In this paper we present new cement systems where the value of elastic parameters are specified to meet the requirement for long term mechanical durability. These systems can also contain expanding additives leading to an optimum configuration to prevent loss of zonal isolation due to change of down hole conditions. The advantages of these systems are demonstrated in field test cases where the cement sheath behaviour as a function of changes of down hole conditons is modeled to determine whether damage could occur or not. Introduction Cement in oil and gas wells is placed in the annular gap between the drilled formation and the steel casing. The main function of the cement is to prevent any fluid communication between these drilled formations to provide long term zonal isolation. Zonal isolation has to be achieved during the life of the well and after its abandonment. However, even if the slurry was properly placed during the cementing job, and initially fulfills its isolation role, changes in down hole conditions can induce sufficient stresses to destroy the integrity of the cement sheath. The consequence will be a loss of zonal isolation1,2,3 which can be detected, for example, by long term gas migration problems or, even worst, by casing collapse. The loss of zonal isolation, in absence of chemical attack, can be due to either mechanical failure of the cement itself, debonding of the casing from the cement or debonding of the cement from the formation. Mechanical failure leads to the formation of cracks while debonding leads to the formation of a micro-annulus. Both mechanisms create a high conductivity path for any fluid. To quantify the deformation mechanism and the amount of damage which are generated down hole, mathematical modeling of cased cemented wellbores was previously carried out3. This modeling determines the properties the cement must have to prevent loss of integrity. To avoid mechanical damage, it was determined that cements with high tensile strength to Young's modulus ratio, and with a low Young's modulus value compared to that of the rock, are the best cements in term of mechanical durability. These requirements are functions of the down hole specific well environment such as well geometry, casing properties, rock mechanical properties and expected loading history. Mechanical damage is either caused by a large increase of wellbore pressure (pressure integrity test, increase of mud weight, casing perforation, stimulation, gas production), a large increase of wellbore temperature (geothermal production, steam injection, HT/HP wells) or the formation loading (creep, faulting, compaction). The weaker the formation, the worse the condition since a weak formation is not able to mechanically support the cement deformation. In the case of temperature increase, the thermal properties of the steel, cement and rock, and the rate of temperature increase have also to be considered. Recently Bosma et al4 follow the same approach and agree that to attain effective zonal isolation a mathematical model based on solid mechanics has to be applied. They also recommend that the selection of the well sealant should be engineered by taking into account the mechanical properties of the "sealant". The compressive strength alone is not sufficient as the quality factor. Finally, mechanical damage can also follow excessive shrinkage of the cement, as demonstrated in Thiercelin et al7, and non shrinking cement is recommended.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractTo improve our understanding of cement debonding linked to microannulus formation and, in particular, to develop better non-shrinking cements to ensure zonal isolation, a well simulating annulus experiment has been developed and tests with expanding cements have been performed. The expansion experiments demonstrate that expanding cements can prevent the formation of a microannulus if the cement properties are designed with respect to the rock properties. A good application for expanding cements is the isolation of gas storage reservoirs. Two examples are shown to demonstrate the efficiency of expanding cements.
In this paper, we predict the bulk shrinkage of oil well cements using a soil mechanics model where compaction is a dominant mechanism. In our approach, the bulk shrinkage is the result of the decrease in pore pressure following cement hydration. We perform triaxial tests on set cements to calibrate the model. We perform laboratory experiments to demonstrate the applicability of this approach to shrinkage prediction. We simulate the downhole conditions to predict the consequences of shrinkage on zonal isolation. P. 329
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractTo improve our understanding of cement debonding linked to microannulus formation and, in particular, to develop better non-shrinking cements to ensure zonal isolation, a well simulating annulus experiment has been developed and tests with expanding cements have been performed. The expansion experiments demonstrate that expanding cements can prevent the formation of a microannulus if the cement properties are designed with respect to the rock properties. A good application for expanding cements is the isolation of gas storage reservoirs. Two examples are shown to demonstrate the efficiency of expanding cements.
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