A well can produce less than it should even when plenty of oil or gas is left in the reservoir. Two common reasons are sand travelling with the produced fluids and water arriving where it is not wanted. Both are easier and cheaper to manage when they are planned for in the completion design than when they appear in a producing well.
Why wells produce sand
Weakly consolidated sandstones, common in young basins such as the Niger Delta, hold their grains together loosely, if at all. As oil, gas and water flow towards the well, the drag on the grains and the stresses around the wellbore can break them free. Sand production often gets worse as reservoir pressure falls and once the well starts producing water.
Once sand is in the system, it is costly. It erodes chokes, valves, pipework and pumps, settles in separators and flowlines, and can fill the bottom of the well and cut its output. Clean-outs and repairs take time, and severe erosion can lead to leaks.
Ways to control sand
Not every well needs active sand control. The decision rests on the strength of the rock and the production rates planned, and the options range from careful management to mechanical exclusion:
- Managing production: limiting rates and pressure drawdown so that the rock is not stressed beyond what it can bear
- Standalone screens: wire-wrapped or mesh screens across the reservoir section that hold back the formation sand
- Gravel packs: sized gravel placed between a screen and the formation, forming a filter that keeps the formation sand in place; a frac-pack combines this with a short hydraulic fracture
- Chemical consolidation: resins injected into the rock around the wellbore to bind the grains together
The right method depends on the size and spread of the sand grains, the type of well and its expected life. Analysis of rock samples from the reservoir is usually the starting point for sizing screens and gravel.
Why unwanted water is a problem
Most wells produce some water sooner or later, as the reservoir is drained and water from below or from injection wells moves in. Some water is expected; unwanted water is the problem. It has to be lifted, separated, treated and disposed of, all of which costs energy and money. Because water is heavier than oil, it also weighs down the column of fluid in the well, which can slow or stop natural flow. It often brings scale, corrosion and sand problems with it.
Common causes include coning, where water is drawn up from below towards the perforations; water moving ahead of the oil through high-permeability layers; and leaks behind the casing where the cement seal is poor.
Ways to control water
The first step is to find out where the water enters the well and why, using production data, well tests and production logs. Remedies then include:
- Shutting off watered-out zones mechanically, with plugs, packers or straddles
- Chemical treatments, such as gels, that block the paths the water takes
- Remedial cementing to repair the seal behind the casing
- Completion designs that allow control later on, such as inflow control devices, which even out the flow along a horizontal well and delay water breakthrough
Plan it at the completion stage
Decisions made when a well is completed set its options for the rest of its life. A sound completion design starts from a good understanding of the reservoir, which is where reservoir evaluation and monitoring comes in. Monitoring after start-up then shows early whether sand or water is becoming a problem, while there is still time to act.
How Nextor can help
Nextor supports well completion operations and production integrity through reliable completion solutions: completion equipment sourcing and support, sand control solutions, water control technologies, production enhancement support and completion operations support. For wells already in production, Nextor’s downhole operations and well intervention services include well diagnostics and maintenance support, production enhancement and production restoration support. To talk through a completion, or a well with sand or water problems, contact our team.
Completion technology and sand/water control
Completion equipment, sand control and water control support for well completion operations and production integrity.
