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 an expected decrease in minimum in-situ stress, resulting in a substantially sub-hydrostatic drilling fluid density being 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 Gas Storage Bergermeer Project.
MPD for the Bergermeer wells is defined as the use of two-phase flow of drilling fluid including nitrogen injection via a tieback casing to maintain bottom hole pressure (BHP) below the anticipated reservoir minimum in-situ stress at a long hole depth. Application of MPD technology in the Gas Storage Bergermeer Project will allow drilling the planned boreholes without exceeding minimum in-situ stress, minimizing the risks 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 133 bar for gas storage operations. By May 2013, the time of drilling the 1st of the new gas storage wells into the Bergermeer reservoir, the formation pressure had been brought up to 81 bar in block 1 and 35 bar in the adjacent block 2.
Due to permitting restrictions, it was not possible to drill a test/pilot well before drilling the 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 equivalent circulating density (ECD) of 0.57 to 0.63 SG. Two wells were drilled during May–June 2013, one S-shaped vertical well in block 1 and one horizontal well into block 2. This was achieved maintaining a constant BHP within the predetermined window using MPD with gasified fluid; in reality it was possible to drill the wells with a very stable BHP with a 0.6 SG ECD. Dynamic formation integrity tests (FIT) were performed to determine the formation rock strength in a controlled manner using two-phase MPD techniques at predetermined depths in the reservoir; results indicated that rock strength was adequate for using conventional drilling techniques. Despite the successful implementation of MPD, future wells will be drilled conventionally although MPD could deliver the wells should the formations turned out to be weak, and it remains as an important contingency in case formation strength turns out to be weak in future wells.
For the Gas Storage Bergermeer 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. This paper documents the key planning considerations required to drill and complete a highly depleted reservoir using two-phase MPD techniques.