Weakly consolidated, highly porous and permeable sandstone seen under a microscope. The complex distribution of sand grains and pore spaces (blue) cover an area of about 2 square millimetres.

Sand occurs when small rock particles and individual sand grains are dislodged from reservoir rocks during oil production, and become incorporated in the wellstream.

The steady increase in subsea production systems, where intervention is difficult if problems arise,  means that an ability to predict sand behaviour is crucial. Questions to be answered include:

• will pipeline erosion occur?
• will the sand be harmlessly transported with the oil and water?
• will it be deposited in a pipeline to cause a partial or complete blockage?

Key parameters are:

the threshold velocity, marking the point when sand transport begins and thereby the danger that it might accumulate as a stationary dune
the considerably higher flow velocity required for all the sand to be carried in suspension.
A variety of equilibrium bed forms of different styles, wavelengths and heights can be created between these limits.

Although the simultaneous flow of fluids and solid materials is common in many industrial applications, modelling such behaviour remains a relatively immature science.

In fact, no accurate models exist for combined gas-oil-water-sand flow.

An industry consortium accordingly launched a big project at the initiative of Norway’s Sintef research foundation in 1999, and Statoil became a participant in this work in 2000.

The Sintef team is experimenting with the transport of different sands in a gas-oil-water flow, while the Norwegian Institute of Energy Technology (IFE) is doing modelling work to capture the critical conditions outlined above.