Summary Acidizing is a common practice that aims to recover the initial skin factor or even decrease it further. Acids tend to create conductive channels (wormholes) through carbonate formations that connect the reservoir to the wellbore and bypass the damaged zones. Optimum wormholes are formed when certain conditions are met, such as optimum acid concentration and optimum injection rate. To thoroughly grasp wormhole creation, several characterization techniques should be performed. Computed-tomography (CT) scan and differential-pressure data are two practices commonly used for characterization of wormholing in laboratories by use of core-plug samples. Differential-pressure data and CT scan can verify the occurrence of acid breakthrough and qualitatively suggest the size and path of a wormhole. Nuclear magnetic resonance (NMR) was introduced in earlier studies as a new characterization tool for wormholes. NMR can detect the changes in micro- and macropores that are caused by acid injection, and it can indicate changes in interconnectivity between different pore systems and detect the materialization of formation damage induced by injected acids. However, the NMR technique cannot detect the new porosity corresponding to the generated wormhole because of an inability to sustain saturation fluids inside the core-plug samples. In this study, the NMR measuring method is further improved by use of a customized polytetrafluoroethylene (PTFE) tube as a container for core-plug samples. Because PTFE material does not interfere with NMR readings, this improvement allows the core-plug samples to maintain full saturation, which enables the detection of the new wormhole porosity. As a result, NMR can indicate different characteristics of the generated porosity, including the size of the wormhole and the changes in diffusion coupling and the distribution of each pore size. This technique can elaborate new aspects of wormhole generation and characteristics in carbonate reservoirs, in addition to assessing the damage caused by the stimulation fluid.

Hydrogen sulfide (H 2 S) scavengers have been used extensively in different field operations such as drilling and acid stimulation treatments. Typically, H 2 S scavengers are preliminarily designed to react effectively at different in-situ conditions. For example, Triazine-based scavengers are designed for neutral-high pH conditions, while aldehyde-based scavengers are intended for low pH conditions. However, reaction products of these scavengers with H 2 S could lead to potential formation damage.The efficiency of two Triazine-based H 2 S scavengers were investigated over a wide-range of parameters: solution pH value, temperature and exposure time. Additionally, the effects of both scavenger concentration and its ratio to H 2 S gas on the scavenging capacity were explored. In this work, the scavenger concentration varied from 1 to 5 vol%, reaction temperatures up to 50°C.Earlier studies have shown that, at low pH, triazine-based have a very low efficiency in scavenging H 2 S because the hydrolysis rate of triazine is faster than the reaction rate with H 2 S. Nonetheless, in this study, it was found that long exposure time between triazine-based scavengers and H 2 S can result in significant scavenging efficiency even at low pH values. Doubling the exposure time had almost doubled the amount of scavenged H 2 S in acidic solutions. In addition, this work, for the first time, highlights the possibility of calcium sulfide (CaS) precipitation in spent acid containing H 2 S scavengers. This precipitation has been observed when low scavenger concentrations were used in spent hydrochloric (HCl) acid. This paper provides optimum design parameters that allow for much effective use of H 2 S scavengers without causing the formation of calcium sulfide scale.

Acidizing fluids perform their functions through complicated processes, hence, require additional additives to achieve wanted goals and prevent harmful side effects. Examples of these additives are diverting agents, corrosion inhibitors, surfactants, de-emulsifiers, and iron control agents. These additives should be designed to perform in a compatible and synergetic way with other additives. In this paper, triazine-based hydrogen sulfide scavenger was investigated for its scavenging capacity in various fluid formulations and its compatibility with several additives. To determine the scavenging capacity of the scavenger, an experimental method was developed. In this method, hydrogen sulfide (H2S) was generated by reacting hydrochloric acid (HCl) with iron sulfide (FeS), passed through a solution containing a triazine-based scavenger, and trapped in cadmium sulfate (CdSO3) solution. Then, the scavenging capacity was measured using gravimetric method. Furthermore, several testing parameters were investigated including amount of generated H2S, flow rate, concentration of scavenger, and type of solvent. All experiments were carried out in and aquatic neutral and acidic solutions at an anaerobic atmosphere and ambient conditions. The second part of investigation was the compatibility of hydrogen sulfide scavenger with common acidizing additives. The objective was to determine the damaging or hindering effect caused by these additives. Tested additives were mutual solvents and quaternary ammonium corrosion inhibitors. Water was found to be a suitable solvent for the tested scavenger. In contrast, the scavenging capacity declined dramatically when HCl was used. Therefore, optimum scavenging capacity, using water as solvent, was determined based on extensive testing that included parameters such as concentration of scavenger, amount of H2S, and contact time. Also, the tested additives were found to reduce the scavenging capacity.

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