Real-Time Images Optimize Drilling

4/18/2014 Real-time images optimize drilling

http://www.epmag.com/EP-Magazine/archive/Real-time-images-optimize-drilling_3648 1/5

Carlos Maeso & Marwan Moufarrej June 4, 2001

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.

Real-time images optimize drilling



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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Real-Time Images Optimize Drilling