where logging accuracy depends on the sensor's proximity to the formation (i.e., nuclear logging), tools are especially susceptible to premature wear. Therefore, these LWD tools must be designed with increased wear capabilities to ensure accuracy and reliability in the face of continuous rotation.

As part of their ongoing effort to improve data quality while minimizing the time spent on data acquisition, the company has recently introduced a number of advanced real-time logging services, including a while-drilling formation pressure testing service. It is a new acoustic LWD service incorporating a proprietary multipole transmitter and an advanced LWD imaging service.

Measuring formation pressure/mobility during drilling

Formation fluid pressure and mobility often determine a field's completion and production strategy. Because of this, nearly all wireline service companies offer some type of pore pressure measurement.
With Inteq's new TesTrak device, rigsite personnel can now measure formation fluid pressure and mobility data during drilling. While the most immediate benefit is rigtime savings, especially in horizontal or highly deviated wells, other benefits include enhanced safety, improved casing placement, updated geological models, early hydrocarbon identification, optimal mud weights and increased ROP.
The testing device measures formation pressure during brief pauses in the drilling process. On command, the tool deploys a pad to seal a small portion of the borehole (Figure 1) while the instrument's onboard computer runs a series of optimized draw downs. Then, smart algorithms interpret the data to compute pressure and mobility using proprietary formation rate analysis. Having completed the tests in as little as 45 seconds, the tool retracts the probe and results are pulsed back to the surface as drilling resumes. Raw data are stored in downhole memory for later retrieval and analysis at the surface.

This real-time pressure data is of great benefit to geologists and drillers alike. Wireline pressure gradient methods have been applied successfully to real-time TesTrak data - allowing geoscientists to derive formation fluid density and readily identify fluid type and contacts. And, by updating geological models with real-time pressure data, experts can quickly identify sealing faults and reservoir connectivity/compartmentalization. Also, these updated models can be used to optimize mud and casing programs and refine wellpaths for maximum pay exposure. The real-time data can also be used to help geosteer in permeable, homogeneous formations by monitoring near-bit formation pressure to identify small true vertical depth (TVD) variations over extended reaches.

In addition to the data's geological benefits, real-time pressure measurements can be used to improve wellsite safety and optimize drilling efficiency. Using direct pore pressure measurements, drillers can adjust mud weight and equivalent circulating density (ECD) to prevent kicks and blowouts, and avoid formation damage or accidental fracturing and lost circulation. The data can also help calibrate predictive pore pressure algorithms. Drilling efficiency can be enhanced by maintaining a minimal differential pressure, or even drilling underbalanced, to optimize ROP and reduce differential sticking risks.

The challenge of real-time acoustic logging

Operators have long used wireline acoustic logs to address key geological, geomechanical and petrophysical issues including seismic-tie, determining acoustic porosity and providing compressional slowness and shear-velocity data for borehole stability and rock mechanic analysis. Various wireline logging methods have been developed to measure formation shear velocities. In fast formations (formation shear is faster than the borehole fluid), monopole tools measure shear using acoustic waves refracted along the borehole. However, in slow formations (formation shear velocity is slower than the borehole fluid), shear cannot be directly measured by monopole logging and necessitate dipole acoustic logging tools.

However, when migrating acoustic technology from wireline to LWD, it was critical that designers properly accounted for the differences between the two logging environments. When using a dipole measurement, strong tool modes traveling along the drill collar will superimpose themselves on the formation response. Furthermore, the LWD dipole-flexural wave, unlike its wireline counterpart, has a significantly lower velocity than the formation's true shear. As a result, LWD dipole measurements require significant corrections to remove the collar signal and account for the lower velocity - compromising measurement accuracy. To address these issues, quadrupole-wave technology was developed and implemented to minimize the contaminating tool-mode and provide a wave velocity to match the formation's shear velocity at low frequency.

Inteq's Acoustic eXplorer service, incorporating quadrupole technology, enables operators to directly measure formation shear-velocities during drilling - eliminating the need for large dispersion corrections and providing a more accurate match than dipole LWD measurements. The service provides real-time and memory compressional (Dtp) and shear (Dts) slowness measurements in both slow and fast formations. In addition to quadrupole data, the new tool incorporates the two traditional measurement modes (monopole and dipole). The monopole waves measure compressional velocity. The dipole waves were initially used to quality control modeling predictions and quadrupole results but are rarely employed in LWD applications unless specifically requested by the client.

LWD imaging confirms geological features and allows true dip determination

Image data can play a critical role in determining optimal well placement in a structurally complex environment. Using LWD image logs, geological correlation can be improved and real-time interpretation of apparent formation dip may be used to improve well placement decisions.

As part of the company's advanced Image eXplorer service, azimuthally sectored densities are transmitted to the surface and an image log is generated at the well site (Figure 2). In order to obtain accurate dip calculations, azimuthal density readings must be obtained from at least three sectors. Traditionally, four oriented density measurements (high side, low side, left, right) have been used for real-time dip computation. When the geology is known to be layer-cake, then simple up-down, two- sector information is adequate for navigation. In reality, Inteq's nuclear logging suite records eight sectors for post-job imaging and offers a choice of two or four sectors for real-time navigation. This full circumferential image log permits rigsite personnel and networked geoscientists to more quickly identify geological events that would not be obvious on a presentation of the four oriented measurements.

If a complete structural interpretation is performed using the tool's full azimuthal memory data, geologists can confirm wellsite results and substantiate existing earth models for further well planning or schedule additional geological investigation.

Understanding apparent and true dip while drilling improves the decision-making process regarding critical well trajectories - resulting in more accurate well placement, valid structural characterization, seismic model confirmation and increased reservoir exposure.

LWD imaging is not limited to density alone. Recent field trials of a high-resolution electrical imaging LWD tool have demonstrated that the drilling environment offers an ideal platform for electrical borehole imaging. While drilling, the borehole wall's rugosity is often minimal and electrical images generated by rotating sensors provide full borehole coverage.

Conclusion

As advances in drilling technology push technical boundaries in extended reach drilling (ERD) wells, over-pressured zones, and deep water, operators are demanding more timely logging data to improve wellsite decision-making. As a result, reliable formation evaluation answers while drilling, including acoustic measurements, imaging and real-time formation pressure testing, have become critical to the well's success. However, the migration of this technology from wireline to LWD is not as straightforward as it may seem and design engineers must take into account all the differences between the two environments when designing and launching new technologies.