A new mud design to reduce formation damage in sandstone reservoirs

Certain polymers are often used during water based mud (WBM) drilling to reduce the filtration loss in permeable intervals. Although they often provide a good performance but cannot totally stop the fluid loss and mud invasion into the reservoir may cause significant formation damage including unfavourable changes of surface wettability. As a result, the two phase relative permeability of the near wellbore region changes and production may face difficulties in the later stages. In this paper, a new mud design is proposed to reduce the surface wettability alteration posed by WBM in sandstone reservoirs. The results obtained from performing a series of contact angle measurements indicated that clean and dirty sandstones are strongly water wet systems but mud invasion can make them a weakly water wet surface. It was also found that Cetyltrimethylammonium bromide (CTAB), as a cationic surfactant, prevents surface alteration of rocks and reduce the formation damage, but it may isolate the clay and creates a huge mud cake around the borehole. It was also observed that the salinity of the mud has a great impact on the surface wettability and adding CaCl₂ can reduce the formation damage in the reservoir intervals during drilling, although caution must be taken to maintain the cost of the mud.