عنوان مقاله [English]
One of the most significant problems during oil and gas drilling is shale formation instability when using water-based muds. During underbalanced drilling due to pressure difference induced between drilling fluid and formation, mud filtrate invades to shale. Moreover, because of the large solid particles in drilling mud in comparison with shale pore throats (about 3 to 100 nm), formation of mud cake to prevent the invasion of filtrate to the shale is not possible. Therefore, drilling fluid filtrate continually penetrates to the formation and will cause an increase in pore pressure and instability of shale followed by lost in sidewall. In order to block the shale nanometer pores and to prevent the invasion of filtrate, a synthesis of amorphous silica nanoparticles using a simple and efficient method of waste material from agricultural industry is performed which was called rice husk, and applied in drilling water-based muds at two concentrations of 3 and 5% weight/volume (w/v). By taking rheological properties of base mud and the base mud with nanoparticles and comparing the test results, it was found that nanoparticles used improved the rheological properties of drilling mud such as thermal stability and reducing the amount of filtration. Then, a pore pressure transmission (PPT) device utilized to investigate the effect of the mud containing the nanoparticles to block shale pores and reduce pore pressure. The results showed that using concentrations of %3 and %5 synthesis nanoparticles cause the reduction of the rate of penetration of fluid into the shale at around 5.6% and 43.7% respectively. This phenomenon can be a factor to prevent the mechanical weakening of shale layers and their loss during drilling by water-based muds.