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Summary

Vibration of casing to improve primary cementing was investigated in the laboratory and in a 200-ft [60-m] deep test well. The effect of vibration is to decrease or to eliminate gel strength of the cement slurry as it is curing. The slurry then flows downward to compensate for shrinkage and reestablishes the hydrostatic head of the cement. This method can prevent pore-fluid entry into the annulus before the cement cures and can increase the radial stress at the interface between the formation and the cured cement.

Introduction

It is very important that the cement pumped around casing in a wellbore achieve a hydraulic seal between the outside of the casing and the exterior rock. Lack of a seal in the annulus of a well can present problems throughout the life of the well. Large volumes of problems throughout the life of the well. Large volumes of hydrocarbons can be lost from flow between zones at different pressures, injected fluids can enter zones not intended to be flooded, unwanted fluids can be produced along with desired hydrocarbons, or unsafe conditions can be created around the well from pressure buildup or flow if the primary cement is not effective.

Two mechanisms have been identified that can prevent a hydraulic seal from forming in the annulus. One mechanism is the lack of mud displacement by cement. This mechanism has been studied recently in some detail. The second mechanism that can limit the effectiveness of primary cementing is associated with pressure reduction in the cement column during the time that cement is curing. This phenomenon was observed in the laboratory several years ago, and more recently, pressure measurements in wells verified that the pressure reduction occurs under field conditions. The pressure loss has been explained as the result of a volume reduction pressure loss has been explained as the result of a volume reduction in cement combined with gel strength development as the cement cures.

The consequences of the pressure reduction in the cement column during curing may be broader than first suggested. Laboratory and field tests showed that one result of pressure reduction is that fluid in contact with the cement column can enter the wellbore and create a path through or along the boundary of the cement if pressure in the cement drops below the external fluid pressure before the cement attains strength. This will leave a permanent defect in the primary cement sheath. But another possible effect of the pressure primary cement sheath. But another possible effect of the pressure reduction in the cement while curing is that the radial stress at the boundary of the cured cement will be decreased. This can also decrease the effectiveness of the hydraulic seal in the annulus. This paper addresses pressure loss and the radial-stress effect. It is paper addresses pressure loss and the radial-stress effect. It is sug-gested that a decrease in radial stress resulting from pressure reduce tion in the curing cement column is also an important parameter affecting the cement bond log (CBL).

During the relatively few years that the petroleum industry has known of the phenomenon of pressure reduction in a curing cement column, several techniques have been suggested to counteract this effect. One approach is to increase pressure to the extent possible at the top of the cement column. This method is beneficial if the column of cement is caused to move. The benefits and limitations of this method of improved primary cementing are discussed in Ref. 4. Other approaches to the problem involve chemicals added to the cement slurry to cause the slurry to expand in volumes or to decrease fluid loss and to change other properties of the slurry. Mechanical approaches have been offered to overcome the problem of sealing the annulus in the form of packers that are included problem of sealing the annulus in the form of packers that are included in the casing string and inflated after cement is pumped. The packers are intended to stop any upward migration of fluids that packers are intended to stop any upward migration of fluids that have entered the wellbore or to improve the hydraulic seal in the annulus by creating a high radial stress between the casing and the wellbore wall.

The purpose of this paper is two-fold. The first is to report experiments in the laboratory and in a test well that clearly demonstrate the importance of cement gel strength to the mechanism causing pressure reduction in cement as it cures. The second is to report pressure reduction in cement as it cures. The second is to report on a mechanical method to eliminate gel strength of curing cement in the annulus, thereby preventing fluid entry into the wellbore and increasing radial stress in the annulus. This method involves vibrating the casing after cement is pumped and before it cures.