A Systemic Approach of Reliability Allocation in the Development of Multizone Well Completion

Reliability allocation allows to assign failure rate or reliability target to different system units in the well solution. The simplest method of allocating reliability is ‘equal’ allocation which distributes reliability equally across all components. But that's not an effective way to allocate reliability for a complex well solution. For developing an effective and systemic approach, reliability allocation methods were classified into two major categories - (a) weight factors-based approach, and (b) cost optimized methods. Weight factor-based approach involves subjective (or objective) assessments to estimate the allocation weight factors and failure rate of systems and subsystems. Whereas cost optimization method involves the use of mathematical programming techniques to estimate reliability allocation which minimizes system and/or development cost under constraints. For well solution reliability modelling, a weight factor-based approach such as Feasibility of Objectives Reliability Allocation (FORA) method was determined to be effective. It's based on the numerical ratings for the criteria related to subsystem design and operations such as system complexity, mission operating time, environment, mission criticality, etc. Ratings are typically based on engineering group's experience and judgement. For every well system, these ratings are multiplied to get an overall rating and are normalized so that the sum is 1. In the new product development for complex well solution, FORA method has certain benefits - (a) its criteria are related to subsystem design and operations, and (b) it can be applied at any level of system hierarchy i.e., systems, subsystems, failure mechanisms, etc. FORA technique was found to provide a structured and unbiased approach of reliability allocation for modelling well solution reliability requirements. This paper highlights a successful application of FORA technique to model well solution reliability and determine reliability targets of well systems and subsystems. Key process steps involved in reliability allocation was (a) use of FORA technique to evaluate weight factors of all well systems, (b) develop well solution architecture for multi-zone completion, (c) develop reliability block diagram for well solution architecture and (d) use system weights to determine reliability targets. The case study of multizone well development presented in this paper describes the effective use, challenges as well as lessons learned of applying FORA technique in achieving multizone well solution reliability targets.