Exploration • Drilling • Field Services
Filter tester revolutionises the measurement of pore sizes in sand screens
Graham Rideal describes a new, highly accurate and repeatable method for certifying the meshes in sand screens to international standards
Graham Rideal define un método nuevo altamente exacto y repetible para certificar las mallas en los deflectores de arena según los estándares internacionales.
Graham Rideal beschreibt eine neue, äußerst akkurate und wiederholbare Methode zur Zertifizierung der Netze in Sandsieben gemäß internationaler Normen.
O
ffshore 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. Tis 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. Te 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. Tis 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). Te 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. Te 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). Te particles passing then reflect the equivalent circle diameter of the filter.
Fig. 1. Unlike a plain weave sieve, pore sizes in sand screens cannot be measured by microscopy screens.
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