Mapping In-Situ H2S in Carbonate Khuff Reservoirs in Central Oman

Abstract H2S is a colorless toxic gas, although harmless in minute quantities, it can pose HSE critical hazards to personnel and facilities if risks are not identified and managed appropriately. Drilling through H2S bearing intervals and production from sour reservoirs require H2S mitigations eg. Emergency Planning Zone (EPZ), Simultaneous Operations Classifications (SIMOPs), materials and to have facilities adequacy assessment in place. Carbonate Khuff reservoirs across Oman exhibit variations of H2S from sweet to sour concentrations up to 8% (80 000ppm). This paper discusses workflow, data gathering, acquisition results and learnings, and thus de-risking of H2S in Khuff carbonates across Central Oman. This study would also be complementing PDO's Guidelines of pre-drill H2S Prediction Best Practices. A review of reservoir fluids and properties, available key H2S data, any drilling well events involving H2S, were conducted and inventoried across nine (9) fields across Central of Oman. Where data gaps were identified, extensive sampling and analyses were conducted, these included surface and subsurface sampling, geo-chemical finger printing and sulphur isotopes analysis. Regional correlations were conducted across these fields on Khuff and its subdivisions and depth maps indicating potential Khuff closures and fluid fills were generated. In terms of understanding fluid fill, pressure gradient analysis has also proven inconclusive to predict fluid type due to the tightness of the formation with mobility <1 mD/cp. While it is expensive, fluid identification and downhole sampling is the most reliable method to test the fluid type in Khuff. All this information was catalogued and integrated to generate H2S map across Central of Oman. Understanding of origins of H2S in Khuff was still challenging. It was observed that there were lateral and vertical variations across the fields in Central Oman. In some cases, even within the same field (Field A), one well was observed to be sweet while the other was sour. Prior to this study all wells’ planning carry the high case assumption of sour Khuff of Field C as their analogues. As a result, all wells penetrating Khuff intervals (total of >200 wells to be drilled in the next 5 years) were classified as DWI-3 classifications and sour well control requirements need to be applied. New data acquisitions identified and executed allowed the team to map H2S across fields in Central Oman. The results concluded that only lower Khuff is sour and upper Khuff is sweet in some fields. Business impact of the study: H2S ranges uncertainties gap had been reduced and appropriate ranges had been used for well design and planning, adhering to H2S standards SP-1190-2, only <15% of the total wells planned in the next 5 years are considered as Sour DWI-3 well classifications, optimizing cost and time.