Well Completions involves a two-step process—starting with the lower completion and followed by the upper completion. It commences during the final stages of drilling, when the associated completions products are lowered into the well as part of the final casing string.
Well Completions—The Lower Completion
The lower completion is initially deployed. Its primary function is to establish contact with the reservoir, and therefore has a direct bearing on the well’s productivity. Hydraulic fracturing is the largest step in this process. Wells are fracture stimulated with multiple fracture stages along the reservoir (i.e. multi-stage fracturing). Multi-stage fracturing is enabled by a range of completion hardware products, depending on the operator’s preference.
Plug-and-perf completions was the earliest multi-stage completion technique. It was used in conjunction within a cemented liner set across the reservoir. Each fracture stage treatment is isolated with “composite plugs”, deployed on electric line. The composite plugs serve as a pressure barrier to facilitate the next treatment in the shallower part of the well. All plugs are finally milled out with coiled tubing after the last fracture treatment is completed.
Open hole completions became popular next because of the ability to deliver higher fracture treatment efficiency, measured as stages per day, or equivalent. An un-cemented liner consisting of “open hole sleeves” is typically deployed across the reservoir. A graduated metallic ball with a progressively increasing diameter is pumped during the penultimate phase of the fracture stage. This ball follows the fracture fluids, eventually fitting into a “seat” designed within the open hole sleeve, isolating the lower fracturing stage, simultaneously shifting open the upper sleeve to conduct the next fracturing treatment. The annular space between the liner and the reservoir is segmented by open-hole packers, with the swellable packers being the more prevalent products used for this purpose. The early generation of ball-based open hole completions eliminate electric line- or coiled-tubing-deployed operations, leading to a faster completions sequence.
A disadvantage of open hole completions is their inability to control the point of fracture initiation. Several operators reported lower well productivity indices (PI) because of this drawback. Coiled tubing-deployed sleeves were introduced to overcome this limitation - controlling fracture initiation, and providing operational efficiency. The fracturing treatment is injected with the presence of coiled tubing within the wellbore. The coiled tubing string is progressively translated to facilitate a sequential fracture treatment along the wellbore length. Zonal isolation is achieved through “coiled tubing sleeves” that also contain a receptacle to hold a resettable coiled tubing-deployed packer, containing the fracturing treatment within the well segment of interest. Coiled tubing fracturing can be done both in an open hole and cased hole well.
Finally, and for cased hole plug-and-perf operations in particular, electric line-deployed plugs and perforating guns cannot be deployed initially because the wellbore represents a “closed system.” No fluid movement, a necessary requirement to “pump” guns, is possible. The first fracturing stage therefore requires perforating with coiled tubing-deployed TCP guns. This is a costly and time-consuming operation. The Toe Initiation sleeve is a specific technology that eliminates this initial coiled tubing-deployed TCP perforating guns that commence a fracturing sequence. It’s use is optional, however, this technology is gaining popularity since these sleeves reduce cost and overall time of the lower completion.
The lower completion has a bearing on the well cost, and its productivity, therefore directly impacting the overall well economics. Its specific choice (plug-and-perf versus sleeves, open hole versus cased hole wellbore) varies across reservoirs, and even within a particular reservoir in select instances. Today, cased plug-and-perf completions are more prevalent in the United States, while sleeves continue to dominate in Western Canada.
Well Completions—The Upper Completion
An upper completion refers to a suite completion hardware products that are deployed in all well types – vertical, deviated, or horizontal.
Broadly speaking, these consist of:
- Packers to seal the lower completion from the reset of the wellbore,
- Safety systems, such as subsurface safety valves to control the well’s flow during unanticipated events,
- Completion accessories that facilitate initial deployment and well maintenance over its producing life,
- Well remedial technologies, including casing patches,
- Gas lift mandrels to support this specific artificial lift methodology, and,
- Permanent downhole sensors to provide data in a continuous manner, without requiring a well intervention. Such information is used to optimize the well’s recovery.
The upper completion design architecture is governed by multiple drivers, including well productivity, cost, choice of artificial lift system, regulatory requirements, anticipated workover and intervention activity, and other considerations.