Reliability of Electric Submersible Pumps ESP & Determination of Weibull Parameters for Failure Characterization, Ecuador Case Study
Authors: Ayala Diego , Padilla Wilson , Carrera Luis 
Published: September 2021

Abstract
The current research focuses on data modeling of ESP (Electric Submersible Pumps) reliability by obtaining, through mathematical calculations, the parameters that define the Weibull shape (β), scale (ɳ) and location (ϒ) parameters. The scale parameter ɳ is the characteristic life at which 63.2% of the population has failed. These parameters can be helpful in characterizing failure behavior and the ESP system run life. This research stemed from the need to better understand failure behavior to improve maintenance program design and enhance equipment reliability.160 wells from four fields (7-21, 56, 57) of the Ecuadorian Oriente basin were analyzed.
Well selection considered only the mechanic failures and excluded other type of failures leaving aside the ESP failures caused by operative issues (reservoir, completions, workover, redesign, zone change). Three mathematical tools were applied to determine the Weibull parameters more accurately.
Another type of analysis could have limited this research since the normal distribution shows limitations with asymmetric data, and the exponential distribution assumes a fixed failure rate, but the ESP failure behavior is asymmetric and the failure rate is variable through time due to factors such as wear and also infant failures (e.g. installation errors). For these reasons, Weibull distribution is the best option because it fits asymmetric data better and it has a variable failure rate.
Determining the value of the Weibull parameters can assist in answering questions such as: what percentage of failures is expected to occur in time? How many failures can be expected before the warranty period? When should regular maintenance be scheduled? Ultimately, Weibull parameters are the basis of any future reliability analysis.
The Weibull parameters obtained in this study can be applied for future ESP reliability analyzes that are being operated in any field in the Oriente Basin of Ecuador.
From the research, two relevant findings were foundThere are a significant number of failures in the initial stage of operation of the pumps,, which could be associated with the installation of the equipment, and the failure risk is drastically reduced in the equipment that reaches life time similar to the characteristic life (ɳ)and pumps that operate without failure to a time similar to characteristic life (ɳ) continue to follow this trend throughout their operating life. Initial stage failures are presumed to be associated with unanticipated conditions: solid binding, design errors, defective equipment, or assembly of equipment with reused and new parts.
The performance of the ESP affects the productivity of the wells and therefore will influence decision-making to develop a field. The reliability of the ESP systems can favor the productivity of a field when the equipment works within its efficiency range without showing recurrent failures. This significantly improves field production costs and profitability.