Insights into interconnections between the shallow and deep systems from a natural CO2 reservoir near Springerville, Arizona

Keating, Elizabeth; Newell, Dennis; Dempsey, David; Pawar, Rajesh

doi:10.1016/j.ijggc.2014.03.009

At a natural CO2 reservoir in Arizona, a permeable fault likely connects the deep gas to a shallow aquifer. Water chemistry suggests that CO2 rises dissolved in saline water that also contains arsenic. In our simulations, most of the gas remains at depth despite the fault.

The problem

A project that stores carbon dioxide (CO2) in deep rock has to show that almost all of the CO2 is retained at depth, and that any CO2 that leaks upward does not harm shallow drinking water. Natural CO2 reservoirs can inform both questions.

The Springerville–St. Johns dome in Arizona is a natural CO2 reservoir 200 to 700 m below the surface (Moore et al., 2005). It is reported to hold up to 445 billion m³ of CO2, measured at atmospheric pressure (Stevens et al., 2005). The Coyote Wash fault zone crosses it. CO2 has leaked to the surface along this fault for thousands of years and formed travertine deposits. The leak rate has since declined, but the shallow aquifer is still rich in dissolved CO2. High dissolved CO2 in the groundwater wells in the fault zone has required their operator to replace the pipe in most of them with stainless steel because of corrosion.

Our goal in this study was to ask (1) whether the fault connects the deep reservoir to the shallow aquifer, (2) what the leaking CO2 has done to groundwater quality, and (3) how up to 445 billion m³ of CO2 can remain stored underground beside a broad, permeable fault.

What we did

We sampled water from 8 wells that Tucson Electric Power pumps for cooling water at the Springerville Generating Station, and from two travertine spring mounds and a shallow well at Salado Springs.

Pumping from the wellfield between 1981 and 2010 lowered water levels by up to 34 m. Monitoring wells on the north side of the fault showed no measurable drawdown. We built a three-dimensional groundwater flow model with the FEHM code (Zyvoloski, 1997) and tested three versions of the fault zone. The first treated the fault zone as no different from the surrounding rock layers ("flow neutral"). The second was a distinct zone reaching only to the base of the deepest production well, and the third extended through the full thickness of the model. For each version, the code PEST adjusted rock permeabilities to best match the drawdown at 21 wells.

To investigate the stability of a shallow gas reservoir beside a wide, permeable fault, we ran multi-phase (CO2 and water) simulations on a two-dimensional cross-section, 100 m thick, taken from the flow model.

What we found

The distinct fault zone through the full model thickness matched the drawdowns best. Its sum of squared errors between modelled and measured drawdown was 598 at the 15 production wells. This is about half the 1,087 of the flow-neutral fault and a ninth of the 5,358 of the shallow fault zone (our arithmetic). At the 6 monitoring wells its sum was 6, against 133 and 10. This analysis demonstrates that the fault that carried CO2 to the surface in the past is still a pathway for groundwater from depth.

In the shallow aquifer, chloride, bicarbonate, sulfate, lithium and rubidium rise and fall together. This pattern is consistent with a mixture of two waters, a saltier, CO2-rich water rising along the fault and more dilute groundwater. Arsenic was the only trace element above the US Environmental Protection Agency drinking water limit of 0.010 mg/l. It reached 0.042 mg/l in one well and exceeded the limit in 9 of the 11 samples (our count from Table 2). Because arsenic rises with chloride in the production wells, we suggest that it arrives with the saline water. The fairly linear relationship with chloride also suggests that CO2 has not released trace metals from the aquifer sediments themselves. However, subtler changes in water quality driven by CO2 might still be occurring here. At Salado Springs arsenic falls below the trend. Microbial processes that precipitate iron oxides could lower it there.

In a simple degassing model, a loss of 40–50% of the dissolved CO2 can account for the carbon isotope ratio and dissolved carbon content of the most concentrated groundwater. We suggest that CO2 moved upward dissolved in brackish or saline water and degassed as the pressure fell or as the water mixed with shallow groundwater.

In the simulations, buoyant gas rose along the fault within the first 300 days. Once the gas in and near the fault had escaped, the rate at which the reservoir lost gas fell to a nearly constant 6 × 10⁵ kg (600 tonnes) per year for the 100 m slice. This is because, at hydrostatic pressure, there is insufficient driving force to move the gas sideways to the fault before it can escape upward. Scaled to a 50 km length of fault, this rate would empty a reservoir the size of the dome in about 2,800 years if no new CO2 arrived from the mantle. That slow leak is probably enough to maintain a plume of dissolved CO2 in the fault zone indefinitely. At these rates, some upward leakage of CO2 may be compatible with storing CO2 at depth and protecting shallow groundwater.

Limitations

The multi-phase simulations are not meant to reproduce the history of the site. In our model the fault was present from the start and did not open after the CO2 was emplaced. Our fault model is calibrated to drawdown data alone, because we know of no site-specific hydraulic tests. Conceptual models that we did not test could be equally valid.

Integrated multi-phase flow and reactive transport simulations, compared against the isotope and trace element data, could test alternative models of the site more rigorously.

Glossary

Carbon dioxide (CO2) storage, or sequestration, is the injection of CO2 into deep rock to isolate it from the atmosphere.

An aquifer is a layer of rock or sediment that holds and yields usable groundwater.

A fault zone is a band of broken rock along which the ground on either side has moved.

Travertine is a limestone deposited from spring water rich in dissolved CO2.

Drawdown is the fall in the water level in a well caused by pumping.

A production well pumps water out of the ground, and a wellfield is a group of them. A monitoring well is used only to measure the water level.

A conceptual model is a hypothesis about how a system is arranged and works, here about the hydraulic properties of the fault zone.

Permeability is a measure of how easily fluid flows through a rock.

FEHM (Finite Element Heat and Mass Transfer) is a computer code that simulates the flow of water, gas and heat through rock.

PEST is a computer code that adjusts model parameters to best match measurements.

The sum of squared errors adds up the squared differences between modelled and measured values. A smaller sum is a better match.

A multi-phase simulation models the flow of more than one fluid, here CO2 gas and water.

Buoyant gas is less dense than the water around it and tends to rise.

Hydrostatic pressure is the pressure of a still column of water.

A trace element is an element present in water at low concentration, such as arsenic or lithium.

Milligrams per litre (mg/l) is a unit of concentration, the mass of a substance dissolved in one litre of water.

Brackish water is saltier than fresh water but less salty than seawater. Saline water has a high concentration of dissolved salt.

Isotopes are forms of an element with different masses. The ratio of carbon isotopes in water indicates the source of its carbon and the processes that have affected it.

Degassing is the loss of dissolved gas from water as a separate gas phase.

Iron oxides are minerals of iron and oxygen that can take up arsenic as they form.

The mantle is the layer of the Earth below the crust.

Reactive transport simulations model the flow of water together with the chemical reactions between the water and the rock.

A plume is a body of fluid, here water carrying dissolved CO2, moving through the rock.

Abstract

If carbon dioxide (CO2) sequestration into deep geologic reservoirs is to be accepted by the public and environmental regulators, the possibility of upward leakage into shallow groundwater should be acknowledged and those processes well-understood. Studies of natural CO2 reservoirs and their connection (or lack thereof) with the shallow subsurface is one way to explore these issues. A natural reservoir near Springerville, Arizona has leaked CO2 to the surface along a fault zone for thousands of years, creating large travertine deposits. In recent times, the CO2 leak rates have declined significantly yet the shallow aquifer is still highly enriched in CO2. In this study, using water level data and simulations we demonstrate that the fault zone likely provides hydrologic communication between the shallow aquifer and the deeper reservoir. It is reasonable to assume, therefore, that the source of the CO2 in wells completed within the fault zone is the deeper CO2 reservoir. We present water chemistry data to demonstrate the geochemical impact of this CO2 on shallow groundwater quality. Interestingly, arsenic concentrations are elevated, but other trace metals concentrations are not. Arsenic and chloride concentrations co-vary, suggesting perhaps an external source of both elements rather than an in situ release of As due to CO2 attack on shallow aquifer sediments. Observations at this site demonstrate that hydraulic communications between shallow and deep layers and upward CO2 migration does not preclude long-term viability of a substantial CO2 reservoir at depth. We present multi-phase flow simulations to illustrate possible mechanisms trapping the CO2 at depth. Collectively, these analyses show that some degree of upward CO2 leakage may not be necessarily incompatible with the overarching goals of sequestering CO2 and protecting shallow groundwater.