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Abstract
It is critical that CO2 injection and storage wells have wellbore integrity to help prevent leakage of CO2 during the injection period, as well as long-term zonal isolation to sustain the loading conditions of pressure testing, completions, injection, shut-in, and stimulation treatment. This paper highlights the salient engineering design features of planning a successful cementing-job operation for extended-reach drilled (ERD), carbon capture and storage (CCS) wells.

Corrosion in Portland cement caused by carbonic acid is a well-documented phenomenon. CO2 injected into geologic formations for underground storage purposes can be converted into various concentrations of carbonic acid with different levels of pH in the formation water surrounding the well, depending on conditions such as temperature, pressure, and the formation rock chemical components. The use of non-Portland cement is often recommended in harsh environments (pH<4) to help avoid any negative impact on long-term well integrity. In addition, the cement should have long-term mechanical resiliency against the
anticipated future loading conditions of pressure testing, completions, injection, shut-in, and stimulation treatment.

A detailed transient wellbore-temperature analysis was carried out for estimating the wellbore and tubular fluid-temperature profiles during the planned well operations. Based on the estimated temperature profiles, wellbore-pressure conditions, and fluid properties, a rigorous cement-sheath mechanical-integrity analysis was performed following a finite-element analysis (FEA) approach. It was found that the use of non-Portland cement alone might not be sufficient for long-term wellbore integrity.

Customized non-Portland cement systems were developed with modified mechanical properties to help ensure appropriate mechanically resilient properties for the long life of a CCS well. Comparative FEA with non-Portland cement and mechanically modified non-Portland cement at the top of tail cement and casing shoe for ERD CCS wells is detailed.

Introduction
Global warming and climate change is one of the biggest concerns in the world today. The increased emission of greenhouse gases, like CO2, has been cited as one of the causes of global warming. Among the various techniques to combat the situation, capturing and geological sequestration of CO2 is believed to be one of the most cost-effective and safest techniques (Santra et al. 2009). It involves injection of CO2 in large quantities in the geological formations and leak-proof retention for hundreds of years. Major companies in Japan have taken a step forward and formed a consortium to research and demonstrate the technologies required for capturing CO2 and its storage in reservoirs. The main purpose of this incorporation is to achieve early massive reduction of CO2 by CCS. Achieving complete zonal isolation in the CO2 environment is identified as one of the important tasks for the overall success of a CCS project.

The main objective of a primary cementing job is to impart zonal isolation for the life of the well. Complete zonal isolation with cement helps protect casing from corrosion, helps prevent sustained casing pressure, helps reduce premature water production, helps limit interzonal communication, and helps reduce the need for remedial work on a well. For successful zonal isolation, the cement slurry should be designed to maintain its chemical, mechanical, and thermal integrity during the life of the well.