Through-Tubing

Key Takeaways

• Through-tubing perforation gun design is constrained by the need to fit within the tubing ID while achieving sufficient perforation performance (penetration depth and entrance diameter) to provide meaningful flow contribution from newly perforated intervals: standard through-tubing perforating guns are manufactured in OD sizes of 1-11/16 inch, 2-1/8 inch, and 2-3/16 inch to fit within common 2-3/8 inch, 2-7/8 inch, and 3-1/2 inch production tubing IDs, with the gun OD constrained to be at least 1/8 inch smaller than the tubing drift ID to allow reliable run-in through any tubing restrictions; the smaller OD of through-tubing guns compared to conventional open-hole or through-casing guns means that the shaped charges must fit within a smaller gun body, limiting the explosive charge weight and therefore the penetration depth and entrance diameter achievable; typical 1-11/16-inch through-tubing guns achieve penetration depths of 5 to 10 inches in API RP 19B concrete targets (compared to 15 to 30 inches for conventional 3-3/8-inch to 3-1/2-inch casing guns), and entrance diameters of 0.20 to 0.30 inches (compared to 0.35 to 0.50 inches for conventional guns), which reduces the initial production response from through-tubing perforations compared to conventional perforations but may be acceptable in formations where the through-tubing perforation serves only as the initiation point for a subsequent acid or fracture stimulation rather than as the primary production pathway.
• Through-tubing logging tool design must achieve the required measurement accuracy and vertical resolution within the constraints of a tool OD small enough to pass through the tubing bore, which typically allows tool ODs of 1-11/16 inch to 1-7/8 inch for common production tubing sizes: the most commonly run through-tubing logging tools include production logging tools (spinner flowmeters, gradiometers, temperature tools, water holdup tools, and capacitance probes that characterize the flow profile within the wellbore to identify which perforated intervals are producing and at what rate and composition), cased-hole formation evaluation tools (pulsed neutron tools, carbon-oxygen logs, and electromagnetic induction tools that evaluate the water saturation of formations behind the casing perforations through the casing and cement to identify bypassed pay and monitor waterflood fronts), and mechanical integrity tools (ultrasonic imaging tools and electromagnetic casing inspection tools that evaluate the condition of the casing and tubing from within the tubing bore, detecting corrosion, perforations, cracks, and scale that affect well integrity); the smaller detector volume of through-tubing logging tools compared to larger-OD cased-hole or open-hole tools reduces the counting statistics for nuclear measurements, increasing the statistical uncertainty in the measured values and requiring slower logging speeds or longer depth-averaged windows to achieve the same statistical precision as a larger-diameter tool.
• Through-tubing wellbore intervention with coiled tubing allows more complex and powerful operations than slickline through-tubing work because coiled tubing can be pumped through with fluids (allowing hydraulic jetting, acid stimulation, and scale dissolution through the coiled tubing while it is inside the production tubing) and can push tools into horizontal or high-angle wellbores where slickline cannot apply the lateral force needed to advance the toolstring: coiled tubing through-tubing operations include matrix acid stimulation (bullheading acid through the coiled tubing and into the formation via existing perforations to dissolve scale, fines, and formation damage around the perforations), mechanical jetting (using a hydraulic jetting tool on the end of the coiled tubing to create new perforations or extend existing ones by eroding channels into the formation rock with high-velocity abrasive-laden fluid), sand cleanout (using jetting and washing tools on the coiled tubing to remove sand accumulations from below the perforations that are blocking production), and scale milling (using mechanical mills or abrasive jetting to remove hard scale deposits from inside the tubing or casing that are reducing the effective flow area); the through-tubing coiled tubing operation is limited by the coiled tubing OD that can fit through the production tubing (typically 1-inch to 1-1/2-inch OD coiled tubing through 2-3/8-inch to 3-1/2-inch production tubing), by the buckling tendency of the coiled tubing inside the production tubing in high-angle wells, and by the friction pressure of pumping fluids through the small-annulus between the CT OD and the tubing ID.
• Through-tubing retrievable bridge plugs and packers allow temporary zone isolation to be established through the tubing bore without pulling the completion, enabling selective zone testing, fracture stimulation of individual zones in multi-zone completions, and fluid displacement operations in specific wellbore intervals without requiring a full workover to gain access to the casing bore: through-tubing bridge plugs are manufactured in OD sizes that fit through common production tubing IDs (typically set in the casing below the tubing and requiring a casing OD-compatible slip and packing system), with their small running OD expanding to the full casing ID upon setting using hydraulic, mechanical, or explosive setting energy from a wireline or slickline setting tool; the limitation of through-tubing packer and bridge plug systems is that the packing element and slip system, which must fit through the small tubing ID in the retracted position, may not be as robust as a conventional packer with the same casing OD when expanded because the mechanical components are constrained by the tubing ID in their unexpanded configuration; the through-tubing plug must provide adequate differential pressure rating in the fully set position (typically 3,000 to 5,000 psi for standard through-tubing plugs) while having an OD in the retracted position small enough to pass through the tubing bore and any restrictions below it.
• Through-tubing sand control remediation addresses the critical problem of failed or inadequate sand control in wells that are producing sand through their existing completion, where the alternative of pulling the completion and installing a new sand control system is prohibitively expensive relative to the production rate and remaining reserves in the well: through-tubing sand control options include chemical sand consolidation (pumping a resin or silicate consolidant through the production tubing and into the perforations to chemically bind the formation sand grains together and create a self-supporting sand pack around the perforations), through-tubing gravel packing (running a small-diameter gravel pack screen assembly through the production tubing and positioning it over the open perforations, then pumping a gravel slurry through the coiled tubing to pack the perforations with gravel), and through-tubing mechanical screens (running expandable screens or pre-packed screens through the tubing bore and positioning them in front of the producing perforations, where they provide mechanical filtration of formation sand); the effectiveness of through-tubing sand control is generally lower than that achieved by a full workover with conventional sand control installation, because the small OD tools available for through-tubing deployment cannot create as complete and uniform a sand control package as conventional-OD tools run in the open casing bore, but the significantly lower cost of the through-tubing approach often makes it the economically preferred option for marginal wells where the production rate does not justify a full workover.