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Multiple Completion

What Is a Multiple Completion?

Multiple completion (also called a dual completion, multizone completion, or stacked completion) is a well design in which two or more separate reservoir intervals are produced simultaneously or selectively through a single wellbore, using a system of packers, tubing strings, and downhole flow control devices that maintain hydraulic isolation between the producing zones. Rather than drilling a separate well to reach each productive interval, a multiple completion allows one wellbore to access several pay zones, reducing drilling costs and surface footprint while enabling independent or commingled production from each zone. The concept applies across conventional vertical wells producing multiple stacked sands, offshore wells with limited slot count, and increasingly in unconventional plays where operators perforate multiple clusters across several laterals from a single pad.

Key Takeaways

  • A production packer is the critical downhole tool in any multiple completion; it anchors to the casing and provides the hydraulic seal that isolates zones from each other and from the annulus.
  • Dual string completions use two separate tubing strings (a long string reaching the lower zone and a short string ending at the upper zone) to produce each zone independently to surface.
  • Commingled completions produce all zones through a single tubing string without isolation, accepting mixed fluid streams but sacrificing individual zone measurement and control.
  • Production allocation in commingled wells requires downhole flow meters, periodic production logs, or reservoir simulation to apportion produced volumes to each zone for regulatory reporting and royalty calculation.
  • In unconventional wells, stacked laterals from a single surface location represent the modern equivalent of a multiple completion, using separate wellbores rather than packers, but targeting multiple benches or formations from one pad.

How Multiple Completions Work

The foundation of any multiple completion is the production packer, a downhole tool that is set against the casing at a precise depth to create a pressure-tight seal. Above and below the packer, the wellbore is hydraulically isolated: one zone produces through the tubing inside the packer bore while the other zone communicates with the casing-tubing annulus or a second tubing string. In a dual string completion, the most common configuration for high-rate wells, the long string of tubing is landed at or below the lower perforation interval, while a shorter parallel tubing string terminates at the upper zone. A dual packer system isolates the two zones from each other and from the surface annulus. Each zone produces independently to separate wellhead outlets, allowing individual measurement, rate control, and workovers without disturbing the other zone.

Single-string commingled completions are simpler but sacrifice zone-level control. Perforations are opened in multiple intervals and all production enters the same tubing string. Commingling is acceptable when the zones have similar pressures and fluid properties, preventing crossflow between intervals through the wellbore. If one zone has significantly higher pressure than another, the high-pressure zone can inject fluid into the lower-pressure zone through the wellbore rather than producing to surface, a condition called crossflow, which reduces total production and can damage the lower-pressure reservoir. Downhole check valves or sliding sleeve valves are used to prevent crossflow when commingling intervals with different pressure regimes.

Fast Facts: Multiple Completion
  • Key downhole tool: production packer (permanent or retrievable) for zone isolation
  • Dual string advantage: independent production, injection, and measurement per zone
  • Single string advantage: lower cost, simpler wellhead, suitable for lower-rate wells
  • Crossflow risk: occurs when commingled zones have different reservoir pressures
  • Regulatory requirement: most jurisdictions require demonstrated hydraulic isolation between zones with different ownership or royalty obligations
  • Gas lift application: dual gas lift mandrels can be run in each string of a dual completion, with independent injection control at surface
  • Offshore application: critical for maximizing production from limited slot count on fixed platforms and FPSOs
  • Unconventional analog: stacked laterals (separate wellbores targeting different benches from one pad)
Field Tip:

Before commingling two intervals, always compare the shut-in wellhead pressures of each zone independently. If the pressures differ by more than 200 to 300 psi, commingling without downhole check valves risks crossflow from the higher-pressure zone into the lower-pressure zone. A simple wellbore pressure buildup test on each zone individually, with the other zone isolated by a temporary bridge plug, quantifies the static reservoir pressure and fluid gradient for each interval and is the most reliable pre-commingling diagnostic.

Packer Types and Downhole Equipment

Production packers fall into two broad categories: permanent and retrievable. Permanent packers are set with a tubing string and require milling to remove; they provide the most reliable long-term seal and are used in high-pressure, high-temperature, or corrosive environments where packer integrity is critical. Retrievable packers can be released and pulled to surface on a workover string, making them preferred for wells where future recompletions or zone changes are anticipated. Hydraulic-set retrievable packers are common in dual completions because they can be set and released using tubing string pressure without rotation, simplifying installation in directional wells. The packer element is typically a rubber or elastomeric compound rated to the expected wellbore temperature and compatible with the produced fluids, including H2S, CO2, and aromatic hydrocarbons.

Downhole flow control in multiple completions ranges from simple sliding sleeve valves that open or close perforation intervals to sophisticated intelligent well completions equipped with inflow control valves (ICVs) operated by electrohydraulic control lines run from the wellhead to each valve. Intelligent completions allow real-time adjustment of production from each zone without a workover rig, using surface-controlled ICVs to optimize zonal contribution, manage water or gas breakthrough, and extend well life. The economics of intelligent completions are justified in high-value offshore wells where a single workover rig intervention can cost several million dollars.

Production Allocation and Regulatory Requirements

When multiple zones with different owners, royalty obligations, or regulatory classifications are commingled, production must be allocated to each zone for accounting and reporting purposes. Regulatory bodies in most jurisdictions, including the Alberta Energy Regulator (AER), the Texas Railroad Commission (TRC), and the Wyoming Oil and Gas Conservation Commission, require operators to demonstrate that commingling is authorized and that a valid allocation method is in place. Allocation methods include downhole multiphase flow meters installed on each completion string, periodic production logging surveys that measure fluid entry by zone, numerical reservoir simulation history-matched to zonal pressure data, and proportional allocation based on periodic well tests of each zone individually with the other zones isolated. Dual string completions with independent surface measurement lines are the most defensible allocation method and are required by some regulators when commingling zones with different working interest owners.

  • dual completion: the most common form of multiple completion, producing two zones through a single wellbore with a dual packer and dual string or single string arrangement
  • commingled completion: a completion in which two or more zones produce through the same tubing string without individual zone isolation at the surface
  • selective completion: a design using sliding sleeve valves or other downhole tools to produce one zone at a time while closing others, allowing sequential or rotational production
  • stacked lateral: in unconventional development, separate horizontal wellbores targeting different benches or formations from a single surface pad, the functional equivalent of a multiple completion

Related terms: production-packer, perforation, intelligent-well, inflow-control-valve

Frequently Asked Questions About Multiple Completions

When is a multiple completion more economical than drilling separate wells?

Multiple completions become economically attractive when the incremental cost of adding a second or third completion string to an existing well is less than the cost of a separate wellbore to each interval. In deep, expensive wells (over 15,000 feet), high-cost environments such as offshore platforms, or areas where surface locations are constrained by topography or regulation, the savings from combining intervals in one wellbore can be very substantial. The economic comparison must also account for the flexibility lost by commingling: commingled wells cannot be individually optimized per zone and may require more complex workovers if one zone needs stimulation or plugback.

What is the risk of packer failure in a multiple completion?

Packer failure in a multiple completion can result in crossflow between zones, contamination of a higher-quality zone by water or gas from an adjacent zone, or loss of hydraulic isolation required by the regulatory commingling permit. Packer integrity is tested at the time of installation by applying annulus pressure and monitoring for bleed-off, and can be retested periodically using surface pressure tests. If packer failure is suspected, a production log or temperature survey can identify downhole fluid entry or exit points. Remediation typically requires a workover rig to pull and re-set the packer or to install a patch liner across the failed element.

How does gas lift work in a dual completion?

In a dual string completion with gas lift, each tubing string is equipped with its own gas lift mandrels and gas lift valves at the appropriate injection depth for that zone's flowing pressure gradient. Surface gas lift injection is split between the two annulus paths using individual wing valves and chokes on the wellhead, allowing the operator to set different injection rates for each zone based on its GOR, productivity index, and flowing bottomhole pressure requirements. The two strings share the casing annulus as the gas lift supply conduit in a standard dual-concentric completion, or use separate casing annuli if the completion uses a two-casing system.

Why Multiple Completions Matter in Oil and Gas

Multiple completions are a fundamental tool for maximizing the value of each wellbore, particularly in mature fields with stacked pay, offshore developments with limited well slots, and unconventional plays where pad drilling makes multi-bench development economically necessary. By producing multiple zones from one location, operators reduce the capital required per barrel of productive capacity, minimize surface disturbance, and lower long-term abandonment liability. The ability to isolate, test, and selectively produce individual zones also gives reservoir engineers higher-quality data for managing depletion and planning infill drilling. As fields mature and operators seek to extract remaining reserves more efficiently, multiple completion strategies become increasingly important to field development economics.

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