A Feasibility Study of Mixing Disposal Water with Aquifer Water for Downhole Injection

Abstract

Abstract Disposal of wastewater produced from various gas oil separation plants is a major concern for many oil companies. Mixing of wastewater with injection water has many economical and environmental advantages. The objective of the present study is to determine the feasibility of mixing produced water with the injection water obtained from a shallow aquifer. Several samples were collected from disposal water and injection water locations. Measurements of hydrogen sulfide and total iron were conducted on site using standard techniques. The concentration of H2S in disposal water was found to be in the range of 550 - 765 mg/L. The aquifer water contains 4 - 5 mg/L of total iron. Compatibility tests conducted in the lab and on-site showed the formation of iron sulfide. Acid solubility tests and XRD analysis confirms that this iron sulfide is present in a non-crystalline, acid-soluble form (amorphous). The presence of iron sulfide in disposal waters is a major concern because it can cause formation damage and subsequent loss of well injectivity. Various chemicals were evaluated to mitigate the formation of iron sulfide. These chemicals ranged from formaldehyde, triazine, to tetrakis hydroxymethyl phosphonium sulfate, to oxidizers (sodium hypochlorite, hydrogen peroxide and sodium nitrite). Similarly, some iron chelating agents, such as citric acid, ethylenediamine tetraacetic acid, and nitrilotriacetic acid were evaluated. This paper will discuss various types of H2S scavengers, reactions with H2S, advantages and disadvantages of each chemical based on extensive lab and field studies. Chelating agents for dissolved iron were also evaluated. Higher dosages of either sodium hypochlorite (hydrogen sulfide scavenger) or citric acid (iron sequestering agent) can mitigate the precipitation of iron sulfide species up to 15 vol% of disposal water in the mixture.