Wells – Processes – Cementing – plugging or consolidating
Reexamination Certificate
1998-09-23
2001-10-02
Bagnell, David (Department: 3672)
Wells
Processes
Cementing, plugging or consolidating
C164S250100, C073S152570
Reexamination Certificate
active
06296057
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for maintaining the integrity of a seal-forming sheath, in particular a cementing sheath, positioned around a metal casing for an oil, gas, water, geothermal or analogous well.
2. Description of the Related Art
An oil, water or gas field is usually exploited via a well into which a metal casing has been inserted and held in place by a cement sheath to fill the space or annulus between the casing and the borehole. The cementing operation, i.e. putting the sheath into position consists in injecting a cement slurry into the casing to cause the drilling mud in particular to rise up and be evacuated via the annulus which is then gradually filled with the slurry. After the slurry has set and hardened, a cement sheath is obtained which prevents any fluid communication between the various formations through which the well passes, and which acts as a support for the metal casing.
Well-cementing is an operation that is very difficult because it requires several parameters to be taken into consideration and kept under control. For example, a slurry with too high a density can cause the rock to fracture, while a slurry with too low a density can cause external fluids to intrude. While slurry density is a parameter which is relatively easy to control, this is not true of its rheological properties. Such problems, which are inherent to any well-cementing operation, are well known to the skilled person, and solutions generally consist in adding various additives to the slurry, the selection of which is not always clear and varies from one well to another.
However, even in a situation where this cementing operation is carried out under good conditions to obtain a sheath which seals and supports once the slurry has set and hardened, it is not long before that sheath is subjected to mechanical and/or thermal stresses which can cause the sheath to deteriorate, and this can culminate in well operating conditions being put into doubt.
Such problems linked to sheath deterioration over the lifetime of the well are not novel in themselves and are well known to the skilled person, but up until now no practical approach has been made to attempt to provide a solution to such problems.
SUMMARY OF THE INVENTION
A principal aim of the invention is to analyse more precisely the mechanical and/or thermal stresses to which the sheath may be subjected during the lifetime of the well, the effects of these stresses and the influence of mechanical and/or physical parameters of the cement, the casing and/or the rock on these stresses, to obtain a solution which can clearly answer these problems of sheath deterioration.
To this end, the invention provides a method which is characterized in that it consists in:
calculating or estimating pressure and/or temperature variations in the well and/or variations in in-situ stresses, which can occur during the lifetime of the well;
for a given sheath, evaluating the various stresses which will be applied to that sheath, in particular as a function of the variations defined above and taking into account the geometrical characteristics of the well and of the casing, and also the mechanical properties of the rock;
from the above evaluation of the various stresses, determining the nature of the stress which is likely to cause sheath deterioration in the first instance;
evaluating the influence of the mechanical and/or physical properties of the sheath, the rock and/or the casing on the above-defined stress;
selecting a sheath with mechanical and/or physical properties which are likely to attenuate the effects of the above-defined stress; and
positioning the sheath as selected in this way around the well casing.
In general, analysis of the data obtained by modelling and which has served as a basis for the definition of the method of the invention has served to identify three main types of deterioration which can damage the sheath, namely cracking due to failure in tension or in shear, or detachment at the interfaces with the casing and the sheath.
An analysis of the influence of the mechanical and/or physical properties of the sheath, of the casing and/or of the rock on these types of deterioration has enabled the method of the invention to be refined to attenuate the risk of these types of deterioration occurring.
Thus, in accordance with two further characteristics of the invention, the method includes:
taking the elastic properties of the sheath into account, and selecting a sheath for which the ratio between its tensile strength and its Young's modulus is as high as possible, and/or
also taking the elastic properties of the rock into account, and selecting a sheath with a Young's modulus which is lower than the Young's modulus of the rock.
With such provisions, the method can attenuate the risk of a crack occurring in the sheath, in particular as a result of an increase in well pressure and/or temperature.
If well pressure increases, the method can also include increasing the thickness of the casing to limit its deformation.
If well temperature increases, the method can also include controlling the increase in temperature to attenuate the effects on the sheath.
In a further feature of the invention, the method also includes placing the sheath in compression while it is being positioned around the well casing.
With such an arrangement, the method can also attenuate the risk of sheath detachment occurring, in particular following a reduction in pressure at the sheath-rock interface.
In general, the experimental data as obtained numerically and/or mathematically on studying the risks of the cement sheath failing under tension or shear and the risk of sheath detachment at the casing-sheath and sheath-rock interfaces as a result of the mechanical and thermal stresses to which the sheath will be subjected during the lifetime of the well have led to the discovery that these risks can all be substantially attenuated, in particularly by adjusting the elastic properties of the cement.
Thus this data has led lo the development of a method which can be used to define the properties required for the sheath, in particular its elastic properties, before proceeding to position it around a well casing.
Cements for cementing sheaths which have the required properties after setting and hardening of the cement slurry are currently selected essentially by adjusting the rheological properties of the slurry. This means defining numerous slurry compositions.
Under such conditions, the method of the invention can also be used as a tool to test slurry compositions and determine, for a given well, their ability to withstand the strains of various mechanical and/or thermal stress systems to which the cementing sheath will be subjected during the lifetime of the well.
An important advantage of the invention is that carrying out the method does not require the well to be equipped with additional technical means to protect the cement sheath.
REFERENCES:
patent: 3927163 (1975-12-01), Gabriel et al.
patent: 4440226 (1984-04-01), Suman, Jr.
patent: 5020594 (1991-06-01), Gill
patent: 5348093 (1994-09-01), Wood et al.
patent: 5355951 (1994-10-01), Allen et al.
French Search Report entitled “Rapport de Recherche Preliminaire”, dated Juin. 26, 1998.
Thiercelin et al., “Cement Design Based on Cement mechanical Response”, 1997 SPE Annual Conference and Exhibition, Oct 5-8, 1997, San Antonio, Texas, pp. 337-348, XP-002068174.
Bagnell David
Dougherty Jennifer R.
Mitchell Thomas O.
Nava Robin C.
Schlumberger Technology Corporation
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