Precise Managed Pressure Drilling Technology for Ultra-High Pressure Brine Layer in the Kuqa Piedmont of the Tarim Oilfield

Ultra-high pressure brine development (pressure co-efficient of up to 2.65) is common in the thick salt formation of the Kuqa Piedmont in the Tarim Oilfield, and the salt formation contains a mudstone layer with a low fracture pressure gradient, which leads to a narrow drilling safety density window, prone to downhole problems such as spill, lost circulation, collapse, stuck pipe, etc. By precisely describing the flowrate change characteristics of drilling fluid circulation system, and quantifying the correlation among the inlet/outlet flowrate difference of the drilling fluid, the spill volume, leakage volume and the elastic deformation volume of high-density drilling fluid, it is possible to realize the rapid and real-time identification of spill and leakage and calculate the formation pressure. By combining the automatic pressure-controlled drainage and pressure-controlled bull-heading, it is possible to accurately control the difference between the formation pressure and bottomhole pressure, and effectively control the appropriate amount of brine return. In this way, it is possible to significantly reduce the downhole risks of both spill and leakage, forming the managed pressure drilling technology by a fine controllable micro-flowrate in an ultra-high pressure brine layer. The technology has been tested in Well Keshen A and B, and drilled through the ultra-high pressure brine layers safely and quickly, which greatly increases the ROP, shortens the drilling cycle and saves the drilling cost. Research and application showed that the managed pressure drilling technology by fine micro-flowrate control in ultra-high pressure brine layer could quickly detect spill and leakage, accurately control formation brine return or lost circulation of drilling fluid, and achieve a controllable micro-spill or leakage, which can reduce the brine discharge time greatly, ensure the borehole stability, and achieve the purpose of drilling through ultra-high pressure brine layer safely and quickly. The technology has provided a technical means for the efficient drilling of ultra deep wells in complex formations.