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Conventional well completions in soft formations (the compressive strength is less than 1,000 psi) commonly produce formation sand or fines with fluids. These formations are usually geologically young (Tertiary age) and shallow, and they have little or no natural cementation. Sand production is unwanted because it can plug wells, erode equipment, and reduce well productivity. It also has no economic value. Nonetheless, formation sand production from wells is dealt with daily on a global basis. In certain producing regions, sand control completions are the dominant type and result in considerable added expense to operations.
Fluid flow from wells is the consequence of the wellbore pressure being smaller than that in the reservoir. The drag force caused by the flow from large to small pressure is related to the velocity-viscosity product at any point around the well. Hence, when fluids flow toward the wellbore, the tendency is for some of the formation material to flow concurrently with the fluids.
Opposing the fluid forces are the restraining forces that hold the formation sand in place. These consist of:
- Natural cementation (compressive strength)
- Friction between sand grains
- Fluid pressure in the pores of the rock
- Capillary forces
The compressive strength of the rock, the primary restraint, is controlled by intergranular cementation that is a secondary geologic process. As a general rule, old sediments are more consolidated than are younger sediments. Young formations commonly have little cementing material and are referred to as being poorly consolidated. Stated another way, they have low compressive strength. Their compressive strengths are usually less than 1,000 psi and may even be so small that their strengths can not be measured. The frictional forces are related to the confining or overburden stresses. The stress that causes the rock to fail includes the mechanical stress that results from the overburden and the drag forces associated from viscous flow of fluids through the rock matrix. The overburden stress is partially supported by the pore pressure, so the net stress (the cause of rock failure, the effective stress) is the difference between the overburden stress and the pore pressure.
Capillary forces can also contribute to sand production; there are numerous examples where sand production occurred when water production began. Sand arches form, on occasion, around the perforations. The questions of when and how arches form are related to:
- Flow rate
- Compressive strength of the formation
- Size of the sand and the perforations
Fig. 1 portrays an arch and the balance between viscous and restraining forces. Unfortunately, sand arches are not stable, and their transient behavior cannot be relied upon for controlling sand production.
Consequences of sand production
The consequences of sand production are always detrimental to the short-long-term productivity of the well. Although some wells routinely experience manageable sand production, these are the exception rather than the rule. In most cases, attempting to manage sand production over the life of the well is not an attractive or prudent operating alternative.
If the production velocity in well tubulars is insufficient to transport sand to the surface, it will begin to fill the inside of the casing. Eventually, the producing interval may be completely covered with sand. In this case, the production rate will decline until the well becomes “sanded up” and production ceases. In situations like this, remedial operations are required to clean out the well and restore productivity. One cleanout technique is to run a “bailer” on a wireline to remove the sand from the production tubing or casing. Because the bailer removes only a small volume of sand at a time, multiple wireline runs are necessary to clean out the well. Another cleanout operation involves running a smaller diameter tubing string or coiled tubing down into the production tubing to agitate the sand and lift it out of the well by circulating fluid. The inner string is progressively lowered while circulating the sand out of the well. This operation must be performed cautiously to avoid the possibility of sticking the inner string inside the production tubing. If the production of sand is continuous, the cleanout operations may be required periodically, as often as monthly or even weekly, resulting in lost production and increased well maintenance costs.
Accumulation in surface equipment
If the production velocity is sufficient to transport sand to the surface, the sand may still become trapped in the separator, heater treater, or production flowline. If enough sand becomes trapped in one of these areas, cleaning will be required to allow for efficient production of the well. To restore production, the well must be shut in, the surface equipment opened, and the sand manually removed. In addition to the cleanout cost, the cost of the deferred production must be considered.
Erosion of downhole and surface equipment
If fluids are in turbulent flow, such sand-laden fluids are highly erosive. Fig. 2 is a photograph of a section of eroded well screen exposed to a perforation that was producing sand. Fig. 3 shows a surface choke that failed because of erosion. If the erosion is severe or occurs long enough, complete failure of surface and/or downhole equipment may occur, resulting in critical safety and environmental problems as well as deferred production.
Collapse of the formation
Collapse of the formation around the well occurs when large volumes of sand are produced. Apparently, when a void is formed and becomes large enough to inadequately support overlying formations, collapse occurs because of a lack of material to provide support. When the collapse occurs, the sand grains rearrange themselves to create a lower permeability than originally existed. This is especially true for formation sand that has a high clay content or wide range of grain sizes. For a formation with a narrow grain-size distribution (well sorted) and/or very little clay, the rearrangement of formation sand causes a decrease in permeability that is not as severe. In the case of the overlying shale collapsing, complete loss of productivity is probable. In most cases, continued long-term production of formation sand usually decreases the well’s productivity and ultimate recovery.
The collapse of the formation particularly becomes critical to well productivity if the formation material fills the perforation tunnels. Even a small amount of formation material filling the perforation tunnels will lead to a significant increase in pressure drop across the formation near the wellbore for a given flow rate. Considering these consequences of sand production, the desired solution to sand production is to control it downhole.
Compaction of the reservoir rock may occur as a result of reduced pore pressure leading to surface subsidence. Examples of subsidence, caused by withdrawals of fluids and reduced pore pressure, are found in:
- Long Beach, California
- Gulf Coast of Texas
- Ekofisk Field in the central North Sea, where the platforms sank about 10 ft
- Penberthy, W.L. Jr. and Shaughnessy, C.M. 1992. Sand Control, Vol. 1, 11-17. Richardson, Texas: Monograph Series, SPE.
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