Well cementing is an important operation during drilling and completion of oil wells. The cement sheath must maintain well integrity behind the casing and provide long-term zonal isolation to ensure safety and prevent environmental problems. Despite recent technological advancement in smart polymeric materials, fibers and self-healing materials, it is still a big challenge to provide adequate long-term zonal isolation in severe oil well conditions. This review provides an overview of challenges faced in oil wells compromising the long-term ability of the cement sheath to provide zonal isolation. Factors controlling the long-term performance of cement sheath are discussed, in terms of shrinkage, tensile strength and flexibility. The use of nanomaterials as cement additive to fabricate flexible, high-tensile strength, and low-shrinkage cement system are reviewed.Introduction of nanomaterials into the cement system is a promising approach to design a sealant for the entire life of the well, thereby avoiding potential remedial costs and environmental impacts.
The failure of cement sheaths to perform as designed in oil wells can result in loss of zonal isolation. Bulk shrinkage of the cement sheath in the annulus is one of the main causes compromising zonal isolation. Conventional cement systems without compensation for bulk shrinkage have a high risk of failure during all phases of well operation. However, when the volume reduction is compensated, without compromising the mechanical properties, the risk of failure is significantly reduced. In this study, nano-sized MgO with designed expansive properties has been introduced to the fresh cement slurry. The expansive properties of nano-MgO were achieved by controlling the preparation condition. A dilatometer with corrugated molds was used to measure the linear strain of samples cured at 40 °C. The reactivity of nano-MgO played a main role in controlling the expansion performance at the required time. The efficiency of nano-MgO with different reactivities for shrinkage compensation of cement system, cured at 40 °C, was studied. Addition of only 2% nano-MgO with appropriate reactivity was sufficient to maintain expansion in the cement system. Controlling the expansion performance of the additive through designing its reactivity is a promising method to limit bulk shrinkage of cement sheaths in oil wells. The results presented show that nano-MgO with controlled expansive properties can be used to design a cement system with short- and long-term zero-bulk shrinkage.
Summary The bulk shrinkage of cement sheaths in oil wells can result in loss of long-term zonal isolation. Expansive additives are used to mitigate bulk shrinkage. To compensate effectively for bulk shrinkage during the late plastic phase and the hardening phase of the cement system, the performance of the expansive additive needs to be regulated considering the actual cement system and placement conditions. This paper presents an introductory investigation on the potential engineering of nanosized magnesium oxide (MgO) (NM) through heat treatment for use as an expansive agent in oilwell-cement systems. In this study, the bulk shrinkage of a cement system was mitigated by introducing NM with designed reactivity to the fresh cement slurry. The reactivity of NM was controlled by heat treatment. A dilatometer with corrugated molds was used to measure the linear strain of samples cured at 40°C and atmospheric pressure. The effect of NMs differing in reactivity on tensile properties of cement systems cured for 3 days at 40°C was examined by use of the flattened Brazilian test. The reactivity of the NM played a key role in controlling the bulk shrinkage of the cement system. Addition of only 2% NM by weight of cement (BWOC) with appropriate reactivity was sufficient to maintain expansion of the cement system. Adding NM to the cement system also resulted in improved mechanical flexibility. The NM with highest reactivity caused the largest reduction in Young's modulus at 3 days and, in general, the ratio of tensile strength to Young's modulus improved through the addition of NM to the cement system. Our work demonstrates that controlling the reactivity of the additive is a promising method to mitigate bulk shrinkage of cement systems and thereby to sustain the mechanical properties of the cement sheath in the oil well at an acceptable level.
We present the preparation of polypropylene (PP)/ fumed silica (FS) nanocomposites via in situ polymerization in this article. The approach includes preparation and utilization of a bisupported Ziegler-Natta catalytic system in which magnesium ethoxide and FS are used as conjugate supports of the catalyst. Catalyst preparation and polymerization processes are carried out in the slurry phase and under argon atmosphere. Scanning electron microscopy images show a good dispersion of the FS throughout the PP matrix. Results from differential scanning calorimetry reveal that the crystallization temperature of prepared nanocomposites increases by increasing FS loading. Also, crystal content of nanocomposites increases as the FS concentration increases up to 3.48 wt%. Nanocomposites containing <3.14 wt% of nanoparticles do not show considerable change in their melting point where with more increment in filler concentration, melting temperature slightly increases. Thermogravimetric analysis shows a considerable improvement in the thermal stability of PP/FS nanocomposites compared to pure PP. Rheological studies indicate that the incorporation of FS into PP matrix results in increment in storage modulus, loss modulus, and complex viscosity of polymeric matrix, particularly in low frequency region. By increasing FS loading, the PP/FS nanocomposites show a transition from liquid-like to solid-like viscoelasticity behavior depicting microstructural changes in their structures. POLYM. COMPOS., 35:37-44,
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