ABSTRACT

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.

INTRODUCTION

Acid stimulation is a frequently used technology to enhance the production rate of hydrocarbons from a reservoir. Depending on the reservoir type (carbonate or clastic) and mineralogical compositions, different fluid compositions and injection schemes are selected to ensure the optimal treatment results. The injected acid dissolves damaged materials accumulated during the drilling or production, and also reactive minerals near wellbore (e.g., calcite). This increases the permeability and decreases the skin value in the vicinity of the production well.

Carbonate reservoirs demonstrate significant heterogeneity (at different scales) both temporally and geographically in the field. Vertical carbonate facies variations controlled by sequence stratigraphy are dominated heterogeneity at meter-scale. Smaller scale heterogeneity may present because of (1) sorting, texture, and chemical composition of grains; (2) diagenesis (e.g., cementation, dissolution and dolomitization) and biological effects (e.g., bioturbation), and; (3) geomechanical effects (e.g., fractures and faults). Due to these heterogeneities, dissolution features such as wormholes may form due to differential dissolution of reservoir rock and formation of selective pathways.

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