Drilling fluids for fractured basement reservoir in Yemen

The rocks are hard with very low matrix porosity and permeability in Yemen fractured basement reservoir. The main flow channels of oil and gas are fractures. For getting high daily production, the deviated and horizontal wells were drilled. During drilling operations, lost circulation was the main potential downhole hazard, and solid particle invasion was the main formation damage factor. By comparing the solids concentration and the ratio of yield point to plastic viscosity (YP/PV), sodium chloride and calcium carbonate were selected as weighting agents, and Duo-Vis was selected for viscosifier. As a result, two kinds of bentonite-free drilling fluids were developed with the mud weight range of 1.01-1.40 g/cm3 and the YP/PV range of 1.0-2.0 Pa/mPa?s. For preventing and stopping the lost circulation, some techniques were proposed, among which the first one was temporary sealing with acid soluble bridge blinding material, and the second one was permanent sealing with chemical cross-linked slurry or thixotropic cement slurry. Furthermore, Aphron was a backup drilling fluids for extreme low pressure formation. The drilling fluids for Yemen basement formations had good rheology and the additives were totally acid soluble or water soluble, so they contained low solid concentration or even no solid, resulting in slight or no reservoir damage. The fluids were successfully used in 32 wells, including 9 horizontal wells.