A new technique for measuring permeability and tensile strength of a curing oil well cement

Gas migration through cement is a problem in many wells around the world. A study of the behaviour of cement slurry during the curing period has been performed in order to relate to slurry properties such as intrusion and migration of gas in the cement. A new method for measuring the permeability and the strength of a curing cement has been developed. At specific times during the curing, water is pumped into the cement paste at a very low rate while pressure is recorded. The pumping is then stopped for a short time while the pressure drops back to normal pressure. The pump is then restarted at maximum rate causing the curing cement to fracture. The fracturing pressure appears as a sharp peak in the pressure against time curve. Using the flow rate and the pressure recording during the first low-rate pumping period, the permeability is calculated. The tensile strength of the cement is proportional to the peak pressure of the second high-rate pumping pressure. The method can be used throughout the whole time interval of interest from initial to final set, and has shown good reproducibility. The behaviour of three parameters has been found to be of importance regarding gas tightness: the permeability development, the build up of tensile strength and the hydrostatic pressure drop.