The production of formation sands within a well is one of the oldest problems of the oil and gas industry, affecting well productivity and equipment lifespan.
This problem is normally associated with young formations with little or no natural cementation, resulting in loose sand grains.
Due to the above, when the wellbore pressure is smaller than the reservoir pressure, dragging forces will act upon the formation sands as a consequence of the production of fluids. If the formation’s restraining forces are exceeded, the sand will be dragged to the wellbore and will plug the well and/or be produced.
Produced sand has, essentially, no economic value. On the contrary, it can plug the well, erode equipment and damage surface valves.
Fig. 1 Sand grain movement due to fluid flow
Fluid flow: the largest force that produces sand usually results from the fluid flow, which is proportional to the smaller pressure between the wellbore and the reservoir. The drag force caused by the flow from large to small pressure is related to the velocity-viscosity product.
Sand production has occurred in almost all of the areas in the world where oil and/or gas are produced from sandstones. Sand production is more common in tertiary age sand formations, due to the fact that they are geologically young and shallow.
When fluids are produced from sandstone formations, drag force acts upon the sand grains, which then tend to flow through the well along with the fluids produced. These forces are caused by different stresses in the formation, fluid drag forces and overburden stresses. When the sum of these forces exceed the formation’s restraining forces, sand will be produced.
Thermal effects: Thermal effects may destroy intergranular bodies and influence sand production. The effects of high temperatures associated to steam injection in heavy oil production projects have shown that many wells experience high levels of sand production.
Consolidación natural: Opposing the fluid forces are the restraining forces that hold the formation sand in place. These stem from intergranular bodies (natural cementation), friction between sand grains, gravity, and capillary forces. Pressure within the pores of the rock (reservoir pressure) helps bear the overburden stress and thus prevent sand production.
In many cases, sand production increases considerably when wells start to produce water or gas and oil. Some theories that may explain this phenomenon are: