The Environmental Impact of Mercaptans in Underground Storage

Abstract

Abstract Mercaptans are added to odorless natural gas for safety reasons. In normal operations, gas companies add mercaptans to natural gas when delivered to the city gates station for commercial usage. Gas transported in gas transmission lines and underground gas storage fields normally does not contain mercaptans or other odorants. However, there are instances when odorized gas from city distribution systems has been injected into underground gas storage fields. Mercaptan solubility in ground water and high adsorption to rock surfaces must be considered to prevent potential underground water contamination. Research efforts using experimental and numerical modeling show possible distributions of mercaptans on rock surfaces and in ground water. A basic simulation study has been conducted with various qualitative results obtained. This paper, a part of an overall effort to study the possible impact of mercaptans to the environment, shows the possible mercaptan distribution under various conditions. Introduction and Background The gas industry utilizes underground storage reservoirs for storing natural gas in summer and supplying consumers with gas in winter. This underground storage most commonly occurs in depleted gas or gas/condensate fields and aquifers near the market. Mercaptans are compounds added to odorless natural gas as odorants for safety reasons. Ethyl Mercaptan (C2H6S) or "EM", is a widely used odorant that is the focus of this paper. EM is added at the city gate station where odorless and dry natural gas enters city distribution systems. In city line networks, EM remains in the vapor phase at low pressure (roughly 20 psia). In some special cases, this odorized gas is recollected and is injected into nearby gas storages. During injection of this odorized gas, the EM is exposed to a porous rock environment with liquid (water or brine) present. In this high pressure environment, EM is absorbed into the liquid and is directly (vapor to dry rock) or indirectly (vapor to liquid to wet rock) adsorbed into rock surfaces. The porous media near a wellbore acts as a filter to partially screen out EM as illustrated by Fig. 1. As a result, EM concentration in vapor phase decreases with increasing distance away from the wellbore As long as there is EM in a gas stream, some EM is dissolved in the contacted liquid. With nearby aquifer movement, dissolved EM may be carried away from the storage site. The deeper EM penetrates into reservoir by injection, the much broader the area of EM contamination becomes. EM distribution around a shallow storage site may affect the quality of drinking or irrigation water. During an injection period, high pressure near the wellbore increases the solubility of EM in water/brine as well as the adsorption ability of rock. In the beginning of withdrawal season, wellbore pressure is reduced due to storage gas withdrawal. Low pressure decreases solubility and adsorption capabilities. In this situation, porous rock near the wellbore releases EM to the gas phase. Therefore, in the beginning of a withdrawal season, EM concentration in producing gas may be higher than the storage gas injection concentration which is normally at a constant 21 mg/m3. Toward the end of a withdrawal season, when low EM concentration gas from outer boundaries arrives, the producing gas has lower EM concentration than the injection gas. Additional EM may need to be injected into the producing gas when delivered to city systems. During this injection/withdrawal cycle, some EM is lost. The remaining underground EM may be an environmental concern. P. 307