Limit analysis of extended-reach drilling considering the coupling effects of hydraulic energy loss and transient cuttings transport

Ultradeep extended-reach drilling has become the most effective engineering method for economically developing marginal reservoirs in the eastern South China Sea. However, complications, including poor hole cleaning, high pressure loss, and high pump pressure, pose serious threats to the safe and efficient drilling of large-hole sections (i.e., 17 1 / 2 -in. and 12 1 / 4 -in. section) with steep inclinations, constraining hydraulic extension limits. In this article, we present a hydraulic extension limit model for extended-reach wells, mainly considering the constraints of rated pump pressure, the safety density window of a formation, cuttings transport, and so on. The key to calculating the hydraulic limit was to obtain the distributions of hydraulic pressure in relation to a cuttings bed throughout an entire wellbore. We then built a mechanical coupling model of annular pressure and transient cuttings transport to overcome the shortcomings of relatively independent calculations of annular pressure and cuttings distribution. A case study showed that variations in drilling parameters during drilling caused corresponding alterations in the cuttings bed distribution. This affected the bottom-hole ECD (Equivalent Circulating Density) and surface SPP (Standpipe Pressure), explaining significant fluctuations in the measurement data. The hydraulic extension limit relates closely to various drilling parameters. Based on the results of a sensitivity analysis, we optimized these drilling parameters. For a 12 1 / 4 -in. section, the recommended mud displacement was 70 L/s, the recommended ROP (Rate of Penetration) was 50 m/h, and the recommended mud density was 1100 kg/m 3 . Using these recommended operating parameters increased the hydraulic limit by about 2.19 km.