This paper reports laboratory test results on early strength development of Portland Class G oil well cement slurries with different W/C ratio at ambient and low temperatures. One commercial slurry has been included for reference to industrial practices. The slurries have been cured and tested under laboratory conditions (+20°C) and at lower temperatures (+7°C, +4°C and 0°C) believed to be more representative of arctic and deepwater seabed conditions. Compressive strength development has been monitored using Ultrasonic Cement Analyser (UCA). The test results quantifies delayed initial setting and hindered development of compressive strength at low curing temperatures and with increasing W/C ratio.
In arctic subsea wellheads the curing condition for lead cement close to seabed will be low in temperature due to cooling from low seabed seawater temperatures. The time to reach 50 psi (0,345 MPa) compressive strength can be more than 24 hrs depending on the slurry temperature and its W/C ratio. Initial setting is delayed significantly with decreasing temperature and with increasing W/C ratio. Rate of compressive strength development is also significantly delayed with decreasing temperature and with increasing W/C ratio. Most lead cement systems used have a high W/C ratio. Typical time between cementing and connection of the BOP/riser in North Sea operations is 24 hrs. Once a subsea BOP/riser gets connected to a wellhead dynamic loading will start, resulting in movements of the surface casing relative to the conductor casing. This is a local effect concentrated to the upper part of the surface casing. If loading of the surface casing starts before the cement has developed ample strength, localized failure of the cement should be expected to occur.