Study on Plugging Mechanics and Model for Ultra-Deep Reservoirs

Ball-sealer staged fracturing is a key stimulation technology for ultradeep reservoirs, but the ball-sealer transport in a wellbore is very complicated. In order to understand how various factors affect the forces on the ball-sealer during fracturing, this paper analyzed the force states of the ball-sealer at three stages (before setting, after setting and after pump-off) in a vertical wellbore. The plugging mechanics model was built depending on the three force states. Ac-cording to the movement and forces of the ball-sealer, the influences of ball-sealer density, fracturing fluid density, fracturing fluid viscosity, fracturing fluid displacement and number of perforations on the force states of the ball-sealer were identified, the correlation between each factor and the force of ball-sealer was analyzed, and the major controlling factors were determined. The results show that less perforations and weighted fracturing fluid with high displacement and high viscosity are beneficial to optimize the force states of the ball-sealer and thus improve the plugging efficiency. The conclusion provides a theoretical basis for formulating and optimizing the scheme of ball-sealer fracturing for deep reservoirs.