Use of a Plume-Modeling Study To Reduce the Risk of H2S Release in an Exploration Well
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper IPTC 13636, ’The Use of a Plume Modelling Study to Reduce the Risk of H2S Release in an Exploration Well to as Low as Reasonably Practicable (ALARP),’ by J.G. Mcllroy, SPE, Petroleum Development Oman, originally prepared for the 2009 International Petroleum Technology Conference, Doha, Qatar, 7-9 December.

With the increasing likelihood of exploration wells being near population centers, a blueprint for the evaluation of the risk of an uncontrolled release of gas reaching such a settlement needs to be quantified. The full-length paper reviews a case study from Oman in which a plume-(gas cloud) modeling study was completed to determine the level of hydrogen sulfide (H2S) away from an exploration well.  

Introduction

Within Petroleum Development Oman’s (PDO’s) Block 6 license, the South Oman salt basin has been widely explored. The discovery of oil-bearing carbonates encased in salt (referred to as “stringers”) led to deeper drilling and more-challenging technical issues, including increasing pressures and the presence of H2S. The well under consideration was notable as the first stringer well to be close to a significant population center. The full-length paper includes background information, key study elements, results, and identified risk-mitigation actions required to reduce the risk to as low as reasonably practicable (ALARP).

Modeling

PDO has been involved in drilling operations in onshore Oman for more than 40 years and has a very high success rate in terms of wells drilled safely. This overpressure 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.

This called for modeling of physical effects (toxic dispersion) of an unconstrained flow from a well. The blowout frequency and probability of blowout type are based on reviews of reports compiled for previous clients. This led to a review of the existing PDO operational Bow-Tie (or risk and outcome) analysis to assess the adequacy of controls (existing and planned) to reduce risk to tolerable and ALARP.

The following key steps were followed in this study.

An assessment of the likelihood of an uncontrolled H2S release (blowout) occurring. This was based on available global statistics.

Computer modeling of near- and far-field H2S concentrations that would result from several release scenarios, which used software from the Energy Resources Conservation Board (ERCB) of Calgary, Canada, and software developed by Shell (for consequence and risk modeling). The Shell dispersion-modeling results were considered more appropriate for the relatively near-field-concentration calculations. It provided more input control than the ERCB software because it allowed entering a large number of parameters including process conditions. This also includes the estimates of the probabilities of the blowout types (with drillpipe and without drillpipe) and the estimate of fluid type.