The Bergermeer Rotliegend sandstone reservoir has been depleted by production. This has substantially reduced reservoir pore pressure and well deliverability. Pressure depletion has been accompanied by a decrease in minimum in-situ stress, resulting in a substantially sub-hydrostatic drilling fluid density required to enable drilling. As a result, Managed Pressure Drilling (MPD) using two-phase fluid has been chosen as the
enabling technology for drilling and completing initial wells for the Bergermeer Gas Storage Project. MPD for the Bergermeer wells is defined as the use of two-phase flow drilling fluid including nitrogen injection via a tieback casing to maintain bottom hole pressure below the anticipated reservoir minimum in-situ stress at or near hole depth. Using MPD technology in the Bergermeer Gas Storage Project will enable drilling the planned boreholes without exceeding minimum in-situ stress, minimizing the risk of differential sticking and drilling fluid losses if natural fractures are present. Reservoir pressure in the Rotliegendes reservoir was originally 238 bar (3451 psi) at 2100 m (6890 ft) subsea. By mid-2009, gas reinjection was started to bring the reservoir up to an operating pressure of 120 to 180 bar for gas storage operations. By May 2013, when the first of the new gas storage wells in the Bergermeer reservoir was drilled, the formation pressure had been brought up to 81 bar. Due to permitting restrictions, it was not possible to drill a test/pilot well before drillingthe first gas injection/production wells to physically determine formation rock strength. Therefore, a decision was made to drill into the 81-bar reservoir with a target BHP of 117 to 127 bar; this equated to an ECD of 0.57 to 0.63 SG. Dynamic formation integrity tests were performed to determine formation rock strength in a controlled manner using two-phase MPD techniques at predetermined depths in the reservoir. TAQA drilled two wells during May and June 2013, one S shaped vertical well and one horizontal well into the two depleted formations. This was achieved maintaining a constant BHP within the predetermined window using MPD with gasified fluid; in fact it was possible to drill the wells with a very stable BHP with a 0.6SG ECD. For the TAQA Bergermeer Gas Storage project, significant planning into the overall system design, equipment selection, techniques, procedures, and training lead to an operation where precise control of the annular pressure profile was achieved and maintained throughout the operation.
