Operators recognized the need for a high-definition LWD imager that could provide both micro-resistivity and ultrasonic images to characterize the subsurface in detail with complementary information. Ultrasonic imaging while drilling was a challenge, especially for designing transducers capable of working in a wide range of mud weights. However, the biggest technology barrier was to overcome the OBM for resistivity imaging.
The high impedance from the mud-layer in the gap between the sensor and the formation was overcome with higher operating frequencies that improve the capacitive coupling through the mud to mitigate the effect of insulating mud.
Both acoustics and resistivity imaging sensors are positioned within a single compact 15-ft drill collar. High-resolution EM pulses are sent through the mud from the sensor to the formation at a wide range of frequencies that produce best-quality resistivity images across the broad range of resistivities encountered in subsurface geological environments. An innovative methodology for data compression was deployed for high-quality real-time data.
The acoustic subsystem emits short ultrasonic pulses and detects the resulting echoes from the borehole wall, providing robust measurements in a wide range of environments, including heavy mud-weight drilling fluids with high attenuation. Frequent sensor sampling and focusing deliver high-resolution images. The four sensors are positioned 90 degrees apart around the collar, orthogonal to each other.
In addition to the dual-physics images, hole shape information is also transmitted from the ultrasonic calipers.
To provide real-time data for both high-definition ultrasonic and resistivity images in mud pulse telemetry transmission, extensive automated processing is required downhole. For the resistivity, the four frequencies are combined into an apparent resistivity image that is transmitted in real time using mud-pulse telemetry.
Compression of ultrasonic images is a challenge because of factors such as standoff effects. Formation features tend to be very fine scale with low contrast in acoustic impedance, and artifacts such as bit scratch marks are common. Ultrasonic amplitude images can be transmitted in real-time through mud-pulse telemetry.
Both image physics collocated in one tool provide a complete solution for detailed geological and borehole understanding in OBM environments while drilling. The images are complementary. The resistivity images acquire details of subsurface geological variations, while the ultrasonic images are more sensitive to drilling effects, stress-related features and borehole conditions.
Real-time full-borehole coverage for unambiguous interpretation helps reduce rig time and costs. In addition to enhancing geosteering in complex environments, the determination of production potential and reserves estimates is accelerated; and subsequent operations, such as wireline runs, are optimized.
The high-resolution EM pulses of the high-definition dual-imaging-while-drilling service enable a wide range of firing frequencies. The service produces the desired high-quality resistivity images across the broad range of resistivities encountered in subsurface geological environments, including clastics, carbonates, shales and evaporites. Operators in the Gulf of Mexico and in Norway have used the service to optimize operations.
Gulf of Mexico
An operator in the Gulf of Mexico drilling in deepwater Miocene reservoirs sought a while-drilling method to acquire high-borehole images without compromising image quality. Steep dips close to salt domes and the presence of potential fluid conduits from faults or fractures created a challenge. Borehole degradation posed risks for open-hole wireline logging.
Using the high-definition dual-imaging-while-drilling service, the operator acquired images with a significantly higher resolution in the OBM environment compared with images provided by previous-generation LWD imagers. This resolution enabled recognition of thin laminated beds, bedding deformation, facies variations and onset of drilling-induced features.
Another operator in the Gulf of Mexico needed to quickly perform a formation evaluation, identify the oil/water contact (OWC) and optimize the completion interval while drilling development wells in an OBM environment. Previous drilling of development wells showed that borehole instability limited the time window to keep the hole open in a thinly laminated reservoir with low resistivity contrast. Conventional triple-log combo logs did not provide sufficiently accurate evaluation of hydrocarbons in place and potential free fluid.
The operator used a combination of the high-definition dual-imaging-while-drilling service and a multifunction logging-while-drilling service for a comprehensive logging suite in a deviated well to be drilled with OBM. This would enable real-time transmission of LWD triple-combo logs, spectroscopy, sigma and resistivity images to identify reservoir sand intervals to make informed drilling decisions.
An efficient interpretation on the recorded-mode images offered full evaluation of the reservoir structure, texture, fluid in place and OWC for the completion design. The system provided geological interpretation and petrophysical evaluation of the lithofacies, which provided understanding of the reservoir architecture and its potential producibility.
High-definition ultrasonic and resistivity recorded-mode image data helped define the geological structure, sand lamination and sand texture. Real-time logs confirmed the conventional hydrocarbon sands interval. The EM sensors of the high-definition dual-imaging-while-drilling service confirmed the OWC in the low-contrast laminated sand. Water saturation was used as the primary fluid volume estimation. Measurements from the multifunction LWD service enabled the analysis of movable fluid in the formation in the drilling pass.
The recorded-mode LWD logs were quickly interpreted and tailored to highlight completion intervals. The solution ended up providing efficient optimization of the completion interval.
In Norway, government guidelines require operators to mitigate risks, decrease operational footprint and reduce critical activities such as the use of radioactive sources for logging operations, especially in sensitive areas.
Using the high-definition dual-imaging-while-drilling service run with reservoir mapping and advanced formation evaluation-while-drilling services, an operator in the Norwegian Sea ran the first global geosteering operation using comprehensive real-time data governed by a chemical-sourceless BHA in OBM. The combination facilitated optimal well placement with higher net-to-gross ratio in a challenging geological environment and enabled the well to produce above predicted rates.
Another operator offshore Norway planned to drill a horizontal well in the upper part of a sand body injected with shales in the Balder Field. Combined with uncertainty in the sand properties and distribution within these zones, the shale stability presented a challenge that required high-resolution borehole imaging to delineate. The use of OBM in the well prompted the operator to select the high-definition dual-imaging-while-drilling service for the use of EM and ultrasonic imaging technology. The service enabled high-definition visualization of complex Balder injectites. The service delineated thin and non-constructed sands injected perpendicular and parallel to shale bedding. This removed ambiguity on conventional log responses and validated questionable reservoir zones for inclusion in completions. The ultrasonic images captured weak-plane failure in the shales down to the scale of millimeters.