Mobility of Ba, Sr, Se and As under simulated conditions of produced water injection in dolomite

Highlights

• Mobility of Ba, Sr, Se, and As on dolomite increases with increasing temperature.

• (co)precipitation/recrystallization lead to irreversible uptake of trace elements.

• XRD analyses at 150 °C confirm the presence of newly-formed mineral phases.

• Dolomite may serve as a low-cost sorbent for toxic elements in industrial effluents.

Abstract

The volume of petroleum produced water (PPW) has increased dramatically over the last decade. PPWs are rich in salts and often contain high concentrations of potentially toxic trace elements such as Barium (Ba), Strontium (Sr), Selenium (Se), and Arsenic (As) which, under the right circumstances, may contaminate freshwater supplies. To avoid these issues, PPW is frequently disposed of in subsurface aquifers such as the dolomitic Arbuckle Group in the state of Oklahoma, USA. This may not be a permanent solution if the injected PPW migrates within the disposal sites to ultimately reach conductive fault zones and/or existing wells with casing and/or cementing failures. In these cases the water could pose an environmental risk to potable groundwater. In order to understand the mobility of these elements under conditions similar to produced water injection, we conducted a series of batch sorption experiments. We investigated the effect of brine salinity and temperature on the sorption of Ba, Sr, Se, and As by dolomite. The results revealed that the sorption of the tested elements on dolomite did not substantially change with increasing salinity from 18 to 90 g/L. However, the sorption for all elements did increase with increasing temperature from 22 to 150 °C. This is likely due to a combination of strong surface complexation or ion exchange reactions coupled with precipitation/co-precipitation on the dolomite mineral surfaces.

Introduction

For every barrel of crude oil produced from conventional and unconventional oil and gas reservoirs, seven to 10 barrels of highly saline petroleum produced water (PPW) are generated (Guerra et al., 2011). The salinity of PPW ranges from a few thousand to 463,000 mg/L (Fan et al., 2018; Liden et al., 2019; Otton and Mercier, 1995; Wenzlick and Siefert, 2020). PPW contains high concentrations of salts and potentially toxic trace elements such as Barium (Ba), Strontium (Sr), Selenium (Se), and Arsenic (As) (Balaba and Smart, 2012; Ebrahimi and Vilcaez, 2017). The concentrations of potentially toxic elements in PPW are frequently higher than the Maximum Contaminant Levels (MCLs) established by United States Environment Protection Agency (Yost et al., 2016). A common management practice to avoid costly desalination and to prevent the pollution of groundwater with toxic trace elements is to inject PPW into deep saline aquifers (Burden et al., 2016; Lutz et al., 2013). Davies et al. (2014) found that well barrier or integrity failures for oil and gas wells ranged from 1.9% to 75%. Because disposal wells are similarly constructed and many oil and gas wells are ultimately turned into disposal wells, it is reasonable to assume similar levels of failures are associated with wells that handle PPWs (Davies et al., 2014). Because of these failures and the possibility of natural fault conduits, the toxic elements of wastewater may ultimately migrate upward from the disposal sites toward potable groundwater sources.

Among many possible water-rock chemical reactions, a combination of adsorption and precipitation reactions (termed here “sorption”) are the primary reactions that retard the transport of many elements in the subsurface (Dube et al., 2001; Landry et al., 2009; Rimstidt et al., 1998; Sarı et al., 2007). Our current knowledge of these reactions is largely constrained to shallow subsurface environments with much lower salinity and temperature as compared to the deeper subsurface, which is the target for PPW disposal (Marley et al., 1993; McCarthy and Zachara, 1989; Muuri et al., 2018). Increasing the salinity of aqueous solutions tends to decrease the sorption of trace elements and increase their mobility (Acosta et al., 2011). Increasing temperature, however, may increase or decrease the sorption of some trace elements on specific sorbates (Zachara et al., 1988) through differences in mineral solubilities and reaction kinetics. It is still unclear how the mobility of some trace elements may be impacted by the high salinities and temperatures of PPW associated with deep subsurface disposal. The lack of knowledge regarding the mobility of trace elements is likely more complicated in dolomite rocks because of reactions associated with the precipitation and dissolution of carbonate surfaces, particularly with increasing temperatures (Ruiz-Agudo et al., 2014).

In this study, we address the challenge of understanding the mobility of Ba, Sr, Se, and As in PPW associated with deep disposal by conducting batch sorption experiments using dolomitic rocks. Dolomite was selected as the substrate because 1) it is likely more reactive as compared to other sedimentary rocks used for disposal of PPW such as sandstone, and 2) the Arbuckle Group dolomite is the most common geological interval where PPW is disposed of in the state of Oklahoma, USA (Murray, 2015). Through these experiments, we aim to understand the mobility of these trace elements in dolomite rock at total dissolved salt (TDS) concentrations of 18 g/L and 90 g/L and at temperatures of 22, 90 and 150 °C. The ranges of salinities and temperatures chosen for this study were selected to span the lower and higher ends of the common ranges found for both PPW disposal sites and dolomitic petroleum reservoirs. The produced waters associated with many disposal sites in the Arbuckle Group have salinities around 90 g/L and the subsurface temperatures of disposal can reach 100 °C (McKenna, 2002; Watney and Rush, 2012). Barium, Sr, Se, and As were selected because they are among the most common potentially toxic elements found in PPW, and experimental work within these elements in PPW-rock systems is limited (Fontenot et al., 2013).