Completion and Stimulation Aspects
The most common types of horizontal well completions are:
- Open hole, or open hole with slotted, perforated or pre-packed liner, and sand control screens with external open-hole packers
- Liner with external casing packer
- Cased, cemented and perforated
- Cased, cemented, perforated, multiple packers for multi-staged fracturing of unconventional shale, tight oil & gas.
- Multi-lateracompletions
Figure 1 illustrates these completion types ((a) open hole, (b) open hole with slotted liner, (c) liner with external casing packer, (d) cemented and perforated ).
Selection of a particular type depends on such reservoir conditions as:
- permeability and permeability anisotropy
- fractures
- section isolation along the horizontal well section
- hole stability
- sand problems
- coning problems
- production control
Open hole completions, where only the vertical section of the well is cased, were the most common completion types in the early years of horizontal drilling, and they are still widely used. They are relatively inexpensive and easy to execute, and they work well in consolidated formations were sand production and hole collapse problems are not matters of concern. In homogeneous formations that are not well-consolidated, operators often run slotted or pre-perforated liners in open hole. Where sand production is a problem, they may substitute pre-packed liners.
The main disadvantage of open hole and slotted liner completions is their inability to isolate zones. Considering that most formations are heterogeneous to at least some degree, this is a significant drawback, particularly if the completion interval is a long one.
In a number of instances, slotted liners equipped with external casing packers (ECPs) have proven successful at isolating formation intervals and providing for selective completion and stimulation. ECPs can be placed at strategic points in the liner string, and then inflated with mud or cement to seal off the liner/hole annulus. Their main drawbacks are that hole eccentricities can make it difficult to form an effective seal, and that they result in a relatively inflexible well design with respect to future workovers or recompletions.
A cased, cemented and perforated completion is the most expensive horizontal well design, and the most difficult to properly execute. At the same time, it is the most effective configuration for attaining zone isolation, and the most flexible in terms of future workover, recompletion or stimulation jobs.
Stimulation of Horizontal Wells
In general, the fluids and additives used in matrix stimulations for horizontal wells are the same as those employed in vertical wells. The formation damage profiles for horizontal wells, however, differ significantly from those of vertical wells. This requires modifications in treatment volumes and placement techniques.
With respect to hydraulic fracturing in horizontal wells, the materials used are likewise similar to those used for vertical wells. The orientation of well trajectories, and of the fractures themselves, represent the critical difference, and require careful planning of fracture stimulations even before drilling begins.
Economides (1993) addresses in some detail the considerations involved in horizontal well stimulation.
ICD Completion
Well completion with inflow control devices (ICD) can delay encroachment from bottom water and/or gas production from gas caps in relatively homogeneous reservoirs. In heterogeneous reservoirs, ICD completions depress intrusions of water and gas from high-permeability reservoir channels or fractures. Different types of ICDs are used in the industry, including orifice types, nozzle types, and helical channel types. Figure 2 shows an orifice type ICD, while Figure 3 depicts an ICD completion.
To delay water and/or gas coning in relatively homogeneous reservoirs, ICDs need to be sized and distributed to achieve a uniform pressure distribution along the sandface.
Figure 4 shows a comparison of annular pressures in a well with and without ICD installations. To postpone water and/or gas intrusion from high-permeability channels/fractures, ICDs need to be sized and distributed to achieve a uniform influx distribution along the sandface.
Figure 5 presents a comparison of influx profiles in a well with and without ICD installations. The result of the ICD design should be checked with reservoir simulation for effectiveness.
ICV/FCV Completion
Well completion with inflow control valve (ICV) or flow control valve (FCV). An active component installed as part of a well completion (Intelligent, Smart) to partially or completely choke flow into a well. Inflow control valves can be installed along the reservoir section of the completion, with each device typically separated from the next via a packer. Each valve can be controlled from the surface to maintain flow conformance and, as the reservoir depletes, to stop unwanted fluids from entering the wellbore. A permanent downhole cable provides electric and hydraulic conduits to relay commands from the surface to each valve. These devices are generally installed in multi-lateral horizontal well completions.
