Briner, Andreas (PDO) | Nadezhdin, Sergey (Schlumberger) | Tessari, Sergio (PDO) | Smit, Jeroen (PDO) | Busaidi, Yazeed (PDO) | Abri, Mohammed (PDO) | Mitri, Joelle (Schlumberger) | Shalaby, Ehab (Schlumberger)
This case study describes the deployment of logging tools on tractor in challenging wellbore conditions in a tight gas field in the Sultanate of Oman. Challenging conditions are defined by a high static temperature of 175°C; high static reservoir pressure of 72 MPa; a long, 5700-m wellbore; openhole configuration; and horizontal well profile. One of the conventional ways of deploying the caliper measurement in horizontal wells is through drillpipe conveyance. Deployment through coiled tubing conveyance is another option; however, this was not available owing to extensive logistical requirements and the operation's complexity. A newly developed openhole tractor designed for the well's extreme conditions was used to efficiently convey the logging tools. Features that enabled successful tool conveyance included the highest tractor force available in the industry, 6-drive tandem wheels, real-time adjustment of the radial force to the tractor arms, and active traction control for improved maneuverability. Calipers were successfully conveyed with the wireline tractor in the 800-m horizontal section of the 5000-m-deep well. The logging job took only 12 hours versus the traditional 60 hours required to convey the same tools on drillpipe. In addition, the tractor conveyance minimized the risks associated with operating at high temperature because it significantly reduced the time of exposure of tools to extreme high temperatures approaching their maximum temperature ratings, hence ensuring top performance and reliability. The fivefold time reduction also helped with the well economics and minimized the overall operational risks associated with the logging operation. The recorded caliper data enabled proper evaluation of the hole conditions for selecting the best location for setting the swellable packers.
To estimate reserves, optimize well recovery, and place future development wells more accurately, operators require increasingly detailed understanding of complex oil and gas reservoirs. High-resolution geological data from the borehole— core samples and wireline microresistivity images—are essential to fully characterize reservoir architecture, guide and constrain reservoir models, and make timely decisions with precision and confidence.
In unconventional resource plays, for example, geologists need to observe complex natural fracture systems, measure fracture density and direction, determine in-situ stresses, and calculate pressures required to initiate and propagate optimal hydraulic fracturing. In high-cost deepwater environments, exploration teams need to interpret a wide variety of sedimentary facies. Reservoirs often consist of thinly laminated sands or channels, which demand high-definition methods for visualization and interpretation. Typically, deepwater formations exhibit very low resistivities— from 1 ohm-m in shales to as low as 0.2 ohm-m in water-bearing sands.
Operators often use whole cores to identify subtle sedimentary facies, but cores can be expensive and time-consuming to acquire. Thus, core samples are usually obtained only in select wells and in the most critical intervals. The resulting lack of geological detail can adversely affect reservoir analysis, interpretation, and modeling.
On the other hand, microelectrical borehole images can be acquired continuously over the openhole interval at any depth or in any formation, with relative ease. Until recently, high-definition borehole imaging has been possible only in electrically conductive water-based muds (WBM). However, most deepwater wells and many unconventional shale wells today are drilled with high-performance, nonconductive oil-based muds (OBM).
Legacy OBM borehole imaging tools, which were introduced in the last decade, are a significant technical advancement over the preceding dipmeter tools but still exhibit certain limitations. For example, spatial image resolution in OBM is nowhere near the quality obtained from WBM imagers. The lack of circumferential coverage of the borehole leaves large gaps that must be filled by inference.
Bloemenkamp, Richard (Schlumberger) | Zhang, Tianhua (Schlumberger) | Comparon, Laetitia (Schlumberger) | Laronga, Robert (Schlumberger) | Yang, Shiduo (Schlumberger) | Marpaung, Sihar (Schlumberger) | Guinois, Elodie Marquina (Schlumberger) | Valley, Glenn (Schlumberger) | Vessereau, Patrick (Schlumberger) | Shalaby, Ehab (Schlumberger) | Li, Bingjian (Schlumberger) | Kumar, Anish (Schlumberger) | Kear, Rick (Schlumberger) | Yang, Yu (Schlumberger)
While they provide a recognized technical advance for wells drilled with oil-based mud (OBM), OBM-adapted microresistivity images of the last 13 years remain far from the geologic interpretability provided by imagers that operate in a water-based mud (WBM) environment. Recently the use of a high-definition WBM imager has been demonstrated in wells drilled with OBM, but its application has been principally limited to high-resistivity formations with excellent hole conditions or to cases where the drilling fluid has been engineered to favor acquisition.
To fill this gap, a new wireline microelectrical imager has been introduced, engineered from the ground up to acquire high-definition, full-coverage images in any well drilled with OBM. The all-new physics architecture includes a strategy to minimize and eventually eliminate the inevitable contribution of the nonconductive fluid and to optimize the mode of operation in accordance with formation parameters. New tool-specific processing steps complement the standard borehole image processing workflow to render highly representative images of the formation.
Examining the measurement response in detail, via both modeling and real-world examples, demonstrates several favorable characteristics, for example, sensitivity to vertical as well as horizontal features, reduction of shoulder-bed effects, and reduced sensitivity to desiccation cracks.
The novel mechanical architecture includes a new sonde design with significant operational advantages. It conveys a sensor array composed of 192 microelectrodes providing 98% circumferential coverage in an 8-in. borehole. The individual microelectrodes are smaller than those of industry-standard imagers for WBM, each with a surface area of only 10.8 mm2, which provides excellent spatial resolution.
From a field test comprising more than 40 operations in various OBM fluids, high-definition images were acquired in a variety of environments, from high-resistivity carbonates to shales and low-resistivity clastics, demonstrating the robustness and widespread applicability of the new tool. The examples include challenging environmental conditions and they explore the limits of accurate measurement. Comparison with legacy images demonstrates that the new physics of measurement coupled with the high-resolution, high-coverage sensor array has achieved much more than a microimaging step change. The new images faithfully reproduce formation geology with photorealistic clarity and promise to revolutionize the geologic interpretation of wells drilled with OBM.