comprehensive review on gas migration and preventative strategies through well cementing

hydrostatic pressure less than pore pressure and the existence of a path to gas migration are two major factors that must be stopped simultaneously to prevent migration. understanding the mechanisms of cement hydration in early times is necessary to investigate these factors. cement hydration can lead to swelling and shrinkage at the same time. at the beginning of cementation chemical shrinkage occurs, followed by swelling and autogenously shrinkage, and the degree of hydration, the water-cement ratio and the fineness of cement. the most important factors that make the annulus pressure less than the pore pressure are: cement placement, cement hydration in liquid state, and cement hydration in solid-liquid state.