document

Abstract

Gas leakage through cement in the annular space around a cemented casing is still a serious problem, although new additives and methods have reduced the problem. The problem is related to migration through the cement matrix itself or to migration along the cement's interfaces with the casing and the formation, caused by poor displacement of mud and/or loss of hydrostatic pressure within the cement slurry during curing.

One solution to this problem is to transmit vibrations to the cement through the casing. The objective of vibration of oil well cement is to counteract the continuous reduction of hydrostatic pressure and to maintain low permeability during hydration of cement. The hydrostatic pressure within the slurry must be higher than the pressure within the gas bearing formation. Vibration tests in a two meter high cement column have resulted in guide lines as to what frequency, amplitude, vibration intervals and duration should be selected to maintain column pressure. For similar vibration parameters it has also been investigated if the short (1 day) and long term (7 days) strength of the cement has been negatively influenced. Different methods for practical implementation of oil well vibrations have also been considered.

Through vibration of oil well cement, not only the cement will become more gas tight, but also the physical bond between cement/casing and the strength of the cement itself will be considerably improved.

Introduction

In order to exclude any vertical communication between formations it is vital that the cement pumped around the casing in a wellbore achieve a hydraulic seal between the cement and the casing/formation. Lack of a seal in the annulus of a well can present problems throughout the life of the well.

The purpose of this paper is to study in general the applicability of vibration during oil well cementing. Experiments in the laboratory will be reported, and they clearly demonstrate the importance of breaking the cement gel strength by means of vibration in order to obtain a pressure reduction as cement cures. In these experiments the selection of vibration energy to obtain optimum pressure reduction is addressed.

Vibration of the casing has never been tried in the field. Several suggestions and methods of how to implement vibrations exist as patents. One further purpose of this paper is therefore to review and discuss such suggestions.

As an introduction to the gas migration problem, previous investigation and solutions to the gas migration problem will be briefly reviewed.

Causes of the gas migration problem.

Three separate routes by which gas flows from the formation to the surface are recognized¹. The first route for gas is via columns of mud or mud contaminated cement left in the cement slurry as a result of poor mud removal. Secondly, gas can flow through microannuli along the cement's interfaces with the casing and the formation. These microannuli develop because of volume reduction of cement as it sets and cures or due to later pressure and temperature variations during the life of the well. While the first two mechanisms bypass the cement sheath, the third provides a route within the cement itself. This mechanism is referred to as matrix gas channeling or gas flows through the microstructure of the cement.

Causes of the gas migration problem.

Three separate routes by which gas flows from the formation to the surface are recognized¹. The first route for gas is via columns of mud or mud contaminated cement left in the cement slurry as a result of poor mud removal. Secondly, gas can flow through microannuli along the cement's interfaces with the casing and the formation. These microannuli develop because of volume reduction of cement as it sets and cures or due to later pressure and temperature variations during the life of the well. While the first two mechanisms bypass the cement sheath, the third provides a route within the cement itself. This mechanism is referred to as matrix gas channeling or gas flows through the microstructure of the cement.