The disappearance of cement bond log response as a result of variations of downhole conditions has been observed in numerous wells. This observation has led to concern about the loss of proper zonal isolation. Stresses induced in the cement, through deformation of the cemented casing resulting from the variation of downhole conditions, are the cause of this damage. This paper presents an analysis of the mechanical response of set cement in a cased wellbore to quantify this damage and determine the key controlling parameters. The results show that the thermo-elastic properties of the casing, the cement and formation play a significant role. The type of failure, either cement debonding or cement cracking, is a function of the nature of the downhole condition variations. Such an analysis allows us to propose appropriate cement mechanical properties to avoid cement failure and debonding. We show that the use of high compressive strength cement is not always the best solution and, in some cases, flexible cements are preferred.
Introduction
The prime objective of cementing the annulus which is present between the casing and the formation is to provide zonal isolation of the formations which have been penetrated by the wellbore No fluid communication should develop during the life of the well between these various formations, whether they are saturated with water, oil or gas. and with the surface. However, even in situations where the cement was properly placed and initially provided a good hydraulic seal, the disappearance of zonal isolation with time is often observed. This disappearance is revealed, for example, by a gas migration problem which was not initially detected, or by the fracturation of a wrong zone during a stimulation treatment. The loss of the cement bond log response with time also creates some concern about the quality of the isolation. Laboratory studies have shown that stresses induced in the cement from the variation of downhole conditions, are the cause of this damage.
Various processes can result in a variation of downhole conditions in a cased section of a wellbore These processes include the drilling of the wellbore, the perforation of the casing, and the stimulation and production of the reservoir. Drilling involves a variation of pressure, if the mud weight has been changed to drill the next section, and a temperature increase of the cased sections when the mud, which has been heated by the formation being drilled, returns to the surface via the annulus. Associated to the drilling process are the various pressure increases which result from integrity and leak off tests. Pressure increase during perforation follows the firing of the guns, and, although it is applied dynamically to the casing (cement is more resistant to dynamic loading than to static loading), can lead to cement damage. The amount of pressure increase during perforation is significant since values in excess of 6000 psi have been measured in laboratory experiments The increase of wellbore pressure during hydraulic fracture stimulation is more damaging to the cement sheath as the fluid injection lasts from minutes to hours.
Increase of pressure and temperature during production concerns mainly the near surface casing sections, where surface pressure is increased from about atmospheric pressure to production pressure, and temperature is increased to about, in some cases, downhole temperature. The pressure variation usually only concerns the production tubing and therefore does not affect the cemented sections, unless a gas migration problem results in an annulus pressure increase. A temperature increase can also lead to pressure increase in the annuli following gas expansion, if the annuli are saturated with gas.
Pressure decrease during production mainly affects the bottom of the hole where downhole pressure, which is controlled by the production rate, decreases from formation pore pressure to downhole production pressure.
Finally loadings other than changes of wellbore pressure and temperature can be applied to the cement sheath during the life of the well. For example, an increase of the pressure on the external surface of the cement represents a situation where the formation loads the wellbore due to creep.
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