Migracion de gas

Society of Petroleum Engineers

SPE 26897 A Unique Experimental Study Reveals How To Prevent Gas Migration in a Cemented Annulus
Soran Talabani, U. of Oklahoma, and G.A. Chukwu* and D.G. Hatzignatiou, * U. of Alaska Fairbanks
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Copyright 1993, Society of Petroleum Engineers, Inc. This paper was prepared for presentation at the 1993 Eastern Regional Conference & Exhibition held inPittsburgh, PA, U.S.A., 2-4 November 1993. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract sUb~itted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The matenal, as presented, does not necessanly reflectany position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE ~eetings are subject to pUblication review by Editorial Co~mittees of the Society of Petroleum Engineers. 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 thepaper is presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., Telex 163245 SPEUT.

ABSTRACT

An experimental study was carried out to investigate the perfonnance of new cement additives in controlling or eliminating gas migration through cemented wellbore annulus. The shrinkage behavior of the proposed cement slurries was obtained as a function of time and appliedpressure at constant bottom hole temperature. Ultra-sonic measurements were correlated with the strength and bulk density of the cement. These measurements were used to screen out cement slurries for further testing in a cement simulation unit This unit consists of a 7" casing pipe placed inside a 10" controlled temperature water bath. The experimental unit was 6 ft long and was used to simulateborehole pressure and temperature conditions. The borehole was modeled by drilling a 2" diameter hole through 2.5 ft long cylindrical core samples.
A large number of additives was tested to evaluate their effects on the cement volume fluctuations which are the primary causes of gas migration. A thorough procedure was followed to evaluate the effectiveness of all tested additives in controlling oreliminating the creation of micro-fractures during cement setting under borehole pressure and temperature conditions. The results obtained from this extensive experimental study show that the use of the proposed additives decrease significantly the cement volume fluctuations during cement setting. The new cement additives proposed in this study reduce the contraction-expansion mechanism effectsduring cement slurry setting, thus minimizing and controlling the creation of micro-fracture.
INTRODUCTION

oil wells in the world. There are over 3,000 oil wells in the United States which have exhibited gas leakage problems and are abandoned because of government regulations. Enonnous amount of money is spent on squeeze cementing to redeem gas leakage without producing a pennanent solution. Thedanger posed by this problem is not only expensive to control temporarily, but also a menace to the environment. Since 1960 attention has been given to the problem of gas channeling through micro-annulus and microfractures to enhance cementing by reducing gas leakage at the surface or between reservoir fonnations.
The three main reasons for gas channeling througb a cemented annulus are (i) themud cake that remains between cement and the penneable formations provides a weak zone for the passage of the water and gas, resul~g to failures in cement jobs, (li) the inability of cement to hold the high fluid pressure at the period of its initial set which may cause water accumulation, resulting to micro-fracture within the cement body, and (iii) the cement's inability to maintain overbalance...