How evolution resulted in the eye is a question often asked. Whatever the answer is, the eye, combined with the human brain, is an incredibly sophisticated and powerful device; the ability to quickly assess a visual image and infer conclusions is astounding. However, a purely visual image has limitations; it has no direct means to measure distances, and optical events can be distorted due to the environment. Additionally, a visual image only looks at surfaces.

In surface oil and gas facilities visual inspection has long played a key part in integrity assessment, particularly when combined with magnetic or acoustic measurements, to investigate beyond where the eye can see. Within the wellbore, visual inspection had not been an option until recent advances in technology. Historically, in-well cameras have been used to a limited extent, with restricted functionality, typically black and white still pictures and the identification of stationary fish at known depths being the main application.

As a result, wellbore integrity has relied on logging with electromechanical instruments such as multifinger calipers and/or acoustic imaging tools and electromagnetic logging tools. In the majority of cases these logs require a sophisticated analysis to produce an “interpretation” of the well integrity status involving specialist skills, and results are typically not available until several days after the intervention. Due to the complex nature of well integrity challenges this, in many cases, leaves questions unanswered and leads to “I wish we had…” conversations.

Better telemetry

With the development of high-speed telemetry running at up to 300 kilobytes per second on monoconductor electric line it is now possible to transmit much larger quantities of data up logging cables during pressure control operations. This higher data density allows real-time diagnosis of wellbore integrity problems by combining quantitative multifinger caliper measurements with full-color high-density frame-rate downhole video in a single tool string.

Typically the tool string is run in the well with the downview camera operating to provide images at up to 25 frames per second, giving an overview of the wellbore in clear fluids such as completion fluids, produced water, condensate or gas. The multifinger caliper is then logged up the well in the normal manner. The latest acquisition and processing software makes real-time corrections for eccentralization and provides 3-D and cross-sectional analysis, meaning that potential issues with integrity can be seen in real time while the logging tool is still in the well.

FIGURE 1. A still picture from the downhole video confirms the split in the casing. (Source: EV)

These data also can be streamed to remote offices, allowing engineers located in the operator’s central offices to make decisions on the logging program as it happens. Typically, when an integrity issue is seen in isolation through a multifinger caliper log, the log may draw concerns about the validity of the data if observing a complex or unexpected response. In a recent operation while inspecting high-strength P110 steel casing, all anxiety was eliminated when the color sideview camera was able to confirm an actual split.

Figure 1 shows a still picture from the resulting video that confirms the split in the casing. As a result, the operator was able to pull the logging string out of the well and rig down, 100% sure of the well integrity diagnosis.

Data from the multifinger caliper and the downhole video camera also can be processed after the job and overlaid to provide an integrated image. Figure 2 shows an overlay of the sideview camera image on top of the 3-D multifinger caliper image.

A different perspective

This example also shows the differences in perspective between a traditional caliper log and a video inspection. A traditional caliper log is typically presented on a compressed vertical scale, meaning small-scale events can be missed completely. With the latest high-speed telemetry, a multifinger caliper will have a vertical resolution of 3 mm when logged at 10 m/min (31 ft/min). At faster logging speeds the vertical resolution will be reduced accordingly. Circumferential coverage will be determined by the number of fingers vs. the pipe internal diameter but is always a fraction of the total circumference.

In addition, when a mechanical measuring arm is pulled up the well, the response to different mechanical features may not be a true representation of the actual feature. Figure 3 is a mosaic obtained by stitching continuous video to create a panoramic style image of an event in the well. The corrosion captured by the sideview camera in this instance, although considerable, was missed entirely by the multifinger caliper due to the size and depth of the pitting. With time, if undetected, it is likely that such pitting could spread to become more generalized corrosion. With the appropriate information, however, measures can be taken to mitigate this risk.

FIGURE 2. Data from the multifinger caliper and the downhole video camera can be overlaid to provide an integrated image. (Source: EV)

These are just a few examples of combined value of qualitative and quantitative measurements. The visual qualitative image provides a level of detail unachievable with traditional logging sensors but relies on a clear fluid and does not provide a direct measurement of features. The quantitative logging sensors provide an overall assessment of the wellbore but lack the detail of the visual image and rely on interpretation. The combination of both sensors into a single tool string provides the best answers.

Work is ongoing to further develop this well diagnosis in real time concept. Specialized software now allows relative dimensioning of features based solely on visual images from downhole cameras. Knowing the internal diameter of the completion in the zone where the camera is, a 3-D mesh can be fitted to the picture, and based on that fit, dimensions can be estimated to an accuracy of 10%. Without replacing tools such as the multifinger caliper this capability adds useful functionality to the downhole video camera.

Similarly, using high-speed telemetry to transmit higher data density to surface, additional sensors are being added to the well integrity diagnosis tool string. Real-time pressure and temperature sensors will shortly be available with the integrated video caliper string. An additional gamma ray tool also can be added to detect scale or naturally occurring radioactive materials in the well. Future work will add thickness and ultrasonic noise tools to the string.

FIGURE 3. The corrosion captured by the sideview camera was missed by the multifinger caliper due to the size and depth of the pitting. (Source: EV)

Well integrity is a complex area with multiple unknowns. In openhole wells the industry relies on multiple sensors to evaluate formation properties due to the complexity; however, relying on a single sensor such as a multifinger caliper to provide a definitive answer to well integrity questions is in many cases simplistic.

A better approach is to develop a single well integrity diagnosis tool string to which multiple sensors can be added and removed as required to fully evaluate the condition of the completion. This results in real-time answers so that well engineers can investigate and diagnose issues there and then, much as a doctor would, allowing the well intervention team to complete operations with full confidence.