Two independent barrier envelopes are the usual requirement used in most well operations to avoid catastrophic accidents. These are classified as primary- concerning preventing the occurrence of a kick, and secondary- concerning controlling the kick to avoid a blowout. Barrier envelopes consist of barrier elements, thus verifying the quality of these elements is fundamental. Barrier elements may be either redundant or mandatory, and these relationships are what constitute the barrier envelopes. In this work, we present a methodology to evaluate well safety by identifying existing barrier elements and barrier envelopes and mapping their relationships through the usage of graphs technique. This technique explicitly states the relationship between barriers and between them and envelopes. It enables a simpler visualization for well designers and allows the development of computer programs to control the safety and integrity of wells, both in the design phase and during drilling. 12 graphs are provided for a 4-phase well (conductor, surface, production, and drill-in), considering both the primary and secondary envelopes. Reasoning for constructing each graph is thoroughly provided. If these graphs are used, reliability values can then be assigned to each barrier element, which results in the reliability of entire barrier envelopes. This can be further extended to analyze the safety of each operation by applying the system to operational sequences and even comparing well designs.
