Design of heavy grout and pressure control method for fine pressure-control drilling

Fine pressure-control drilling is a new technology for efficient and safety drilling in the stratum with narrow-density windows. The technology enables precise control of annular pressure and realizes constant bottom hole pressure. During pressure-control trip out, drill is lifted to preset well depth, and heavy grout is filled before conventional trip out. The back pressure control method during the previous heavy grout filling is not operable. On one hand, the influence of heavy grout from nozzle to annular space by internal pressure of drilling tools is not taken into consideration, thus design contents are incomplete. On the other hand, there is no method for real-time pressure control. Bottom hole pressure control is further optimized according to actual engineering requirements and considerations of heavy grout in nozzle of drilling tools under internal pressure, actual borehole conditions, as well as stratum features (near-balanced pressure control may be adopted for carbonate rock stratum, and slightly-over-balanced pressure control may be adopted for hydrogen sulfide stratum). For calculation of relation between heavy grout filling and back pressure at wellhead, segmented real-time pressure control curve is established to realize stable control over bottom hole pressure during round trip, effectively avoid possible overflow or leakage during pump shutdown, and improve drilling safety and overall efficiency.