Abstract

With the increasing likelihood of exploration wells being located near population centres, a blueprint for the evaluation of the risk of an uncontrolled release of gas reaching such a settlement needs to be quantified. This paper reviews a case study from Oman in which a plume (gas cloud) modelling study was completed to determine the level of H2S away from an exploration well. Future wells drilled in Oman and other countries can use this work as a model for reference with which to compare and to assess whether further detailed risk analysis is required.

Within Petroleum Development Oman's (PDO's) Block 6 licence, the South Oman Salt Basin (SOSB) has been widely explored figure 1. The discovery of oil-bearing carbonates encased in salt (referred to as ‘stringers’) lead to deeper drilling and more challenging technical issues including increasing pressures and presence of H2S.

The well under consideration in this article was notable as the first stringer well to be located close to a significant population centre figure 2. This article includes background information, key study elements, results and identified risk mitigation actions required to reduce the risk of to As Low As Reasonably Practicable (ALARP).

Risk levels are demonstrated to be tolerable and significantly below PDO risk tolerability criteria. Only one of the sixteen scenarios modelled described greater than 50 ppm H2S concentration at the settlement. Additionally, five scenarios suggested that the H2S levels exceed 50 ppm at the road near the well location. This result will now be put into context.

Introduction

PDO has been involved in drilling operations in onshore Oman for over forty years and has a very high success rate in terms of wells safely drilled. This over-pressure stringer well represents another well drilled to a target within the salt, and as such is "business as usual" from an exploration perspective but the presence of the nearby settlement requires a more complete approach to risk analysis for the safety case.

This modelling of physical effects (toxic dispersion) of an unconstrained flow from a well has been generated. The blow-out frequency and probability of blow-out type (either vertical/horizontal, with drill pipe/without drill pipe) and is based on reviews of reports compiled for previous clients. This led to a review of the existing PDO operational Bow-Tie analysis to assess the adequacy of controls (existing and planned) to reduce risk to tolerable and ALARP.

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