Abstract Damage due to CO2 corrosion related processes is a cause of major concern regarding the structural reliability of subsurface installations used in oil and gas production. As to well completions, the effective control of CO2 corrosion includes the complete characterization of the operating environment since corrosion rates and CO2 corrosion morphology are closely related to variables such as the presence of multiphase flow, temperature and produced water composition.
In the present paper an a priori damage assessment methodology is used to analyze casing failures observed in deviated wells with electro submergible pumps (ESP) in the Oritupano oilfield (located in eastern Venezuela). The study analyzes the assessment of CO2 corrosion damage and determines the subsequent probability of failure in relation to the structural reliability of the well. The method proposed is based on the mechanistic CO2 corrosion and multiphase flow modelling in order to deliver a comprehensive quantitative description of the expected corrosion rates within the context of the actual well operating environment and completion design.
The probability of failure is quantified by assessing two independent probability events: damage distribution and the fracture mechanics associated failure probability. The corrosion related damage distribution is calculated by using stochastical simulation of the CO2 corrosion mechanisms, linked to the deterministic corrosion study. The fracture mechanics associated probability is quantified by defining the main operating parameters (such as temperature and bottom hole pressure) as stochastically sensible independent variables within the context of a Level 2 Failure Assessment Diagram (FAD), as defined in API 579.
The results obtained explain the difference observed in the reported work over rates between vertical and deviated wells in the specific oilfield (Oritupano A) using the same extraction methodology. It is also possible to predict consistently the reliability curves obtained from actual field data and discriminate among the possible causes of failure.
Introduction The production of oil and gas from mature reservoirs involves a series of risks related to the particular characteristics associated with the corrosive environments managed and the economic as well as technical limitations.
A common scenario found is the use of waterflooding to enhance oil and gas production in conditions where deviated wells are required due to the nature of the geological formations.
In this condition the formation of complex multiphase flow patterns can be generated, which in turn produce severe corrosive environments that affects the integrity of the subsurface installation as a whole 1,2
Another aspect of this scenario involves the use artificial lifting technologies for the oil and gas up to the surface. Where it is necessary to provide in the well design for the maximum possible suction depth, particularly when deploying electro submersible pumps (ESP).
A common practice involves the conduction of the produced fluids through the exterior or production casing of the well, which intends to supply the necessary conditions for the optimal operation of the ESP within the restrictions of the well bore and the well design.
As can be foreseen this involves the risk of exposing a relevant structural component of the well to the corrosive environment. Inducing the subsequent failure of the casings due to either collapse or burst, which involves the need for expensive fishing or other work over operations, that impacts the overall downtime for the field.
Therefore the development of methodologies that can be used to assess the corrosive severity of a given production scheme, together with an expectation of the risk associated to that scheme, can potentially be applied to an effective well design or production policy, that aims at minimizing those risks by increasing the well structural reliability.