Conveyance: Downhole Tool Delivery Methods in Well Intervention

What Is Conveyance?

Conveyance (also called tool conveyance or downhole conveyance) is the method used to transport logging instruments, perforation guns, wellbore intervention tools, and completion equipment to a specified depth in the wellbore, encompassing wireline (electric line and slickline), coiled tubing, drill pipe, and tractor-assisted delivery systems. The choice of conveyance method is determined by well inclination, wellbore fluid environment, target depth, required tension or compression capability, and whether real-time data transmission to surface is needed; no single method is optimal for all conditions, and selecting the wrong conveyance is a primary cause of stuck tools and failed intervention runs.

Wireline and Slickline Conveyance

Electric wireline (e-line) remains the dominant conveyance method for open-hole and cased-hole logging in wells with inclinations below 60-70 degrees. A monoconductor or multiconductor armored cable connects the downhole tool string to the surface logging unit, transmitting power down and data up simultaneously. Seven-conductor cables allow multiple tools in a string to communicate with the surface computer in real time; single-conductor cables use pulse-position modulation to multiplex data from several sensors on one channel. Modern wireline cables are rated to tensile loads of 14,000-30,000 lb, depths of 25,000-35,000 ft, and temperatures up to 350°F (175°C) or higher on high-temperature variants. Logging speeds for standard formation evaluation tools range from 900 m/hr (gamma ray, resistivity) to 3,600 m/hr (simple casing collar locators), and a complete openhole log suite in a 3,000-m well can be acquired in 8-16 hours of rig time.

Slickline uses a solid single-strand wire (0.072 to 0.125 inch diameter) with no electrical conductors; it transmits no data and provides only mechanical depth and weight control. Slickline is the preferred conveyance for routine cased-hole interventions — setting and retrieving plugs, shifting sleeves, gauge running, and sand bailer operations — where the low cost and simple surface equipment justify the absence of real-time telemetry. Modern memory gauges can record pressure and temperature data downhole for retrieval after the run, partially compensating for the lack of real-time transmission. Slickline units mobilize in a single truck and can rig up in 2-4 hours versus the 6-12 hours needed for an electric wireline spread, making slickline the default choice for any intervention where real-time data is not required.

Pipe-conveyed wireline bridges the gap between wireline and drill pipe conveyance. The logging tool string is attached below the drill pipe and run to depth on pipe; an insulated electric wet-connect at the tool top mates with a conductor in the drill string to establish real-time telemetry. This method is used in highly deviated wells where the logging string cannot reach total depth on wireline alone but where the operator wants the data quality and real-time capability of electric wireline rather than the memory-mode recording of drill pipe logs. Pipe-conveyed logging adds $50,000-$150,000 to rig time costs compared to conventional wireline due to the additional make-up and break-out time for the drill string.

Conveyance Synonyms and Related Terminology

Conveyance is also referred to as:

• Tool delivery — the operational term used in intervention planning documents and wellsite programs
• Deployment method — used interchangeably in completion and intervention engineering contexts
• Well access method — emphasizes the barrier-crossing and wellbore entry aspect, particularly in live-well operations
• Running method — shorthand used in completion procedures describing how a specific tool is run to depth

Related terms: wireline, coiled tubing, slickline, well intervention, perforation, downhole tractor

Frequently Asked Questions About Conveyance

Why does wireline fail in highly deviated wells and what replaces it?

In wells inclined more than 60-70 degrees from vertical, the component of gravity acting along the wellbore axis falls below the friction force between the tool string and the casing or formation wall. The tool string stalls or hangs up before reaching total depth regardless of how much cable slack is paid out at surface. Three alternatives exist: coiled tubing physically pushes the tool string from behind and can negotiate horizontal sections up to 6,000 m lateral in favorable conditions; downhole tractors grip the casing ID with motorized wheel pads and crawl forward, pulling the wireline tool string behind them at 100-400 m/hr without changing the surface equipment; and drill pipe conveyance uses the rigid column of the drill string to push tools to any depth the drill bit can reach, at the cost of full rig time.

What is the main limitation of coiled tubing conveyance in long horizontal wells?

Coiled tubing buckles helically when the compressive force in the lateral exceeds the critical buckling load, which is proportional to tube stiffness (EI) and inversely proportional to the annular clearance between the CT and casing. Once helically buckled, additional pushing force at surface is lost to friction against the casing wall rather than advancing the tool, creating a depth limit called the lockup point. For 2-3/8 inch CT in 5-1/2 inch casing, lockup typically occurs at 3,000-5,000 m of lateral. Larger-diameter CT extends reach but is heavier and slower to rig up. Hydraulic drag-reduction fluids, friction reducers pumped through the CT, and real-time tension modeling can push the practical reach to 6,000-7,000 m in optimally planned operations.

How is conveyance method selection documented in a well program?

The conveyance selection is formally documented in the well intervention or logging program as part of the operation-specific section, which includes the well trajectory survey, the expected friction factor calculations, the maximum and minimum loads on the conveyance string at each critical depth, the contingency conveyance method if the primary method fails, and the wellbore pressures and fluid weights that affect equipment ratings. Regulatory bodies in most jurisdictions require that the intervention program be reviewed and signed off by a petroleum engineer before operations begin. For offshore wells in the North Sea and Gulf of Mexico, well integrity documentation must confirm that the conveyance method is compatible with the installed well barriers, particularly for live-well operations where killing the well before intervention is not planned.

Why Conveyance Matters in Oil and Gas

Conveyance is the enabling technology for every downhole measurement and intervention operation across the life of a well. The choice of conveyance method directly controls whether a logging program can be completed in a single trip or requires multiple runs, whether perforation guns can reach the target zone in an extended-reach horizontal well, and whether a wellbore integrity inspection can be completed safely while the well is on production. As the industry drills increasingly complex horizontal and multilateral wells with laterals exceeding 3,000-5,000 m, the engineering challenge of delivering tools to total depth has grown substantially and now rivals the drilling itself in planning complexity. Advances in downhole tractor technology, real-time friction modeling, and embedded-telemetry coiled tubing are steadily extending the operable envelope, but conveyance selection remains one of the first and most consequential decisions in any well intervention planning process.