Expansion of oilwell cement after placement in the annulus is a promising route to improve wellbore sealing.Expansion can potentially provide benefits such as:closing a microannulus,reducing the tendency for cracking/debonding,and improving cement logging evaluation.To properly evaluate the performance of expanding cement formulations in a laboratory setting,the cement should be hydrated under confined conditions similar to a wellbore.We conducted measurements using a new confinement cell developed in our laboratory.The cell provides radial confinement and elevated temperature as in a wellbore.The availability of water to the cement can also be controlled to simulate tight or porous formations.In the axial direction the cement sample contacts a piston that either confines the sample while measuring the axial stress development,or permits linear expansion that is measured.Using blends of oilwell cement with magnesium oxide(MgO),a standard expanding additive,we measured the axial stress development and linear expansion under different curing conditions.To better understand and optimize cement expansion we also characterized the hydration kinetics of the MgO additive alone and in the presence of the cement,using isothermal calorimetry.Finally,we measured the acoustic response of cement formulations with and without a commercial expanding agent with a standard ultrasonic pulse-echo technique.This testing confirmed the benefits of expansion for the logging response.
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