Iron Sulfide and Removal in Scale Formation Sour Gas Wells

Abstract

Scale formation has been a persistent challenge in many sour gas wells producing from one of the world's largest gas reservoir in Saudi Arabia. Accumulation of scale deposits on downhole tubular and in wellhead manifold interferes field operation, limits well accessibility and decreases well productivity. Extensive efforts have been devoted to understand the scale formation process and to develop cost-effective mitigation strategy. This paper discusses the up-to-date knowledge on the scale formation in these prolific gas wells and presents the descaling technologies deployed and currently considered.Scale composition analyses have been performed for a large number of deposits collected during well workovers and interventions. Wide range of mineral phases were identified and their distribution showed significant variations with samples. Scale often consisted of several different mineral phases. Iron sulfides were usually the dominant components, these included pyrrhotite, troilite, mackinawite, pyrite, marcasite and greigite. Ferric iron scales, such as hematite, magnetite, akaganeite, goethite and lepidocrocite, were also common in the scale mixtures. Common mineral scales, especially calcite, were often found. In addition, iron carbonate and other ferrous iron compounds were also identified. The relative abundance of these minerals showed wide-ranging variations from well to wells. Those variations also changed and with depth and time in the given wells. A more interesting phenomenon was the layered structure in the scale deposits, with two distinct layers having very different compositions. These results provided critical information for the understanding of scaling formation process.Scale removal with chemical method had limited success in past. Scale dissolvers, based on HCl acid, caused severe tubular corrosion and formation damage. Different mechanical techniques have been tested and implemented over the years. These field experiences are reviewed in the paper. Also, challenges and requirements for scale dissolvers are discussed.

Density-Functional Theory Investigation of Barite Scale Inhibition Using Phosphonate and Carboxyl-Based Inhibitors

“…8 Other common scale types include different forms of iron sulfides (pyrite, greigite, and marcasite) and ferric compounds depending on reservoir conditions. 3 However, the most common types detected are barite, calcite CaCO 3 , gypsum CaSO 4 •2H 2 O, and dolomite CaMg(CO 3 ) 2 . Different groups of researchers have studied the iron sulfide scale formation using both theoretical and experimental techniques.…”

“…For example, at around 2 km on the downhole tubing, the scale starts to be noticeable, and the severity increases with the depth. 3 The main factors that affect scale formation are temperature, pH, operating pressure, flow velocity, permeation rate, salt content, and the presence of other metal ions in the system. 4 From a thermodynamic point of view, precipitation becomes feasible when the solution is supersaturated; that is, ion concentration in solution is higher than its saturation limit.…”

“…Different wells may have scales that are different in composition, severity, and depth. For example, at around 2 km on the downhole tubing, the scale starts to be noticeable, and the severity increases with the depth . The main factors that affect scale formation are temperature, pH, operating pressure, flow velocity, permeation rate, salt content, and the presence of other metal ions in the system .…”

“…Many studies were conducted to evaluate scale types in the oil and gas industry. , In general, scales are composed of sulfate, phosphates, carbonates, and other salt forms, such as alumina silicates . Other common scale types include different forms of iron sulfides (pyrite, greigite, and marcasite) and ferric compounds depending on reservoir conditions . However, the most common types detected are barite, calcite CaCO 3 , gypsum CaSO 4 ·2H 2 O, and dolomite CaMg­(CO 3 ) 2 .…”