A Brief Review of Gas Migration in Oilwell Cement Slurries

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.

Possibilities of Limiting Migration of Natural Gas in Boreholes in the Context of Laboratory Studies

“…Limitation of uncontrolled natural gas flows from the ring and/or annular space of a borehole is one of the crucial aspects during sealing of casing in boreholes drilled in formation with a higher risk of gas migration (the so-called exhalation). It relates to the main objective of casing sealing, i.e., ensuring durability and tightness between the lowered casing, on the one hand, and the drillhole wall and the previously cemented casing string, on the other [1][2][3][4][5]. As there are cases of gas invasion into the structure of the bonding slurry, resulting in leaks in the structure of the cement coating, it is necessary to carry out research and development work aimed at continually improving the quality of cement slurries formulations and the slurry injection technology [2,[5][6][7].…”

“…It relates to the main objective of casing sealing, i.e., ensuring durability and tightness between the lowered casing, on the one hand, and the drillhole wall and the previously cemented casing string, on the other [1][2][3][4][5]. As there are cases of gas invasion into the structure of the bonding slurry, resulting in leaks in the structure of the cement coating, it is necessary to carry out research and development work aimed at continually improving the quality of cement slurries formulations and the slurry injection technology [2,[5][6][7]. During cement slurry bonding, in the process of its hydration, gel structure is created and at that time a clear decline of hydrostatic pressure in the ring space of a drillhole is observed.…”

“…During cement slurry bonding, in the process of its hydration, gel structure is created and at that time a clear decline of hydrostatic pressure in the ring space of a drillhole is observed. When slurry changes its structure, it is very likely that gas moves in the gelled slurry structure, which may consequently result in gas migration both through fresh and hardened cement slurry [2,3,5,[8][9][10][11][12][13].…”

A Prediction Model of Pressure Loss of Cement Slurry in Deep-Water HTHP Directional Wells

“…The annular channeling formed in the cementing stage has also become a problem to be solved urgently in the oil, natural gas and hydrate industries [1,2]. During the waiting on cement stage, with the progression of the hydration reaction, the gas will gradually enter the annulus space and cause gas channeling, which occurs when the liquid column pressure is lower than the formation pressure [3][4][5][6]. At present, there are three mainstream views on the causes of annular channeling both at home and abroad.…”

Remediation of Annular Gas Migration along Cemented Wellbores Using Reactive Mineral Fluids: Experimental Assessment of Sodium Bicarbonate and Sodium Silicate-Based Solutions

“…While designed to have a low permeability, the Portland cements, typically used to create hydraulic barriers against unwanted fluid seepage along wellbores, may become impaired by permeable defects, such as fractures within the cement or debonding micro-annuli along its interfaces with the casing pipe or rock formations. Such defects can have various causes, including (i) ineffective cement placement [9][10][11], (ii) autogenous shrinkage and debonding upon setting of the cement [12][13][14][15][16][17][18], or (iii) mechanical damage sustained by the set cement [19][20][21][22][23][24][25][26]. The defects offer possible routes for annular gas migration, potentially leading to sustained casing pressure (SCP), surface-casing vent flow (SCVF), or other sealing integrity issues [27,28].…”