Iron sulfide scaling has been a persistent problem for oil and gas production from sour reservoirs. To study iron sulfide deposition in near-wellbore formations, an advanced numerical geochemical method, reactive transport modeling (RTM), is applied in this paper to simulate the iron-bearing scale formation, especially during and post acid stimulation. The code Toughreact is used for simulation. The model results allow an insight into the spatial and temporal distribution of iron-bearing scale precipitation, wormhole formation, as well as reservoir pressure and water chemistry evolution.
Results of an acid treatment scenario in a hypothetical high H2S carbonate reservoir demonstrate that the scales tends to be concentrated in the wormholes. Pyrrhotite and siderite are the most abundant Fe-bearing scales. However, the latter will gradually convert to the former during the normal production stage. The acid selectively dissolves calcite while leaving the dolomite component largely undissolved. FeCO3 (e.g., siderite) as a form of Fe-bearing scale is recognized in this study. This new understanding will contribute to optimizing acid treatment design and formulation for iron-bearing scale prevention in sour gas wells.
Key words: iron sulfide, oilfield scale, acid stimulation, numerical modeling