ABSTRACT:

Establishment of reliable zonal isolation is one of the principal objectives of primary oil well cementing. Failure of cement seal under formation condition is common in oil fields, especially the cement plugs in high temperature, high pressure and corrosive environment, such as CO2 or H2S gases. Porosity, Permeability and Shear bond strength are important properties of cement that determines the life of oil and gas wells. Cement-casing samples are prepared to mimic the downhole cement plug-casing assembly. Curing experiment data show the permeability is inverse to the shear strength, the shear bond strength of cement-casing samples increases with the curing time. Corrosion experiment of cement-casing samples were aged in CO2 environment (0.54MPa) under both formation temperature (140°C) and room temperature (25°C). Experimental result showed that interface cementation is related to downhole temperature and CO2 concentration. Porosity, permeability and shear bond strength were tested, and the experimental data of shear strength is similar to previous research. Shear bond strength of cement increases with the expansion of cement under high temperature, CT scan show that there are parallel penetrating channels at the cement-casing interface. The results of shear bond strength of cement-casing interface under different conditions are as follows: blank samples (at 25°C without CO2)<25°C-CO2<140°C-without CO2<140°C-CO2. The most important influencing factor of shear bond strength is temperature. Permeability results of cement-casing test treated with CO2 are twice than those without CO2 at the same temperature. The interface normal stress between cement plug and casing inner surface was simulated by finite element method, the maximum normal stress at the interface of the blank sample after 3 days' curing is 11.65 MPa.

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