Study of the Influence of Cement Slurry Composition in the Gas Migration
Authors: Tavares Frederico , Rocha Jose , Calado Veronica 
Published: June 2014

Abstract
Guaranteeing zonal isolation in front of a gas zone is a complex task. It depends on the complete removal of drilling fluid, drilling fluid filter cake and cuttings from the annulus, on the proper design of a cement slurry and on the long term sheath integrity. This work focus on designing of gas tight slurries and on the methods to evaluate them. A factorial design with center points was performed. The factors studied were the concentrations of four components:water;latex based additive;fluid-loss control agent anddispersant.
The slurries were evaluated in a gas migration simulator and the following properties were determined: ultrasonic static gel strength, fluid loss and bulk shrinkage or expansion under impermeable condition.
The slurries with latex presented better results. No correlation between gas migration and static gel strength development was observed. The results showed that slurries with fluid-loss control agent alone (without latex) can stop gas migration if enough dispersant is added. Among these slurries, when dispersant concentration increases, the compressive strength increases, the bulk shrinkage also increases and the risk of gas migration decreases. This relationship indicates a more advanced cement hydration when the dispersant concentration is higher. On the other hand, it is a consensus that slurries with high shrinkage present higher gas migration risks in a real well (by fractures or micro-annulus formation). Therefore, it's important that a cement slurry presents low shrinkage, but on condition that it is " impermeable" to gas as well.
This work showed a relationship between the gas migration simulator results and the slurry composition and its properties. The criteria of design slurries with short transition time and low bulk shrinkage are not enough to guarantee gas tight slurries.

Introduction
Many works were published about gas migration. However, there are not consensus among researches about its causes, methods of simulating its mechanisms and practical solutions to avoid its occurrence [1]. Many authors indicate the hydrostatic pressure decline of the cement column as the cause for gas enter in the well and the buoyancy force as the responsible for gas rise to the surface, creating a path that will damage the hydraulic seal [2][3][4]. Measured of pressure and temperature in cemented well annulus proved the occurrence of this pressure decline and showed that the pressure begins to decrease soon after the slurry displacement finishes, trending to the formation pore pressure. Then, just before the temperature rising, due to the exothermic heat from cement hydration, the annulus pressures decrease faster [5]. Gas just will invade the well when its hydrostatic pressure is lower that the adjacent formation pore pressure. The most accept model to predict the pressure decline determines it is induced by a volume reduction of the cement (that occurs due to the filtrate loss to the adjacent formation and cement shrinkage), occurring in an environment with low compressibility, simultaneously with sufficient gel strength of the cement to avoid the downward movement of the cement column[2]. This gel was named critical static gel strength (CSGS) [6]. The bigger CSGS, the lower the risk of gas migration. The practical solutions to increase the CSGS are the increasing in the overbalance between hydrostatic and pore formation pressures, the reducing of the length of cement column and the increasing of the annulus equivalent diameter [7]. A cement slurry design for be gas tight should has low shrinkage, low filtrated loss and short transition time. However, testes do not indicate that the SGS is critical for preventing gas migration in a laboratory simulator.[8]
On the other hand, an experimental study verified that a gel strength as low as 25 Pa (50 lb/100 ft2) can be enough to stop the gas upward movement [1]. Hence, the same gelification that contributes to the hydrostatic decline, stop the gas to rise, resulting in damage in the cement sheath just in few meters ahead the gas zone. Likely this damage wouldn't be long enough to create a hydraulic seal failure.