A Study of Completion Practices To Minimize Gas Communication
Authors: Carter Greg , Slagle Knox 
Published: December 2007

The first step towards preventing leakage of gas through a cemented annulus is recognizing that a potential problem exists. Important factors such as fluid column density and filtration control, as well as such ordinary considerations as pipe movement and centralization, must be taken carefully into account if the well completion is to be a success.

Introduction
The leakage of gas through a cemented annulus was first recognized as a significant problem in the mid-1960's in connection with gas storage wells. The problem was thought to be related primarily to cement-casing-formation interfaces and the bonding between them. As a result, considerable work was performed by both producing and service companies. performed by both producing and service companies. Other reasons why gas may enter a cemented annulus were not evaluated until the latter 1960's, and investigations were still predominantly concerned with bonding, density, and displacement or prevention of channeling - properties that will not be considered here.
Gas communication through a cemented annulus becomes more evident in deep well completions across gas intervals, where it causes a pressure buildup in the annuli of the production and intermediate casings. To understand this occurrence of gas leakage more completely and to determine some of the factors responsible for gas migration in wells, data have been gathered from laboratory model studies simulating down-hole conditions.
Variables considered in these experiments were (1) the type of cementing composition, (2) the relationship of mud and cement density, (3) temperature, and (4) pressure differential.
Possible Causes of Gas Leakage Possible Causes of Gas Leakage One of the more popular explanations of why gas enters a cemented annulus is that the cement column cannot effectively transmit full hydrostatic pressure to the zone containing the gas. Gas diffuses into a cement column for many reasons, but only a few characteristics of cement are significant enough to warrant mention; for example, density, setting, gelation, dehydration, and bridging. These specific factors will be considered separately, even though more than one may actually occur in a well at one time.

Density
The density of the cement slurry and of other fluids in the wellbore is very important because it determines the hydrostatic pressure exerted at any particular depth. In the case of gas well completions, particular depth. In the case of gas well completions, the density of the drilling mud, chemical wash, or flush and cementing composition, either separately or in combination, must exceed the formation gas pressure to prevent the gas from entering the annulus. pressure to prevent the gas from entering the annulus. Before cementing, the well must be circulated to condition the mud and to help remove any trapped gas bubbles at the bottom of the hole. After 1 hour of circulation, the pumps should probably be shut down for a 5-minute period and then started again to help remove any microscopic gas bubbles that may be trapped in the washout areas or that may be adhering to the walls of the hole. This gas could be considered as a potential second gas kick, and if it is not removed before the cement is placed, it may lower the density of the fluid column.
JPT
P. 1170