Analysing the Suitability of the American Petroleum Institute Filter Press Test for Polymer Support Fluids

The American Petroleum Institute (API) filter press test is widely accepted in the construction industry as providing a reliable and repeatable assessment of the fluid loss of bentonite slurries and the associated filter cake thickness and texture. Whilst some project specifications adopt it as a de facto test for polymer support fluids, there is a lack of published evidence on its suitability given the very different mechanisms by which polymers achieve excavation support. This paper presents the results of a laboratory investigation into the suitability of the API test for the assessment of clean (i.e. free of dispersed soil) polymer support fluids. The investigation shows that the API test results for polymers are highly sensitive to the filter paper used, whereas those for bentonite slurries are not. Repeatability testing has revealed a substantial variation in the polymer fluid loss rates attributable to three primary parameters: (a) the pore size of the filter paper, (b) damage to the filter paper, identified by optical microscopy, and (c) apparent ‘clogging’ of the filter paper pore space. Of the filter papers investigated, the quantitative, hardened Whatman Grade 54 paper is shown to achieve the best repeatability and reliability. It is noted that the Whatman Grade 50 is deemed to be a suitable paper for the test; yet, produced a higher coefficient of variance than several other papers. Furthermore, a paper supplied by Baroid, an equivalent paper to the Whatman Grade 50, produced a much lower coefficient of variance.