The Inability of Unset Cement to Control Formation Pressure
Authors: Stone William H. , Christian William W. 
Published: December 2007

This paper was prepared for the Society of Petroleum Engineers of AIME Symposium on Formation Damage Control, to be held in New Orleans, La., Feb. 7–8, 1974. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon requested to the Editor of the appropriate journal, provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Introduction
The migration of gas through a cemented annulus was not appreciated until the problem arose during the mid 1960's in gas storage wells. The communication of gas in these wells resulted in wellbore analysis to determine the apparent cause of leakage. This analysis developed the idea that the problem was related to cement-casing-formation interfaces and the bonding between them. At this time, the solution was thought to be better displacement of drilling fluids by cement and prevention of channeling. However, in the past few years, with the advent of deeper well completions across high pressure gas zones with small or negative pressure differentials, the problem of formation damage and annular blowouts or pressures has become more prevalent and new pressures has become more prevalent and new theories evolved. This paper will describe a blowout that illustrates this problem.
The most accepted theory to this problem is the inability of the cement column to effectively transmit the hydrostatic pressure to the formation containing the gas. The physical characteristics of cement such as physical characteristics of cement such as density, setting, dehydration, bridging and gelation are the determining factors for gas migration. Any one of these properties may cause the migration even though more than one may actually be occurring in a well at the same time.
The density of the fluid column must exert a pressure greater than the formation pressure of a permeable zone to prevent liquid migration into the wellbore. In order to have gas cutting of cement, it is necessary for the gas pressure to exceed the pressure exerted by the hydrostatic head of a liquid cement column. Therefore, it is necessary for the density of the cement column and the drilling muds and flushes, either separately or in combination, to exceed the formation gas pressure to prevent it from entering the annulus. This is generally known.

LABORATORY DATA
Laboratory tests have indicated that it would be impossible to have gas leakage through a column of cement when the hydrostatic pressure is greater than the gas pressure. As one would expect, though, if the gas pressure is increased above the hydrostatic pressure while the cement is in a fluid state, leakage into the well-bore can occur, but will cease upon a decrease in the gas pressure toward the annulus. However, when the gas pressure is higher than the hydrostatic pressure after the cement has taken an initial pressure after the cement has taken an initial set, a channel may be formed and gas will continue to migrate up the annulus with a decrease in gas pressure as a result of the low hydrostatic pressure in this gas flow channel.