Top-hole drilling in the Caspian Sea has historically been very problematic: severe drilling problems are normal when drilling the 26" top-hole sections conventionally – an unconsolidated, over-pressurized sand stratum in the Shah Deniz Field has been one source of these problems. A narrow pore/fracture pressure window, shallow water flow, dissolved gas percolation and mud losses have resulted in extensive non-productive times and an inability to set casing at the desired depth.
Rate of Penetration (ROP) was excessively limited due to cuttings build-up in the hole as the result of low annular velocity. The pressure increase in the wellbore due to the cuttings loading was sufficient to cause losses to the formation.
A new managed Pressure Drilling (MPD) system, developed for drilling subsea wells, uses proven subsea mud pumps from the Riserless Mud Recovery (RMR) system to adjust fluid level inside the riser to maintain a constant Bottom Hole Pressure (BHP)/constant Equivalent Circulating Density (ECD). This compensates for changes in the wellbore pressure profile that may occur due to standard drilling practices and problems such as cuttings loading and annular frictional pressure loss. This technology, called EC-Drill, is also integrated with a reliable early kick/loss detection system which allows the driller to respond quickly before a situation escalates should influxes or fluid losses occur.
Top-hole cement jobs also have been reportedly unsuccessful under such challenging conditions. Losing cement to the formation, plus the added problems of shallow gas and water influx, have been common elements in such failures. To slove the problem, a closed-loop circulation system was established to provide an excellent level of control over the return at the wellhead, so that wellbore pressure could fall inside the pressure window during placement of cement slurry. This cementing technique, called Managed Pressure Cemeneting (MPC), was successfully used in the Caspian Sea.
This paper covers the principle of EC-Drill and MPC methods as well as field data for successful top-hole drilling and cementing operations.