Need to design well completion from the least-skin option
Kelvin Smejkal
Baker Hughes INTEQ
A few years ago, a major operator commissioned a year-long program to identify the most efficient type of well completion. Efficiency, for the purpose of the study, was defined in terms of production, injection, cost, longevity, and ease of workover.
Within a short period of time, all available data pointed toward open hole as being the most efficient completion type.
Since it is documented that open hole completions have higher productivity and greater longevity than their cased hole counterparts, the question becomes: "How can we account for these differences?"
The most important difference, as always, is skin. The reservoir (stimulated or unstimulated) dictates the maximum rate possible at its present state of depletion. Potential chokes to a maximized production rate include:
- Drill-in and completion formation damage
- Tubulars
- Surface facilities
- Precipitates from reservoir fluids
- Phase changes
- Fluid contacts
- Fines production
- Wellbore configuration
- Compaction.
Of these chokes, only drill-in and completion formation damage, precipitates, phase changes, and fines production are skin driven. Compaction may show up as skin but would probably be the same for both types of completions since it is a depletion phenomenon.
Skin development
Each of the skin-driven chokes is time dependent in the sense that a completion begins the moment the drill bit enters the producing formation. This also is the time when drill-in skin damage begins.
Drill-in damage can be minimized with the use of a properly selected drill-in fluid. These fluids are designed to deliver good drilling fluid properties while protecting producing formations against skin damage. These fluids protect the payzone with a thin filter cake that is easily removed by production. However, drill-in skins are only part of the story and other opportunities for additional skin damage continue to present themselves all the way through to final completion.
In terms of induced damage (as well as some non-Darcy skin effects due to inefficiencies inherent in perforating), open hole completions usually exhibit less skin than cased holes on initial completion. This explains the open hole's higher initial production rate.
The cause of the cased hole's higher skin and lower initial production is most easily explained by the fact that 12 perforation shots per ft will not deliver 12 damage-free perforations. This results in cased hole with a reduced and damaged flow area. In turn, this creates higher pressure drops and fluid velocities than found in an open hole completion. A secondary problem is debris preferentially going into the open perfs and damaging them further.
While skin has a less pronounced effect on initial production from high capacity well bores or reservoirs with low viscosity fluids, it becomes a major impediment to low capacity well bores or reservoirs with viscous crude. In a high permeability reservoir, a skin of 7 might not significantly impact initial production rates. However, this same skin can cut initial production 50-80% in low permeability reservoirs.
Unfortunately, stimulation treatments designed to remove drill-in and completion induced skin are often inconsistent. One well may respond and the next show little or no improvement. Once a well is damaged, it is almost impossible to get it back to a skin of one or less.
Skin constancy
Well skin rarely remains constant over the life of the reservoir. In fact, skins usually increase over time with cumulative production. This is most often the result of workover phase changes, precipitates, and fines, all of which are depletion dependent. Their development is a function of pressure drop, particle size and concentration, reservoir fluid composition, mineralogy and velocity (shear). The initial skin serves as a seed for the subsequent skin development.
For example, drill-in and completion induced skins create pressure drops, increase shear, and provide a filter medium. Pressure drops and increased shear can, at certain stages of depletion, induce the precipitation of paraffins, asphaltines and minerals.
Filtration skin can translate into the formation of new filter cake. This internal filter is created by fines in the reservoir fluid bridging on (and in) the skin. Permeability may also drop with any phase change. It is easy to see that these phenomenon have a synergistic effect that can sometimes rapidly increase skin with time.
Minimizing skin
Since open hole completions exhibit lower initial skins than cased hole, it follows that their skins will increase more slowly over time. Unlike the effects of initial skin on high and low permeability reservoirs, depletion dependent skin can effect any reservoir regardless of the wellbore capacity. Therefore, no matter what type of well, one usually benefits by minimizing initial skins.
Regardless of whether wells are completed within the same reservoir, open holes last longer and have higher productivity. This is true for both gravel packed and non-gravel packed well bores.
In addition, open holes cost less to complete and their workover costs are comparable to cased hole completions. With all data pointing toward open hole as the most efficient completion type, the question becomes, "Why isn't open hole the most popular completion type?"
Open-hole obstacles
While operators familiar with the process support the efficiencies of an open hole completion, interviews with others revealed three major obstacles to this completion path. These include
- Reduced wellbore stability
- Insufficient zonal isolation (during completion or workover)
- Lack of awareness.
Let's first focus on two of these objections: reduced wellbore stability and insufficient zonal isolation. In most cases, these objections can be addressed at the time of completion with proper techniques, completion fluids, and equipment selection.
Although the potential for reduced wellbore stability exists, most open holes are more than stable enough to permit an initial completion and later workovers. The majority of "train wrecks" are often a result of poor drilling or completion practices. These failures could have been avoided with proper preplanning and:
- Identifying problem shales during drilling
- Isolating known troublesome shales
- Maintaining a proper fluid weight throughout drilling and completion operations.
Open hole completions are far more amenable to zonal isolation and fluid shut-off than most realize. Gravel packing is effective in achieving zonal isolation on both initial completions and later workovers. Another option involves isolating intervals within a gravel-packed wellbore using coiled tubing. While effective, this procedure requires preplanning for the proper screen size/type, tubulars and nipples.
Within certain limits, even intervals in non-gravel packed open holes can be shut-off or stimulated through pressure transient placement or plug back techniques. Pressure transient placement is an old, but effective, technique that uses differences in transmissibility to place material in a given zone. It requires treating the highly contrasting permeabilities between intervals, measuring the phenomenon before (and during) treatment, and open access to the formation. For a plug back to be effective no protective liners or screens that would prevent placement of the plug back material can be present.
Awareness
The remaining obstacle to open hole completions is a lack of awareness. This was noted time and time again during the year-long completion efficiency study. As part of a simple test, operators and service company personnel were asked to design a completion for a normal-pressured, single-producing, black oil horizon. The only information they were given was that it would take three strings of casing to get to the pay. They were then asked to draw the completion.
After a few questions (e.g. Is this well onshore?, Is the formation consolidated?, etc.), they each produced a cased hole completion. Even those working in the open hole country of West Texas drew cased hole completions. Over the course of the one year period, no one ever drew an open hole completion or, in fact, anything other than a cemented, perforated, cased hole completion.
There are many efficient open hole completion paths, but because of a stereotyped cemented cased hole consciousness, they are never considered. The rejection of an open hole approach is compounded by the lack of knowledge surrounding their completion and work over.
Since every completion starts out open hole and it is the most efficient completion, this seemed a logical place to begin our guide. Through a series of questions, the reservoir (and well conditions) are used to determine the proper completion type. These completion types include open hole, cased hole and through-tubing. As with any flow chart, its purpose is to stimulate or generate out-of-the-box thinking rather than become a blind guide. No artificial intelligence mechanism can cover every possible well scenario.
REFERENCES:
Leibach, R.E. and Cirigliano, J.: "Gravel Packing in Venezuala," Proc., Seventh World Pet. Cong., Mexico City (1967) Sec. III, 407-18.
Penberthy, W.L. Jr. and Shaughnessy, C.M.: "Sand Control," SPE Series on Special Topics Volume 1 - Henry L. Doherty Series, 1992
Halliday, W.S.: "Drill-in Fluids Control Formation Damage," World Oil., December 1994.
Copyright 1995 Offshore. All Rights Reserved.
