Annular pressure reduction during cementing is the major factor causing annular gas flow. It has been accepted and proven experimentally that the pressure reduction phenomenon results from the shear stress opposing downward motion of slurry undergoing to volume reduction. Mathematical models have been proposed to describe this process based on the gel strength (or shear stress) development in time. However, the models can only simulate pressure reduction process observed in the lab where compressibility of the system is very small. The models can not explain the variety of pressure reduction patterns observed in the field operations when time-dependent effects of compressibility significantly contributes to the process.
The paper presents a mathematical model combining the effects of gel strength, volume reduction, and compressibility on pressure loss in the cement column. Results from the model, shown in the paper, compared very well with the data from the laboratory and in field tests. The simulated results explained discrepancies in the pressure loss patterns observed in these tests. Main contribution of this study is to quantify the interaction between friction and de-compression mechanisms and their extend in the cement column after placement.
Annular and interzone gas migration after placement continues to be a major problem of well integrity and sustained casing pressure. In primary cementing gas migration occurs due to two mechanisms. The first mechanism stems for poor bonding of the cement to either pipe or rock surfaces. The second mechanism involves entry of gas into the cement slurry column. Gas may enter the cement column when the formation pressure exceeds the hydrostatic pressure at the depth of invasion. It has been recognized that the early mechanism of invasion is the percolation of gas through the cemented annulus leading to the development of vertical channels in the cement sheath. Early gas migration has proved to be difficulty to predict and, on occasion, extremely dangerous when migration of H2S-containing gas is an issue. On the other hand, late-time gas migration causes excessive casing pressures that are persistent and costly to repair.
To date, most of the reported research has related annular gas migration to hydrostatic pressure loss in the cement column caused by two primary factors: cement slurry gelation and volume reduction. Water loss due to filtration and chemical shrinkage (hydration) is two required mechanisms of volume reduction. Gelation restricts the free downward movement of slurry needed to compensate the volume reduction. As a result, the compensation is not complete which resulting in reduction of pressure. The amount of pressure lost is controlled by compressibility of the annulus. Thus, qualitatively, compressibility, gelation, and volume reduction should all be included in mathematical model of this process.
The propose of this study is to develop such a model and use the model to explain various patterns of hydrostatic pressure loss at depth.