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Good cement bond at the casing-cement and cement-formation interfaces is essential for effective zonal isolation. Poor bonding can lead to underground fluids and gases to enter the annulus and create sustained casing pressure (SCP), jeopardising the working envelope of the well and limiting its production. One of the causes of a poor cement-formation bonding is attributed to a cement shrinkage. Cement systems that expand after setting can help improve primary cementing job results by sealing microannulus. The enhanced bonding is the result of enhanced shear bond and adhesion of the cement against the pipe and formation. Cement expansion is achieved by addition of the expanding additives into cement system. The mechanism of expansion is based on set cement volume growth over initial volume post setting. This is driven either by gas bubbles created during chemical reaction or by crystal growth within set cement matrix. Careful optimization of the cement slurry designs with an addition of the expansion additives to conventional and complex blend systems allowed greatly improving the cement bond evaluation log results without compromising other mechanical properties of cement. This paper outlines the successful application of expanding cement to seal different sizes of wellbore; the study evaluates the effect of the expansion by comparing the cement evaluation log from numerous cementing jobs. Examples included in the comparison are cemented production strings (casings and liners) with different types of cement systems used across 9 5/8-in. production casings and 7-in. and 4 1/2-in. production liners.
Good cement bond at the casing-cement and cement-formation interfaces is essential for effective zonal isolation. Poor bonding can lead to underground fluids and gases to enter the annulus and create sustained casing pressure (SCP), jeopardising the working envelope of the well and limiting its production. One of the causes of a poor cement-formation bonding is attributed to a cement shrinkage. Cement systems that expand after setting can help improve primary cementing job results by sealing microannulus. The enhanced bonding is the result of enhanced shear bond and adhesion of the cement against the pipe and formation. Cement expansion is achieved by addition of the expanding additives into cement system. The mechanism of expansion is based on set cement volume growth over initial volume post setting. This is driven either by gas bubbles created during chemical reaction or by crystal growth within set cement matrix. Careful optimization of the cement slurry designs with an addition of the expansion additives to conventional and complex blend systems allowed greatly improving the cement bond evaluation log results without compromising other mechanical properties of cement. This paper outlines the successful application of expanding cement to seal different sizes of wellbore; the study evaluates the effect of the expansion by comparing the cement evaluation log from numerous cementing jobs. Examples included in the comparison are cemented production strings (casings and liners) with different types of cement systems used across 9 5/8-in. production casings and 7-in. and 4 1/2-in. production liners.
Wellbore construction practices are complex. Achieving dependable zonal isolation is a critical and challenging process to optimizing asset life and minimizing future well intervention. Sustained casing pressure challenges related to poor zonal isolation are well documented and can impact production that may require significant remedial well intervention. The need to address these challenges called for revisiting cementing operational practices and lead to the development of a Basis of Design (BoD) document as a tool to help manage well design practices and standards. As common industry practice to prevent fluid migration, slurry designs should be gas tight. To avoid unwanted wellbore fluids to migrate to surface, two things are required: less time to initiate migration and the lack of a flow path. With analysis of the current cement slurries, designs were unable to perform successfully on inflow tests due to low temperature of the zones to be cemented, increasing the transition time for a slurry to move from liquid state to solid phase. With extensive laboratory testing it was concluded that current designs were not addressing the bulk shrinkage phenomenon of cement, which could lead to the creation of micro annulus creating conduits for fluid migration. This paper will discuss the detailed analysis and testing of the current and new designs and techniques validated by laboratory tests and field executions (Cement Bond Logs) to prevent fluid migration and ensure that dependable long-term zonal isolation was delivered. Mud displacement mechanics needed to be optimized to reduce the risk of mud on the wall phenomenon. This included the design of spacers that increased the annulus mud displacement efficiency, improved standoff and finally optimum displacement rates to create sufficient annular velocity. A comprehensive look at all the pertinent steps in the construction of the well starting from the drilling phase, through cement job design, preparation and execution was required to ensure that the best practices are adopted to achieve the best results. Slurry selection, spacer formulation, centralization, hole cleaning and excess volumes were all at the center of the improvements that were necessary to achieve optimum results. A BoD document was developed as a roadmap for cementing to further enhance wellbore integrity. It formalized the planning, design and job execution practices and specified the slurry design, placement, and verification criteria for each casing section. Several cement jobs have been executed using the newly implemented practices that resulted in excellent zonal isolation results, which were verified through cement integrity logs. Since the implementation in all subsequent wells, no casing-casing annulus pressure issues have been reported.
Ordinary Portland cement has been the prime material used for well cementing and permanent well- abandonment. Occasionally, the properties of the Portland cement can be inadequate to fulfil the necessary requirements. Hence, alternative barrier materials have been suggested for use in primary cementing and permanent well-abandonment. Among these, geopolymers, expansive cement, pozzolan based material, and thermosetting resins can be good alternatives. This article presents the fluid properties and rheological behavior of the above-mentioned alternative materials. API neat class G cement is used as a reference material to compare the obtained results. Through this study, the bottom-hole circulating temperature (BHCT) and bottom-hole static temperature (BHST) are selected to be 65 and 90°C, respectively. The pressure is 2500 psi. These conditions are representative for most wells on the Norwegian continental shelf. The initial results include viscosity profile at BHCT, the effect of pressure on thickening time, and intensity of static fluid loss. Our results show that the barrier materials have their pros and cons. One material might be good for an application such as a squeeze cementing job while the other one may not fit this application. The viscosity measurements showed all the candidate barrier materials to be shear-thinning to some degree. Except for the pozzolan based sealant, all other selected materials presented right angle set and pressure- dependent consistency.
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