LWD dual-physics imager for OBM applications enables real-time geological characterization

For almost three decades, borehole images have been used widely across many applications, including drilling, geomechanics, reservoir characterization and production optimization. One challenge that operators have faced for many years is acquiring high-resolution borehole images in oil-based mud (OBM). Experience suggests that geological features respond better to resistivity-based imaging. Nonconductive OBMs pose a barrier for microresistivity imaging of the subsurface.

A new LWD tool captures high-resolution images of complex geological formations in OBM, in real-time.

Recent technological advances, such as photorealistic reservoir geology imaging, have made it possible to acquire high-resolution borehole images in OBM wells during wireline logging. However, this conveyance is not always possible, due to economic and operational challenges.

When it comes to logging while drilling (LWD), no options were available until recently for acquiring high-resolution images, due mainly to coverage constraints against the downhole environment and lack of contrast for the geological measurements. This severely restricted any real-time applications of this data, upon which operators could make critical geosteering decisions for proper well placement and risk mitigation. Real-time data transmission capability of while-drilling images adds a new dimension. More and more often, geosteering operations need high-resolution images of the drilling bottomhole assembly (BHA) to navigate through complex geological structures and maintain a high net-to-gross for the well being drilled.

To enable operators to obtain photorealistic borehole images in nonconductive mud while drilling, Schlumberger introduced the TerraSphere* high-definition dual-imaging-while-drilling service. This first-of-its-kind LWD dual-imaging tool advances borehole image acquisition in OBM with deployment of multi-sensor, multi-physics technologies of electromagnetic (EM) and ultrasonic measurements on one small sub.

Four operational frequencies of EM measurements are used for resistivity imaging to cover different resistivity ranges in the subsurface, while ultrasonic images are made at two frequencies to account for variation in mud density and borehole wall rugosity during drilling. This enables feature identification of subtle variation in texture, structure, sedimentation and diagenesis style.

HIGH-RES LWD IN OBM

The development of high-resolution imaging during the drilling phase in OBM wells has learned from the advances in imaging from wireline conveyance and the techniques available from azimuthal measurements from LWD. Since their development in the 1990s, a variety of LWD measurements have been made with azimuthal capabilities. In OBMs, these include density, photo-electric factor (PEF), natural gamma ray and ultrasonic measurements. The initial application for these measurements was primarily petrophysics, with azimuthal capabilities, allowing measurements to be made with minimum standoff.

Fig. 1. LWD borehole images of density and PEF showing gross variations. These images are good for some applications, but are not suitable for geological characterization or critical real-time decisions. The resistivity images from the LWD dual-imaging tool (3x right) show more geological features at higher resolution in the same interval. (Image courtesy of Schlumberger)

Very quickly, these measurements were displayed as borehole images and used widely. There is an inherent low resolution for most of these measurements, with four to 16 azimuthal bins and 10.16 centimeters (4 in.) to 15.24 centimeters (6 in.) along hole, which is the typical measurement resolution. The main applications for these measurements have been real-time geosteering (positioning relative to bed boundaries) and basic geological interpretations (helping understand structure in high-angle wells). However, these images are often not sufficient for geological characterization or many real-time applications, leaving a lot to be desired, Fig. 1.

The LWD dual-imager tool builds on the experiences of wireline and LWD tool design and imaging, and provides ultrasonic and microresistivity images at resolutions an order of magnitude higher than previously available for LWD in OBM. Ultrasonic and resistivity images are made at various frequencies to ensure that the features do not escape from the multiple measurements made. The comparison of the new LWD images is shown in Fig. 2 with wireline images to emphasize the high-quality LWD borehole images available while drilling, thereby enabling real-time, wide-scale geosciences applications, Fig. 2.

Fig. 2. Wireline images and the new multiple-measurement LWD imager. RAP = apparent resistivity image, US = ultrasonic/amplitude image. (Image courtesy of Schlumberger)

Approximately 9.14 m (30 ft) of interbedded sands and shales are shown in Fig. 2. The layering is clearly seen on the resistivity tools from both conveyances, including a series of thin beds in the shalier facies. In addition, an erosive surface is present at the base of the sand at 76.81 m (252 ft), and the overlying sands are clearly cross-bedded. Although the wireline imager has higher resolution and can identify a number of thinner beds, the resolution of the resistivity images from LWD allows both the structural and sedimentological information to be identified.

Two differences are seen between the resistivity images by different conveyances. On the LWD images a speckled appearance is seen in the shale below 76.2 m (250 ft). This is an interval of enlarged hole where conductive cuttings are likely to be common. On the wireline image, there is a subvertical conductive zone seen in the sands around 74.68 m (245 ft), roughly in the north and south orientations. This is an induced feature that developed after the LWD logging time and so is not observed on the LWD image. The layering is not as apparent on the ultrasonic amplitude images. This is likely, due to the low acoustic impedance contrast between the high-porosity sands and silt-rich shales.

For this section, the clearest development of layers seen in the ultrasonic image is at the sand shale boundaries. In these locations, the sands are probably cleaner and have a higher contrast with the interlayered shales. Figure 2 also illustrates a number of fractures. These are high-resistivity and also high acoustic amplitude, suggesting a cement-filled fracture. They are not observed on the travel time images. The fractures are most clearly observed on the ultrasonic amplitude images with three fractures seen at 72.85, 73.46 and 75.29 m (239, 241 and 247 ft). The uppermost fracture is clearly visible on the LWD and wireline resistivity images, however,
the lower two are difficult to detect on the resistivity images. In addition, the detail of the shape of the upper fracture is also most clearly seen on the amplitude images.

Multiple measurements for LWD borehole imaging. The design of any LWD tool must meet the needs of the drilling operations while delivering the quality required for the measurements. Several challenges were overcome in order to build the new LWD dual-imaging tool. As with all LWD tools, there is limited space within the collar for sensors and electronics boards. For the dual-imaging tool, these have been fitted in a tool that is just 14.6 m (15 ft) long.

The resistivity imaging sensors are positioned on a stabilizer. This minimizes the stand-off between the tool and the borehole wall, which equates to the least amount of resistive mud that the EM signal passes through. The tool has two resistivity sensors that are diametrically opposed. Four EM frequencies are used, covering kilohertz to >100 megahertz, to read beyond the capacitive drilling mud. The range of frequencies enables apparent resistivity imaging of formations across a wide range of low-to-high resistivity.

For the ultrasonic imaging, four sensors are positioned at 90° intervals around the collar. Travel time and amplitudes are extracted from the reflected signals, with filtering of the wideband acquisition for a low-frequency (~100 kHz) and high-frequency (~400 kHz) measurement, Fig. 3.

Acquisition is effectively simultaneous at each firing. This means that each opposing sensor can be combined directly to calculate a borehole diameter. In addition, the two pairs of sensors are offset in diameter, and therefore experience different stand-offs and travel times. This information is used to calculate the velocity of the mud downhole, which is used to calculate the hole size and shape, including where mud properties are changing.

Fig. 3. Dual-physics imager showing four ultrasonic sensors for pulse echo measurements. The tool system also has two EM sensors for guarded electrical impedance measurements across a wide range of subsurface resistivity environments. (Image courtesy of Schlumberger)

To provide real-time data for both ultrasonic and resistivity images, extensive automated processing is required downhole. For the resistivity, the four frequencies are combined into an apparent resistivity image. This is done by compensating for the mud properties and using an equation based on the Padé approximation. The resulting, apparent, resistivity image is compressed and streamed to the surface, using mud-pulse telemetry. Where bandwidth is high or ultrasonic images are preferentially required, the ultrasonic amplitude images can be transmitted, as well. In addition to resistivity or ultrasonic amplitude images, hole shape information is also transmitted from the ultrasonic calipers, be it average or directional.

DUAL-IMAGER RESULTS

Data from the LWD dual-imager tool has been acquired from a wide variety of environments around the world. In most cases, it has proved beneficial to have the two different types of physics, resistivity and ultrasonic measurements. In general, the resistivity measurements have a high sensitivity to variations in formation properties. Formation resistivities commonly vary over decades in value when changing from layer to layer (e.g., from shale to hydrocarbon-bearing layers). This makes the resistivity image very good for structural and sedimentological dips.

The resistivity images provide rich information relating to structure and sedimentation, highlighting the sedimentary layering, textural variation and diagenetic imprints. Ultrasonic images are more sensitive to natural fractures and drilling induced features, such as induced fractures and break-out. Operators in United Arab Emirates and Norway have used the service to optimize their operations.

UAE case study. An operator offshore United Arab Emirates wanted to place a long lateral section through the most permeable layer of the carbonate reservoir and avoid high gas/oil ratio (GOR) zones that could not be identified using conventional logs. The operator also sought to identify and avoid high water saturation zones and map possible faults.

The high-definition dual-imaging-while-drilling service was used in a long horizontal 21.6-cm (8½-in.) section at a measured depth of more than 8,230 m (27,000 ft). The service helped geosteer a lateral length interval longer than 3,962.4 m (13,000 ft) without compromising ROP in a TVD of more than 2,514.6 m (8,250 ft). The novel data compression technique, coupled with the mud pulse telemetry capability, transmitted the high-definition images in real time for an extended-reach drainhole. The service provided real-time identification of large vugs that were linked with high-permeability interval and validated with the mobility estimation from StethoScope* formation pressure-while-drilling service pressure measurements. This service also helped the operator understand and validate the structural and facies control of fault and bioturbation on the real-time fluid log data acquired using FLAIR* real-time fluid logging and analysis service. The integration of technology within one BHA, guided by high-definition images, enabled real-time decisions during a single run in the 21.6-cm (8½-in.) section.

Norway case study. 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 its 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 in a challenging geological environment and enabled the well to produce above predicted rates with a higher condensate-to-gas ratio.

Another operator offshore Norway planned to drill a horizontal well in the upper part of a sand body injected with shales in 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.

A third operator in Norway required high-definition images while drilling a sidetrack offshore. The operator expected the well to run through a complex clastic shallow marine and fluvial sequence. Delineation of the expected thin and variable interbeds of sandstones, claystones and coals in the OBM created challenges with conventional LWD tools. The operator used the high-definition dual-imaging-while-drilling service to acquire high-resolution data in the interbedded formation. The EM and ultrasonic technologies highlighted faults and fracture orientations, layer bedding details, cross-bedding and irregular surfaces. The data acquisition while drilling enabled detailed imaging of the coal layers and breakout phases.

CONCLUSION

The case studies discussed illustrate how high-definition dual-imaging-while-drilling service improves reservoir understanding and augments well integrity. Singular measurements would often miss a host of geological intricacies and could still leave ambiguity about the feature classifications that are observed. Even high-resolution measurements may miss out on picking some features that do not provide enough contrast to be observed; and resolving of features is possible only when it is observed.

Wide-scale geoscience applications require both: definition and resolution of downhole features on the borehole wall, manifesting either formation or the drilling conditions. The high-definition dual-imaging-while-drilling service provides the data required for all such applications in real and relevant time previously not possible for OBM-drilled wells.

^{*Mark of Schlumberger}