Abstract
Gas migration in oil and gas wells is defined as gases and/or fluids from adjacent formations invading a freshly cemented annulus. During well completions, gas and/or fluids can migrate to zones with lower pressure or even to the surface. Static gel strength (SGS), related to the yield stress of the cement, is a widely accepted measurement used to predict and minimize gas migration. In this review article, we look at the mechanisms and some possible solutions to gas migration during oil and gas well cementing. The use of static gel strength (SGS) and experimental measurements for SGS and wellbore pressure reduction are discussed. Rheological properties, including the yield stress and the viscosity of cement slurries, are also briefly discussed. Understanding the rheological properties of cement is complex since its material properties depend on cement type, as well as the shape and size distribution of cement particles. From this brief review, it is evident that in order to reduce free water and settling of the cement particles, to lower fluid loss, and to develop compressive strength in the early stages of cementing, an optimal cement slurry design is needed. The SGS test is a standard method used in estimating the free water in the well and could be a reference for gas migration reduction for oilwell cement slurries.
Highlights
Cement is, in general, a non-linear complex fluid that sometimes behaves as a viscoelastic material and sometimes as a viscoplastic material
static gel strength (SGS) is used in determining the transition time, traditionally defined as the time it takes a slurry to change from a fluid-like material to a solid-like one and no longer transmitting the hydrostatic pressure to where the slurry has developed a gel strength that resists gas percolation through the cement column [2]
Researchers have offered some solutions to the problem of gas migration: using special seals, using cement containing filtration control, high slurry compressibility, high fluid density, having a shorter cement column, applying an annulus pressure against the pressure drop in the formation, ways to lower the free water which is a layer of water on top of the cement, using cement with thixotropic agent for customized jobs, modifying the viscosity of gelation both chemically and mechanically, using chemicals to reduce the gas flow, and using salt cements [2,11,12,13,14,15,16]
Summary
In general, a non-linear complex fluid that sometimes behaves as a viscoelastic material and sometimes as a viscoplastic material. Researchers have offered some solutions to the problem of gas migration: using special seals, using cement containing filtration control, high slurry compressibility, high fluid density, having a shorter cement column (stage cementing), applying an annulus pressure against the pressure drop in the formation, ways to lower the free water which is a layer of water on top of the cement, using cement with thixotropic agent for customized jobs, modifying the viscosity of gelation both chemically and mechanically, using chemicals to reduce the gas flow, and using salt cements [2,11,12,13,14,15,16] It is a challenge for the petroleum industry to (1) prevent the gas migrating into the cement slurry during the hydration process and (2) maintain the annular cement seal and well isolation for the long term applications [17,18,19,20,21,22]. In the past few decades, a great deal of research has been performed to study and solve this issue, the exact failure mechanisms of the problem are complicated and gas migration problems continue to occur [23,24,25,26,27,28,29,30,31,32]
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