Salaheldeen Almasmoom, Drilling Engineer at Saudi Aramco
Naif Rubaie, Oil and Gas Facilities Supervisor at Saudi Aramco

Introduction

Designing a horizontal well in an unconventional play always comes with a challenge to adequately balance the geosteering requirements with an optimized drilling engineering design that delivers the best drilling performance while acquiring all the desired data. The main requirement was to acquire high-resolution resistivity images to support geosteering operation, and to interpret fractures, geological features, and events in the formation. This is to be done while drilling with an oil-based mud (OBM) system as the drilling fluid to overcome the drilling challenges. The logging while drilling (LWD) density image has its limitations in terms of geosteering due to its poor vertical resolution. It was the best available option until the recent development of the new generation of LWD imaging tool. This recent      development was the answer to enable both acquiring high-resolution images and supporting geosteering in an OBM environment      without jeopardizing the well and logging plans. A collar mounted sensor LWD technology, able to deliver both ultrasonic and resistivity images in real-time in an OBM environment, was utilized to achieve the well objectives, Figure 1

Figure 1: The new-generation dual-image OBM environment LWD imaging tool. The sensors send the pulses through the drilling fluid using the electormagnetic subsystem. These pulses are then sent into the formations at a wide range of frequencies to produce the resistivity images of the subsurface geological structures of the formations. The short ultrasonic pulses taken from the formations affected by the OBM are removed by the acoustic subsystem of the sensors. Combining both features helps make the new LWD imaging tool capable of recording high-resolution resistivity and ultrasonic images. The continuous sensors’ sampling of the LWD tool of the ultrasonic and resistivity images improves the resolution of the produced images to surpass the images recorded by wireline tools1.

Engineering Planning

Bottomhole Assembly Design: Along with the modeling-based data correction done to the measured acquired logs, reducing the severe logging sensors’ motion while drilling by having the best drillstring stabilization is essential to mitigate the lateral motion effects in the logging measurements2. In addition, lowering the      borehole tortuosity in the wellbore will result in lower downhole dynamics, such as the phenomenon of stick-slip. Lowering the changes in dogleg severity (DLS) while drilling a directional wellbore will result in having a smoother lateral profile, and therefore lower      borehole tortuosity3.

The planned lateral drilling BHA included the following LWD sensors; gamma ray (GR), density, neutron, resistivity, sonic, density imager, and the new LWD OBM dual imager to measure ultrasonic and resistivity image      logs. The addition of the newly developed tool added three main challenges:

  1. The tool’s addition adds approximately 15-ft sub (or short pipe) to the normal LWD BHA run. The total length of the BHA becomes approximately 505 ft. 
  2. The tool’s addition adds two more stabilizers to the planned drilling BHA. Therefore, the drilling BHA will have six stabilizers, compared to four stabilizers in the normal logging-while-drilling BHA typically run in the field. 
  3. The placement of the tool in the planned drilling BHA to preserve both the quality of the recorded logs, and the real-time transmission throughout the run. 

The drilling engineering team ran several drilling BHA designs. These designs were mainly simulated for torque and drag (T&D), axial displacement, and stick-slip magnitude to select the best engineered drilling LWD BHA design. Figure 2 shows the selected drilling LWD BHA design configuration.

Figure 2: The LWD BHA configuration, including the new LWD OBM dual imager, while drilling with OBM in the wellbore.

Well Execution 

Well Placement & Geosteering: The real-time transmitted density image log is normally the tool used for well placement, along with the GR log if an OBM system is used     . The resistivity image log could not previously be provided. With OBM, the mud-cake behaves similar to electrical insulators which obstructs the current flow, so using the same LWD tools to acquire data in a water-based mud (WBM) environment      is not feasible to use in an OBM system4. Furthermore, existing and drilling-induced fractures filled with OBM misrepresent the image by scaling fewer sedimentary features5

The real-time density image log is      used for well placement through identifying the bedding planes while drilling through the lateral section. From the shape of the bedding layers recorded real-time, the well path can be guided to keep the bit inside the target zone. Despite that, the bedding planes shown in the resistivity image log recorded real-time from the new dual LWD imager tool show higher resolutions when compared to the density image. Therefore, providing more data to improve goesteering. 

Figure 3 shows the better image resolution of the resistivity image compared to the density image log. For the first time ever in this field, the real-time transmitted resistivity image was used for well placement along with the density image log. In addition, it was the first time ever to transmit in real-time all three image logs. 

Figure 3:  All three image logs (resistivity images on the left side in two different modes, ultrasonic images the next two in the middle in two different modes, and the density image on the right side) were transmitted real-time together when drilling commenced. The gap in some logs is due to excessive ROP and some noise in MWD telemetry. This real-time transmission was done for the first time in the world.

Prior to drilling the well, the expectation from the offset horizontal wells was to have the target formation almost flat. Therefore, the initial directional plan was designed to maintain the well inclination after landing in the range of 89.7 to 90° while drilling through the lateral section. The resistivity image log transmitted real-time from the new dual LWD imager tool obtained while drilling through 96 to 98 lb/ft3 OBM, combined with the density image log helped to identify the dipping behavior of target formation. The formation starts flat for a short interval, then it dips downwards for most of the lateral section. Then, toward the end of the lateral section, the downwards trend decreases until it is almost flattened out, Figure 4. 

The resistivity image log helped correct the well path back to the target zone. Figure 4 shows that the actual well path was out of the target zone for a small portion of the lateral, then was corrected back to the target zone. Then, the well path was maintained inside the target zone throughout the lateral section of the well by using the real-time resistivity image log.

Figure 4:  The actual well path relative to the zone of interest. The target formation is dipping down initially, then flattens out at end of the lateral.

 

Images Vertical Resolution and Quality Assessment

LWD Images vis-à-vis Wireline Images: One of the main concerns while acquiring LWD imaging logs in an OBM environment was always the vertical resolution when compared to wireline imaging tools. This additional acquisition of wireline logs significantly      increases the budget of not only the logging program, but also of the drilling part of the project as it      adds additional rig time. Unfortunately, acquisition of wireline image logs in horizontal wells does not always provide image data with a suitable quality for the tasks they are acquired. This is due to the challenging nature of the acquisition in such environments      e.g. hole condition, solids in mud, and so on.

The OBM LWD resistivity and ultrasonic images acquired in this well while drilling were compared with wireline images, and with the LWD density image. The results were      promising; the image quality and image resolution of the OBM LWD resistivity and ultrasonic images were excellent, and permitted the users of the data to identify more geological events than with the wireline images, Figures 5 and 6. Several natural fractures and faults were clearly identified, many of which could not be observed with the wireline image logs.

Figure 5: Compressed plot over the entire curve and lateral sections of a carbonate interval. Static image with LWD (density, oil-based resistivity and ultrasonic) and wireline (oil-based resistivity and ultrasonic) images. Note that at this compressed vertical scale dip planes are clearly more evident on the LWD oil-based resistivity image if compared with the rest of the image logs. Also note that without the image data, the low porosity zones of this section will be considered with low flow potential, when in fact they will be the major contributors to the hydrocarbon flow due to the presence of the natural fractures (red tadpoles).

 

Figure 6: (Expanded scale plot). On the LWD OBM resistivity and ultrasonic images several natural fractures are observed (red tadpoles). Some of these events are hardly or none observed on the wireline images.

Conclusions

The overall process summarized in this paper strived to optimize the well construction operation in unconventional drilling. The engineering design and fit for purpose LWD dual imager in OBM enabled a faster delivery in completing the horizontal well. The ability to make fast decisions in real-time based on full information and borehole images improved the geosteering operation. The process from the pre-job phase      and well engineering design toward the execution phase reflects a success story delivering such a challenging well.

The LWD oil-based dual imager (dual physics) provided resistivity and image logs that were better than the wireline resistivity and ultrasonic image logs in many sections in terms of quality and resolution. It allowed for real-time decision      making to adjust the well path and thus improved well placement.

Acknowledgment

The authors would like to acknowledge and thank management and other contributors to this technical project and paper; Javier O. Lagraba Penaloza form Saudi Aramco and Gagok I. Santoso and Jamal S. Alomoush from Schlumberger.

References

  1. https://www.slb.com/drilling/surface-and-downhole-logging/logging-while-drilling-services/ terrasphere-high-definition-dual-imaging-while-drilling-service. 
  2. Haji, A., Tisdale, C., Muslem, M. and Abouzaid, A.: “Quantification and Correction of Lateral Motion Effects on NMR Logging-while-Drilling,” SPE paper 203456, presented at the Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, UAE, November 9-12, 2020. 
  3. Shen, Y., Zhang, Z., Zhao, J., Chen, W., et al.: “The Origin and Mechanism of Severe Stick-Slip,” SPE paper 187457, presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, October 9-11, 2017. 
  4. Laronga, R.: “Borehole Imager Provides Valuable Data in Oil-Base Muds,” Journal of Petroleum Technology, Vol. 53, Issue 11, November 2011. 
  5. Bourke, L.T. and Prosser, D.J.: “An Independent Comparison of Borehole Imaging Tools and Their Geological Interpretability,” paper presented at the SPWLA 51st Annual Logging Symposium, Perth, Australia, June 19-23, 2010. 

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