Offshore oil exploration is probably one of the most expensive business investments in the world. Hiring a drilling rig costs in excess of US$250,000 a day. On top of this there is the cost of sinking stainless steel pipes up to 10,000 metres in length to reach the oil seam. It is therefore not unusual to have spent over US$20 million on a single site before a drop is brought to the surface.
Contrary to popular belief, oil does not always spurt to the surface as a fountain of 'black gold' once the well is punctured by the drill. This only occurs where the seam is pressurised by the prevailing geology, for example, compression by tectonic plates.
In most other cases, the oil is not under pressure and has to be pumped to the surface. And herein lies the problem. The oil is often in the form of a sand slurry, so a filtration process must be employed to separate the oil from the sand.
If the sand screen used has apertures that are too large, the abrasive sand can pass through and destroy the pumps and/or erode the pipeline. If the sand screen pores are too small, flow rates are uneconomically low and in the extreme case, the filter can blind or plug. Such a situation effectively 'junks' the complete oil rig. This scenario has happened in the past, not just on a single well but on a whole oilfield at a cost in excess of US$150 million. It is therefore of paramount importance that the sand screens are specified for optimal performance.
Sand screens are usually made from a complex weave of stainless steel wire so, unlike a simple test sieve, it is not possible to measure the pore sizes by microscopy (Fig. 1). The only other methods of measuring pore sizes are porometry and challenge testing using particles of known size. However, the accuracy of Porometers decreases rapidly above about 100 microns. Furthermore, it is only a derived method based on air flow rates so does not give 'absolute' results.
Although porometry and microscopy have their uses, they measure the geometric size of the pores, usually in terms of the equivalent circle pore diameter. Defining a filter medium by a more performance related criterion, such as the cut point, may be more helpful. The cut point is measured by challenging the filter with real particles and measuring the maximum particle sizes of the particles passing.
For the most unambiguous results, the challenging particles should be spherical and have a narrow particle size distribution (Fig.2). The particles passing then reflect the equivalent circle diameter of the filter.
By accurately measuring the particle size distribution of the filter standards both by microscopy and a precision electroformed sieving method, a calibration graph is constructed where the percentage of the beads passing an unknown mesh can then be used to calculate the filter cut point (Fig. 3). As the results are traceable to international standards such as the National Institute of Standards and Technology (NIST), the derived filter cut points are also NIST traceable. Having a precision range of traceable microspheres is only the first stage in measuring filter cut points. There must also be an accurate and repeatable way of presenting or challenging the filter medium with the standards.
The sonic filter tester (Fig. 4) is a unique system that uses intense sonic energy to produce an oscillating column of air, which flows through the body of the mesh. This process energises the individual microspheres at rates of 3600 cycles per minute rather than mechanically shaking the mesh as in conventional sieve shakers. It therefore has the potential of sifting down to five microns. An on-board computer programmes the entire test sequence thus eliminating any operator bias. Woven sand screens can be measured in a few minutes using the sonic sifting device.
A known weight of the calibrating microspheres is placed on the sand screen under test and sifted for one minute. From the per centage of the standard passing the filter, the calibration graph is used to determine the cut point of the mesh. Because the particle size distribution of the standard is so narrow, a five per cent error in weighing only results in a two micron difference in cut point. Once the accuracy, repeatability and confidence levels have been established it is then possible to use the method as a quality control instrument in the weaving process.
In one production facility, 25 random samples were taken from several rolls of sand screen meshes having target cut points 270µm, 230µm and 150µm. The results showed excellent consistency over a prolonged period and illustrated the precision that can be achieved in the weaving process.
The sonic filter tester has had a dramatic effect on the measurement of pore sizes in sand screens. There is now an accurate and repeatable method of certifying the meshes to international standards. Having an independent laboratory issuing the test certificates gives users, from sand screen fabricators to oil companies, confidence in the results. It is hoped that there will never again be a US$150m disaster from wrongly specified sand screens.
Dr Graham Rideal is Managing Director, Whitehouse Scientific Ltd, Chester, UK www.whitehousescientific.com