New hardware correlates depth and allows unobstructed coiled tubing (CT) procedures for more accurate placement and job confidence.

As CT well intervention operations have broadened in their use and applications, so has the need for greater downhole depth accuracy.
To meet this need, a new wireless, CT-based correlation tool uses pressure telemetry to transmit, in real time, depth correlation data to surface, eliminating the limitations and added requirements of current methods. The acquired data generates a casing collar locator (CCL) log, which can compare and correlate well depths with established well logs, that meets American Petroleum Institute (API) standards. This real-time information verifies depth accuracy quickly, helping operators make critical decisions during a CT job.
Depth correlation technology
Depth-critical reservoir remedy solutions using CT have presented challenges to the industry. Depth measurement errors can cause discrepancies between the formation's location targeted for a CT job and the placement of the tool. While CT control at surface has improved pipe measurements, industry experience has confirmed this is not enough.
Surface CT depth measurements determine the pipe length deployed in the well, which may, for several reasons, differ from true depths. Errors can stem from several sources, including pipe length measurement inaccuracies, erroneous base log depths or errors in procedures or references. Also, environmental effects and events associated with deployed CT can result in pipe distortion and inaccurate depth estimations. Such factors also include pressure and temperature effects and CT-related issues like elastic and plastic deformation and sinusoidal and helical buckling. While these phenomena are common, experience shows downhole depth correlation can eliminate depth errors.
Correlated depth measurements ensure that operators perform depth-critical CT jobs at a specified formation target, as mapped on a previous well log. For this reason, a downhole tie-in is necessary when conducting well intervention operations for accurate placement of depth-specific CT applications. Practices that correlate and correct depth measurements include memory reference tools, nipple and tubing end locators and wireline inside CT. However, each method has its drawbacks and added requirements, such as additional memory tool runs, particular well configurations and possibly added runs due to tool incompatibilities, or complex logistics associated with CT-inlayed electric lines.
The depth correlation dilemma
The new wireless, CT-based correlation tool uses pressure telemetry to transmit depth data uphole. The Schlumberger DepthLog tool detects casing anomalies and uses the resultant fluid pressure changes within the CT to send real-time pressure signals to surface, thus eliminating the need for electric line. This acquired data is used to produce an API CCL log (Figure 1), which enables the operator to correlate well depths accurately in reference to the baseline log. Since the results are provided in real time, the depth verification process allows the operator to make critical decisions during the job. The result is more timely and convenient baseline cased-hole log comparisons.
The tool has four sections (Figure 2):
• a traditional, passive CCL tool that is made up of a coil and passive magnet and has flow-through capability;
• a downhole processor that monitors the CCL output and issues commands to the signaler when detecting anomalies;
• a signaler that contains a primary flow path and secondary path with an orifice through which flow is diverted when receiving a processor command, sending a pressure pulse to surface; and
• a lithium battery that powers the processor and the signaler.
Integral to the correlation tool is the CCL, which uses two important properties of magnetic fields to locate casing collars and other hardware. First, metal close to the magnet influences magnetic fields. Second, a changing magnetic field induces an electric current in any circuit within the changing field.
During an intervention, as the tool is lowered into the wellbore, its magnetic field senses the casing. When it detects varying casing thickness (due to a casing collar, for example), the produced magnetic field is interrupted, which then induces an electric current in the CCL circuit. This induced electric current signals to the processor the presence of a casing collar. The processor then sends a command to the signaler.
Once the tool detects a casing collar, the signaler communicates the information to surface. The tool uses hydraulic pressure for signaling. Within the signaler, downhole flow diverts from the primary flow path to the secondary path that contains the orifice. The resulting flow restriction causes pumping pressure to rise at the surface, where it is detected. The pressure change signaling is carried out when a piston, activated by casing collar detection, closes off the primary flow path to the signaler and redirects flow through the secondary path and its orifice.
This signaling capability is introduced into the flow being pumped through the bottomhole assembly (BHA), without diverting flow to the annulus or causing pressure increases in the tool string below. As a result, neither the job design nor its implementation is impacted because the flow is temporarily restricted rather than diverted or shut off entirely. Also, this makes the correlation tool virtually invisible from the standpoint of the BHA tools below, thereby minimizing the possibility of interaction among downhole tools and their functions.
The tool's flow-through capabilities provide the effect of unobstructed, open-ended tubing. Thus, it acquires depth information in real time without interfering with other downhole operations or fluid flow through it or the CT, permitting an unobstructed well intervention. As a result, the CT's BHA position is determined quickly and accurately without restricting whatever CT application is under way, such as packer setting, fluid treatments or squeezes and perforating gun placements. In this respect, it adds value to the job without adding compromises to the primary job tool, such as a plug setting tool or a firing head.
The 21/8-in. correlation tool has a minimum flow-through inner diameter of 0.656 in. with shock-, acid- and H2S-resistant features. The tool enables crews to correlate bottomhole depths accurately in vertical, deviated and horizontal completions. It operates at up to 13,500 psi and 300°F (149°C). A higher-temperature battery pack is available, boosting the tool's temperature threshold to 350°F (177°C).
Intervention applications
Certain well interventions demand particularly precise depth accuracy. These include zonal isolation, selective stimulation, perforating gun conveyance and plug setting. The tool helps overcome the challenges of these depth-sensitive well interventions. For example, thin, closely spaced zones that require an isolation procedure to shut off a lower water-producing zone while still maintaining hydrocarbon production of the upper zone would require precise depth measurements.
Traditionally, wireline-inlaid CT would perform this depth-critical operation, enabling the operator to know in real time the bridge plug's location so it could be set accurately between the two zones. However, while this method achieves the basic objective - accurate placement - it presents logistical challenges and service quality risks by requiring a wireline inside the CT string, preventing the passage of process balls as well as acid and gel treatment fluids. The wireless correlation tool allows process balls and fluid treatments to pass through in the same run, while supplying real-time knowledge of the plug's location for the setting procedure.
In addition, the tool not only increases intervention process accuracy, but it reduces operations costs by eliminating the need for electric line (and thus, the added logistics to perform other operations) or the extra correlation runs required with memory-based logging tools.
The tool applied in a new well
The correlation log was used to support an intervention in a new well that produced only gas and hydrates after two flowing attempts. The suspected problems were damage in the horizontal section and gas leakage through an unclosed 1-in. cementer port in the 41/2-in. liner at 10,650 ft (3,248 m). Cleaning up the horizontal section and producing the well required isolating the orifice with a tubing patch.
Accomplishing this required a job design that would set a bottom packer in a pup joint below the leaking port and a top packer in a solid joint above. Given the temperature, tubing size and estimated buckling and stretch, the use of memory correlation and flagging pipe would not have been accurate enough for precise patch placement. After careful review, the operator selected the new tool to correlate depth so as to accurately place the tubing patch within an 8-ft (2.4-m) window. The BHA length extended 45 ft (14 m), and the borehole deviated 88° in the setting window.
Positioning of the tubing patch tool was correlated in real time to the well baseline
log. A subsequent well log confirmed the tubing patch had been set at target depth. The well produced 1,300 b/d of oil after the tubing patch job.
Reliability the key
When performing depth-critical CT intervention operations, accurate correlation prevents depth errors associated with incorrect tubing length measurements (from tubing deformation) and incomplete or inaccurate well references and histories. Accomplishing this requires improved CT control at surface along with a reliable method to correlate the BHA position to the target depth identified for the reservoir remedy solution. Downhole depth correlation using data generated by this new tool ensures accurate depth-sensitive CT procedures without restricting their implementation.

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