Ultrasonic Imaging Technology Evaluates a Lateral-Entry Module
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Several attempts to enter the upper lateral of a multilateral well operated by a major oil company in Alaska had been unsuccessful. In May 2017, an ultrasonic imaging technique based on medical ultrasound imaging was used to inspect the lateral-entry modules (LEMs). This paper presents the data collected by an ultrasound downhole scanner, demonstrating a novel method for diagnosing multilateral wells.
The West Sak field is a viscous-oil deposit within the Kuparuk River Unit on the Alaskan North Slope. The reservoir was deposited in a lower shoreface to inner-shelf marine environment and consists of highly unconsolidated sandstones that have a gross thickness of approximately 500 ft and an average net thickness of 90 ft. The reservoir consists of three major producing intervals, the West Sak D, B, and A sands.
Since original development in 1997, West Sak has been an active waterflood site. As technology improved and additional studies were completed, field development transitioned from a sand-exclusion strategy to a sand-management strategy. Completion strategy shifted to both producers and injectors with multilateral horizontal slotted liners.
These multilateral completions have higher productivities than vertical wells but are subject to producing sand, and, over time, matrix-bypass events (MBEs) have been noted. These events occur when the rapid breakthrough of offset water injection through nonmatrix flow causes the affected producer’s water cut to jump instantly to 100%. Once an MBE occurs, the affected injector is shut in and the affected producer is left online to dewater. Eventually, the oil production returns, but, because of the lack of waterflood support, oil-production rates rarely return to those seen before the MBE.
An MBE occurred between a West Sak multilateral injector and producer well pair. The standard diagnostic process of locating the MBE began. An injection profile log (IPROF) of the main bore in the injector determined that the MBE occurred in the D-sand (upper) lateral. An IPROF of the D-sand lateral was attempted using coiled tubing (CT) but was unable to exit the lateral junction. To gain lateral access, several different CT bottomhole assemblies were used and cleaning of the junction was attempted. These attempts were unsuccessful.
The decision was made to run a downhole video to investigate the D-sand lateral junction better. Evaluation of the video data indicated that a portion of the LEM may have shifted, covering the upper section of the lateral window. This slight shift in downhole equipment may have been the only issue impeding CT from entering the lateral. Analyzing the data led to the idea of creating a specialized whipstock to allow CT to mill the obstruction to provide access to the lateral.
During the planning process, it was determined that exact depths and locations of the LEM jewelry would be required to ensure success. To provide such information, the decision was made to log the D-sand junction using the ultrasound downhole scanner. As a reference of a working lateral junction, logging of the B-sand junction in this well also was completed. Any differences observed between the two junctions would provide additional detail as to why access into the D-sand lateral had been unsuccessful.
The ultrasound downhole scanner uses established technology applied in medical ultrasound imaging to obtain images and measurements of downhole completion components. A 288-element, 3.3-MHz circumferential ultrasound transducer array combined with electronic beamforming allows the flexibility to optimize image quality for different tubing sizes with no moving parts. The transducer operates in pulse/echo mode. Logging is performed dynamically, with images obtained in real time.
The possibly defective LEM was investigated by the scanner. A reference scan of a fully functional LEM in the same well also was made, and the results from the two were compared. The ultrasound data, visualized both as 2D grayscale images and 3D-rendered images, clearly show that the upper LEM assembly was not aligned properly with the window of the lateral. Measurements were made directly on the ultrasound images to document the findings. The results from the survey helped the customer understand the situation of the well and provided valuable information. The measurement principle of the technology is provided in the complete paper.
Application, Equipment, and Process
Application. In the target well, two LEMs had been installed. The lower LEM (B) was operating as normal, but, in the upper LEM (D), attempts to access the lateral had been unsuccessful. The two LEMs were similar around the access window. The minimum inner diameter (ID) of the LEM assemblies was 3.688 in. The deviation was 75°. The length of the access window was not stated. Images from a downhole camera suggested that the latching collet had slipped down over the access window, preventing access to the lateral in LEM D.
Equipment. The ultrasound scanner is 8.46 ft long and has an outer diameter (OD) of 2.125 in. A telemetry unit was connected to the scanner directly above it. To centralize the tool string, inline centralizers were added at the bottom and above the telemetry and two spring bow slip-over centralizers were added on the ultrasound scanner and telemetry bodies. The OD of the slipover centralizers is 3.125 in.
The ultrasound scanner is controlled in real time. Images captured by the scanner were monitored in real time, and parameters were changed by the operator to optimize data and make sure that sufficient data were collected before the tool was pulled out of the well. Two of the important parameters controlled by the operator are the radial start of scan and the radial scanning range.
Process. Because of the deviation at target depth, the ultrasound-scanner tool string was run with a downhole tractor. Scanning was not performed while the tractor was running, so all results were collected with the tractor switched off and wireline pulling up at approximately 4 ft/min over the two LEMs and 10 ft/min in between. The line speed was selected to optimize vertical resolution.
No special preparation or well conditioning was needed before deployment. Each frame of data from the ultrasound scanner is made up of 128 samples in radial depth independent of tool settings; thus, a narrow scanning range increases resolution in radial depth and, therefore, ID measurements. However, a long scanning range also was required to obtain a full understanding of the situation. To overcome this challenge, two main scans and two repeat scans using different radial start-of-scan and scanning ranges were performed with both LEMs.
The first scans were completed with a scanning range from immediately outside the sensor to 9.5 in. radially from the tool center to provide an overview of the situation, covering the distance from center to 6 in. out from the OD of the access window. The second scans were completed to 5 in. radially from the tool center to obtain a better resolution of the LEM and latching collet, covering the distance from the center to 1.5 in. out from the OD of the access window.
Some stick and slip of the tool string was experienced, resulting in a higher uncertainty in the longitudinal measurements and giving the ultrasound images a stepped look.
The entire inspection was completed in one run; the ultrasound scanner was in the well for 15 hours.
A discussion of how to interpret the presentation of the ultrasound scanner data is presented in Fig. A-1 in Appendix 1 of the complete paper.
Comparing LEM D (Target) and LEM B (Reference). A longitudinal cross-section of LEM D is shown in Fig. 1. Fig. 2 shows a longitudinal cross section of LEM B from a corresponding depth range (from 1.5 ft above to 4.5 ft below the access window). The cross-section planes are, respectively, from the center of the access windows and through the center axes for each LEM. The distance to the geometry outside the access window in LEM D increases with measured depth but increases considerably less than in LEM B. This indicates that the access window in LEM D might not be aligned correctly with the lateral. Some deviations with regard to relative measured depth, or longitudinal measurements, are seen, but these are probably caused by stick/slip movement while scanning. The overall shape and dimensions of the two LEMs coincide well, except through the access windows.
From circumferential cross sections, the distance to geometry outside LEM D is much shorter than the corresponding distance in LEM B.
The data collected by the ultrasound scanner show that LEM D, which should secure access to the D lateral, was not aligned correctly. Furthermore, the two LEMs were oriented differently compared with the high side, but the orienting profiles in each LEM are 180° from each respective access window.
The latching collet, which was suspected of preventing access to the D lateral, had slipped and was covering the upper 2 ft of the access window in LEM D, but, because this also was the case in LEM B, which was fully functional, this phenomenon could not explain the lack of access to the D lateral.
Ultimately, on the basis of the results shown from the ultrasound scanner, a more-complex milling operation would have been required to gain access to the D-sand lateral. Because of the risks involved, and the potential total loss of an injection well, the decision was made to abandon milling operations. Until a plan can be developed to treat the MBE without knowing its location, the D-sand lateral has been isolated and injection was restored to the B- and A-sand laterals.
For a limited time, the complete paper SPE 189947 is free to SPE members.
Ultrasonic Imaging Technology Evaluates a Lateral-Entry Module
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