What is sand production?
Sand production occurs when sand eroded from the producing formations enters the wellbore and rises to the surface along with the hydrocarbons.
Sand production is not desired at any well site as it can lead to a host of problems. These include:
- Pipe scouring: Sand continuously rushing along the walls of the borehole scours the inside of the casing. If this continues unchecked, the pipe can be eaten away until a hole forms. This compromises the integrity of the casing, which can lead to lost circulation and the contamination of groundwater aquifers.
- Surface equipment damage: Most surface production equipment is not designed to deal with sand entrained in the hydrocarbon stream. Sand can build up in the machinery, scour parts, and clog pipes.
- Downhole washouts: Sand erosion can lead to washed-out zones downhole. Washouts are bad because they destabilize the borehole and tend to make sand production worse as they get bigger.
Even if the well is equipped with machinery designed to remove sand, the cost of disposing of the produced sand can be quite high, since the sand is soaked in formation fluids. In general, producers strongly prefer to eliminate sand production.
Predicting Sand Production
Fortunately, not every well will produce sand. Normally, sand is only produced if the well passes through unconsolidated or poorly consolidated sandstone units or if the proppant is expelled by formation pressures. Operators should have learned about the presence of these units during the drilling and formation evaluation phases of production. The best way to estimate the probability of sand production is to take a sample of the formation, for example a core sample, and simulate underground pressures in the laboratory. Under these simulated conditions, mechanical strength tests can be used to predict sand production.
As long as potential sand-producing formations are identified before the completion process occurs, steps can be taken to minimize sand production.
When does Sand Production Occur?
Sand production is most likely to occur when sudden changes are made to flow rates. If flow is constant, sand grains tend to eventually form a stable “bridge” across the casing perforations. However, when flow rates change, the equilibrium is disturbed, and sand production rates will be elevated until flow stabilizes at the new level and a new bridge forms.
In uncased holes, bridging doesn’t occur because fluids can approach the wellbore from all directions. Although this can increase sand production in very soft formations, in some cases sand production is actually lower in an open hole completion because flow isn’t concentrated and accelerated through perforations, but can instead approach and enter the wellbore from any point in the surrounding rock. Since spot flow rates are lower, sand is less likely to become entrained.
Methods for Mitigating Sand Production
There are two ways to mitigate sand production: production control and filtration.
One strategy is to limit the maximum rate of production – by keeping production below the maximum sand-free rate, sand production can be prevented.
Another is to ensure that changes to flow rates occur slowly and infrequently.
This limits the collapse of sand bridges, which prevents sand from entering the wellbore as frequently.
These methods limit the rate at which hydrocarbons can be produced from the well. They also make it harder for operators to have flexible control over the well’s production rate.
For these reasons, most operators prefer filtration methods. In these methods, sand production is prevented by a filter installed as part of the well completion. This method requires planning ahead and prediction of which zones will produce sand before production begins. However, if employed successfully, it is much more effective and economical than methods that depend on limiting and controlling the flow of hydrocarbons in the well.
There are countless types of sand filters which are each best suited to specific conditions. Here, we’ll cover some of the most popular.
Gravel Packing
About 75% of wells that require sand filtering employ the gravel packing method. In this method, a bottomhole screen is installed across the producing zone. Then, gravel is pumped to the bottom of the well, where it is packed between the screen and the borehole wall or perforated casing.
The optimum gravel size is determined by looking at available information on the formation collected during formation evaluation. If the gravel is too small, the permeability around the wellbore will drop, leading to a decrease in production. If it’s too big, it won’t stop sand from entering the production tubing.
Resin Injection
In resin injection, a specialized resin is pumped to the sand producing zone and forced into the formation. Then, a catalyst is pumped into the formation. The catalyst reacts with the resin, causing it to harden. Ideally, the resin binds the sand grains together without clogging the pores too much. In reality, this method tends to decrease the permeability of the producing formation by 25% or more.
Despite this, the method is so effective in problem wells that it is used in 10% of wells requiring sand control.
Resin-coated gravel
This is similar to the resin injection method, except that grains are coated with resin at the surface, then pumped down the well. Instead of cementing the sand in the downhole formation, this method cements gravel pumped into the borehole from the surface.
Usually, high temperatures at the bottom of the well are sufficient to activate the resin, causing it to cure. If not, steam can be injected into the well to heat the resin.
Slotted Liners / Prepacked Screens
Slotted liners and prepacked screens are essentially pre-made filters that are fitted to the production tubing and extend across the problem zone. While simple and cheap, these liners and screens can easily get clogged up by fine sediments such as silt or mud.
Surface Sand Traps
As a last resort, sand traps can be installed at the surface to help reduce the amount of sand reaching the production equipment. Surface sand traps may only be needed for a few weeks to several months after a well comes online. As the well declines in natural pressure, sand production at the surface will usually stop.
Images: “Yellow Dials” by Michael Black