Abstract
Valhall is a fractured chalk reservoir in the Norwegian sector of the North Sea which has been producing mainly under compaction and solution gas drive since 1982. Seawater injection in the crestal area started early 2006. Due to high porosity and low permeability the risk for chalk influx is high. It takes approximately one year from when a new well is put on production before it is fully open on the choke. It is also believed that the chalk has been weakened by compaction due to loss of cohesion.[7] The mitigation strategy to stop scale from forming on seawater breakthrough in the Valhall producers was based upon downhole scale inhibitor squeeze treatments. A scale inhibitor qualification program was put in place to find a non-damaging squeeze chemical that will stop the wells from scaling. The qualification program involved creep tests in a triaxial coreflood rig to monitor the chalk weakening effect, dynamic tube blocking tests and core flooding to test inhibitor performance. A robust analysis method for residual scale inhibitor based upon HPLC has also been developed.
It is well established that water itself will weaken the chalk significantly. A significant extra weakening effect was seen on the chalk when tested against traditional scale inhibitors. It is believed that the pH of the scale inhibitor has a significant contribution to the extra weakening seen, probably due to dissolution of the chalk. Non-aqueous versions have been lab tested without success due to high pressure build up during injection into the core. The water based scale inhibitor that was eventually chosen for field trial was a chemical that had a proven track record in chalk reservoirs and was pH neutralized, compatible with seawater and formation water and demonstrated good scale inhibition performance. The experience so far is that the chemical performs very well in terms of preventing scale from forming. The scale treatment itself increases the risk for chalk influx significantly due to the increased pore pressure in the influx area due to the pumping operation itself, the introduction of water and squeeze chemical and by rough opening of the well post squeeze treatment (needed to get the well back on production).
