ABSTRACT
Upstream oil and gas companies operate natural gas and crude oil gathering systems comprising flowline networks and process facilities that transport the produced fluids from the wells to a main processing plant. Increasing frequency of corrosion related leaks is a common problem facing oil and gas producers, despite that a corrosion inhibitor is injected into the flowlines. Root-cause analyses conducted by several companies in both conventional and unconventional fields revealed that severe internal corrosion was caused by a low fluid flow velocity, an increasing water cut, and the presence of sulfate-reducing bacteria (SRB) in the production streams. Nevertheless, it was not clear why some of the flowlines leaked (a few lines developed multiple leaks) while others did not leak, despite the composition of produced fluids, principal design parameters (diameter and length), dosage of corrosion inhibitor, and environmental conditions of the flowlines were similar. A diagnostic analysis of flow-induced corrosion in the flowlines was carried out to gain an understanding of different corrosion rates in similar flowlines. The methodology used for the diagnostic analysis comprises 1) Ultra-High Definition simulation of 3-phase or 4-phase flow of gas, oil, water, and solids; 2) 3D imaging of phase distributions inside critical sections of the flowlines as per NACE ICDA methods; 3) mapping adverse operational conditions; and 4) the determination of probability of failure of the flowline sections based on criteria depending on the severity of operating conditions inside and outside the flowlines. It was found that multiple flowline sections were exposed to stagnant water and/or had a fraction of internal surface area covered by a stationary bed of solids (proppant or formation solids produced from the well). The identified causes of potential leaks comprise the following failure mechanisms: a) metal loss caused by colonies of SRB, b) composed load acting on the pipe wall, and c) cyclic” thermal expansion/contraction of the flowlines due to seasonal ambient temperature variations. One of the surprising findings of this study was that a shorter flowline with a lower water cut may have multiple leaks while a longer flowline with a higher water cut may have not a single leak approximately for the same period after commissioning. This result was explained with help of maps of adverse operational conditions constructed for the two groups of flowlines. Immediate corrective mitigation actions and preventive actions were proposed to reduce leak frequency, including the installation of a novel automatic flushing system.
