Design and application of foamed spacer to mitigate annular pressure induced by fluid thermal expansion

摘要

Annular pressure build-up (APB) caused by fluid thermal expansion is a severe risk in deepwater wells. Foamed spacers can alleviate APB by carrying nitrogen into a sealed annulus. To properly design a foamed spacer, a prediction model based on the nonideal gas assumption is proposed for obtaining the optimal nitrogen quantity. The proposed model includes the pressure increment caused by nitrogen migration and fluid thermal expansion. A laboratory experiment is carried out to validate the proposed model. Traditional methods based on the ideal gas equation are compared with the proposed model; the results show that the error of the proposed model is less than that of the traditional model. A case study shows that more than 5% nitrogen should be used in the annulus, But, when the injected nitrogen quantity is greater than 20%, the annular pressure hardly decreases with the increase of nitrogen. For widespread applications, formulations of dimensionless pressure increases are developed using the Pi theorem, and the dimensionless graphs are presented. The pressure increase at different initial conditions can be obtained. By referring to the dimensionless graphs, engineers could determine the migration pressure caused by gas dissociation and the nitrogen quantity to mitigate the annular pressure increase.