Abstract

Quality annular cement is critical to ensuring adequate zonal isolation as well as casing protection and support during the lifespan of a well. The exploration of unconventional resources creates unique challenges with regards to cement integrity. Casing pressure fluctuation such as that observed during hydraulic fracturing operations, carbon dioxide sequestration, polymer injection and so on, has been shown to cause cement degradation via the initiation and propagation of cracks and micro-annuli. In this work, computed tomography scanning and advanced 3-D image processing algorithms were applied to the investigation and quantification of cement integrity evolution with time due to cyclic casing pressure fluctuations.Lab scale wellbore models were constructed using shale rock, four different cement formulations and stainless-steel pipes with simulated perforations running from the pipe to the rock face for fluid injection. With water as the hydraulic fluid, the models were subjected to a cyclic pressure schedule and CT-scanned at fixed intervals. The 3-D images obtained from these scans were then processed and analyzed to quantify the damage observed in the cement and an attempt was made to relate the data to the physical properties of the rock-cement-pipe system. The results of the study demonstrate the highly detrimental effect of cement shrinkage as well as pipe expansion and contraction during cement hydration. The defects and void spaces in the annular cement were quantified and denoted as the Damage Index. The cyclic injection tests resulted in a sinusoidal pattern of Damage Index decrease and increase for high and low-pressure injection pressure respectively, with the pattern eventually levelling out at a late stage of the pressure cycle. Cement containing styrene-butadiene copolymer latex (SBR) showed the best performance of all the cements tested and maintained the best formation bond. Overall, computed tomography scanning proves to be a very insightful tool for cement integrity research.

Introduction

The quality of the annular cement sheath often determines whether a well will be productive and profitable or a financial and environmental burden for an operator. Cement is placed in the annulus of a hole section to ensure zonal isolation and crossflow prevention, protect the casing from corrosive wellbore fluids, and physically support the casing. When the annular seal is degraded either by debonding at the cement/rock or cement/pipe interface, cracking within the cement sheath, or invasion by corrosive downhole fluids, its capacity to effectively perform the functions listed above is greatly reduced. Despite significant advances over the years with regards to hole conditioning, slurry design, and slurry placement, annular sealing problems are still prevalent in many wells worldwide. There are still gaps in our understanding of the relationship between the physical properties of the components of a cemented wellbore system and the resulting evolution of the annular seal integrity from placement to setting and through the operating life of a well. Also, the extreme downhole conditions that accompany hydraulic fracturing, carbon sequestration, developments in the arctic and high-pressure/high-temperature (HPHT) assets present unique challenges with regards to cement integrity and emphasize the need for reliable methods of predicting downhole cement mechanical response and the design of more effective annular cements.

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