Drilling into a hydrogen-sulphide (H2S) rich formation presents risks that the highly toxic gas might be carried to the surface and released, thus creating a serious hazard. To mitigate this risk, it is standard practice to run the drilling mud at high pH and to include a sulphide scavenger. Commonly, such scavengers are based on iron oxide or on zinc compounds. Iron oxide has the advantage of being environmentally friendly but, at high pH, has a slow reaction rate. In contrast, the zinc-based scavengers have a faster action. In particular, chelated zinc, which is water soluble, has a very fast reaction rate. The action of the zinc in any form is to precipitate the sulphide as a zinc salt. Precipitation prevents release of hydrogen sulphide gas, but it contaminates the mud and cuttings with zinc. In many parts of the world, disposal of material containing zinc is a problem because it is regarded as a toxic heavy metal. For example, discharge is forbidden in many international waters and cuttings for disposal on land may have to be treated as special waste.
E&P Forum recommends product substitution as a source reduction method of reducing waste disposal problems and associated costs. Accordingly, a less toxic alternative was pursued.
A new scavenger has been developed that relies on the affinity between iron and sulphide. The active chemical is a form of ferrous iron complexed with a carbohydrate derivative. It is highly soluble in the brine phases of water-based mud and will not precipitate at a pH over 12. Because the iron is in solution, the reaction with sulphide is instantaneous and complete. The paper deals with the environmental implications of zinc vs. iron and the development of the new scavenger. The paper will present details of case histories concerning operations on a reservoir containing 25% H2S. Even in these demanding conditions, no H2S was detected at the surface.
It is well known that the presence of free H2S in subterranean deposits can represent a serious safety problem while drilling. The circulating drilling mud brings the toxic, flammable gas back to the surface where it can be released and create a severe hazard to rig personnel and others.
Release of gas can be controlled to some extent by keeping the pH of the mud above 10; this helps ensure that most of the sulphide is held in the ionic sulphide form rather than as the covalent H2S form.1 The low concentration of covalent H2S in the mud will be reflected in low vapour pressure of the gas so release will be less of a problem. However, elevating pH is not a permanent solution because the mud system will have only a finite capacity and will not be able to hold an indefinite quantity of H2S or cope with an influx of the gas.1,2 Furthermore, increased lime is not a satisfactory solution to the problem. Calcium hydroxide readily reacts with sulphide but calcium sulphide readily dissociates3 and allows release of H2S if the pH decreases due to depletion or an influx of carbon dioxide or hydrogen sulphide.
Ideally, any preferred method of preventing release of H2S should be capable of removing the sulphide by converting it rapidly in a nonreversible way to a safe, innocuous form. For many years, zinc compounds that readily form zinc sulphide have been used. This reaction immobilises the sulphide very effectively unless the pH of the system is allowed to fall below about 4, which is unlikely.
Zinc compounds can be used in solution form, for example, chelated zinc or more commonly as insoluble solids such as zinc oxide or basic zinc carbonate. Zinc chelate remains stable in solution in drilling mud at high pH until it contacts the sulphide ion, whereupon the chelate decomposes to precipitate zinc sulphide. This reaction is very rapid and efficient. In the case of solid zinc oxide or carbonate, the overall sulphide reaction is significantly slower. The reaction with the surface of the solid particles is rapid but diffusion of the sulphide ions into the interior of the solids is required thereafter, thus slowing the subsequent reaction. In addition, at pH 11 or higher, solid zinc scavengers can contribute to rheological or filtration properties as a result of formation of zincates2 and release of carbonate.