Drilling with water based mud (WBM) generally has its green and cost effective advantages but also comes with its own challenges, which raise the need for more vigilance, planning and timely reaction when WBM is to be used for any hole section. On FAQ1X well, the 16″ top hole section was drilled to 9,500fttvd with KCl polymer WBM; while circulating hole clean in preparation for tripping out, erratic mud rheology properties were observed. It took up to 3days to recondition the drilling fluid specifications. An investigation was carried out to explore the possible causes. The behavior bore no similarity to equivalent sections in previously drilled wells, though the mud system was different. The fresh water for building the mud was optimum; the additives were in good condition and within acceptable operating conditions. The analysis for any carbonate contamination by Alkalinity Analysis (which is rather unreliable) gave an indication of possible carbonate contamination though no source of carbonate was detected. The bentonite used was not in excess yet there was an increase in the Cation exchange capacity of the mud. The other source of Low gravity solids is formation solids that have been very well grinded by the bit and dispersed in the mud such that it becomes difficult to separate them from the mud system via the mud cleaning equipment. They lead to severe gelling of the mud and consequently high mud rheology and hole-swabbing tendency. This significant distortion in rheological properties of the mud was as a result of high solid content in the mud and possibly carbonate or bicarbonate. A dilution recipe was developed and applied to tackle both possibilities and the mud properties were restored.
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