Experimental Study on the Corrosion Behavior of Casing Steels Under Oilfield Wastewater Reinjection
Authors: Wang Yangang , Feng Yongcun , Li Saxing , Yu Bo 
Published: August 2024

ABSTRACT:
This study conducted corrosion experiments on three types of oilfield casing steels commonly used in wastewater re-injection environments: N80, L80-3Cr, and L80-13Cr. It was discovered that N80 casing steel exhibited severe pitting corrosion, L80-3Cr casing steel showed uniform corrosion, and L80-13Cr casing steel experienced extremely minor corrosion. The uniform corrosion rates for N80, L80-3Cr, and L80-13Cr casing steels are 0.38 mm/year, 0.03 mm/year, and 0.0011 mm/year, respectively, with the pitting corrosion rate for N80 casing steel being 1.27 mm/year. The findings indicate that N80 casing steel is unsuitable for wastewater re-injection environments, whereas both L80-3Cr and L80-13Cr casings meet the anti-corrosion requirements for such environments. However, due to the higher cost of L80-13Cr casing, it is recommended to use L80-3Cr casing in field applications where anti-corrosion requirements are met. It characterized the morphological features and corrosion rates before and after corrosion for the three types of casing steels, clarifying their corrosion patterns in the wastewater environment. This provides a valuable reference for the optimal selection of casing materials in wastewater reinjection sites.

1. INTRODUCTION
As oil and gas extraction continues and subsequent development measures are implemented in oil and gas reservoirs, the water content in the produced fluids during the middle and late stages of field development gradually increases. Currently, most oilfields in China have entered these later stages of development, with some fields reporting water content in produced fluids exceeding 80% (Zhu et al.,2020; Wang et al.,2020;). This results in the generation of a substantial amount of oilfield wastewater after the separation process of oil, gas, and water, with China's oilfields producing over 400 million tons of wastewater annually. Direct discharge of oilfield wastewater into the environment would lead to severe pollution (Nasr-El-Din et al.,1998; Costa et al.,2022). Reinjecting wastewater into underground reservoirs represents the most cost-effective and convenient treatment method (Pichtel et al.,2016; Rozell et al.,2012; Sun et al.,2019). This approach not only avoids surface pollution but also supplements reservoir energy and is thus adopted by most oilfields worldwide. Before reinjection into the reservoir, oilfield wastewater requires treatment; however, the mineralization level of the treated wastewater remains high, containing a large concentration of chloride, sodium, and potassium ions (Li et al.,2019; Liu et al.,2014). Moreover, while produced water from reservoirs generally does not contain dissolved gases, during the water injection process, air components such as CO2 and O2 can easily dissolve into the water, significantly increasing the concentration of corrosive agents in the reinjected wastewater (Elgaddafi et al.,2021; Zhu et al.,2011). This creates an environment where casing steels, especially in high mineralization wastewater containing CO2 and O2, are prone to severe corrosion. Numerous instances of casing corrosion perforation have been reported in both domestic and international oilfield wastewater reinjection sites, posing a significant threat to the safe and smooth operation of wastewater reinjection (Nielsen et al.,2008). This study obtained original samples of reinjected wastewater and three types of casing from H oilfield, conducting corrosion experiments on three commonly used oilfield casing steels, N80, L80-3Cr, and L80-13Cr, in the wastewater reinjection environment. It characterized the morphological features and corrosion rates before and after corrosion for the three types of casing steels, clarifying their corrosion patterns in the wastewater environment. This provides a valuable reference for the optimal selection of casing materials in wastewater reinjection sites.