Experimental investigation of hydrogen sulfide scavenging capacities and mechanisms in iron-bearing minerals

Reservoirs that produce oil and gas containing no H2S (“sweet” systems) frequently turn “sour” (i.e. produce H2S) after some period of seawater (SW) injection. Certain minerals e.g. siderite (FeCO3), iron oxides (FexOy), and iron-bearing clays may react with produced H2S and retard its progress towards the producer wells. Despite the impact that these parameters may have on the development of reservoir souring models, there are no published experimental studies that have sought to explain the mechanisms of H2S scavenging or to quantify scavenging capacities for commonly occurring iron minerals. Focussing primarily on siderite but with additional data from field core samples, a combination of static bottle tests and dynamic sand pack experiments were used to identify and quantify H2S scavenging. This study has observed two proposed mechanisms for H2S reservoir scavenging, namely dissolution/precipitation, which depends on the stability of the Fe-bearing minerals, and surface displacement, which depends on the relative solubilities of the resultant precipitates. These mechanisms were rationalised using a suite of analytical techniques; ICP-OES, ESEM-EDX, pH data and particle size analysis. Capacities in the order of 0.5 to 13 mg/g (mg H2S per gram of active substrate) were calculated over a range of initial pH values and temperatures.