Modeling the effects of silica deposition and fault rupture on natural geothermal systems

Dempsey, D. E.; Rowland, J. V.; Zyvoloski, G. A.; Archer, R. A.

doi:10.1029/2012jb009218

We modelled a geothermal field in New Zealand's Taupō Volcanic Zone where cooling water deposits silica. Over 120,000 years, a low-permeability layer about 200 m thick formed at the surface. It diverted water sideways and warmed the reservoir. A fault rupturing the layer took at least 10,000 years to reseal.

The problem

Hot water rises at more than 20 geothermal fields in New Zealand's Taupō Volcanic Zone (TVZ) (Bibby et al., 1995). As the water cools in porous volcanic deposits near the surface, its capacity to hold dissolved silica falls. The excess silica precipitates as quartz in the pore space. This reduces the permeability of the rock. Sealing can change how much heat a field stores at depth and where its heat reaches the surface. Both are relevant to geothermal energy and to exploration for fields with little surface activity.

Our goal in this study was to quantify how silica deposition changes the flow of heat and water through a geothermal field. Geothermal activity in the TVZ also correlates with faulting (Wood, 1992; Wood et al., 2001). So we modelled an earthquake rupturing a fault through rock that silica had already sealed.

What we did

We used FEHM (Zyvoloski, 2007), a computer code that simulates the flow of heat and water through rock, to model a block of crust 15 km square and 8 km deep, heated from below. After 300,000 simulated years, a single plume of hot water rose through the centre. The geothermal field above it had a heat output of 120 MW and an area of 1.4 km². We defined the field as the area where heat flow from the ground exceeds 1 W/m², over 14 times the continental average (Chapman, 1986).

We assumed the water everywhere holds the maximum amount of silica it can at its temperature. The rate of deposition or dissolution then depends on the flow rate, the temperature gradient, and the angle between them. Deposition is fastest when water flows straight towards cooler rock, and falls to zero when flow is at right angles to the temperature gradient. Water flowing towards hotter rock dissolves silica instead.

We converted the silica deposited into a loss of porosity, and then into a loss of permeability. In the volcanic deposits, we assumed that permeability varies with the cube of porosity (Zhang et al., 1994). A fall in porosity from 20% to 2% then lowers permeability a thousandfold. However, we did not allow permeability to fall below 10⁻¹⁶ m². This limit is intended to represent the permeability that faulting and fracturing maintain in the TVZ.

In a second model, we ruptured a vertical fault through the centre of the sealed field. The rupture reset permeability on the fault to its original values.

What we found

Silica deposition was fastest at the surface above the plume. This is because temperature gradients and flow rates are largest there, where hot rising water meets the cool atmosphere. After 120,000 years, a circular low-permeability cap 1 to 2 km across and up to about 200 m thick had formed. This process is known as self-sealing (Facca and Tonani, 1967).

The cap reduced the field's heat output from 120 to 80 MW within several thousand years. Hot water from depth continued to arrive and was deflected sideways beneath the cap. The field area more than tripled. Beneath the cap, the heat stored in the top 2 km increased by about 20%. In our model, sealing improved the field as an energy resource. However, conduction through a thick cap can also cool the centre of a field and leave little surface sign of the hot reservoir beneath.

Before rupture, the field in the fault model had an output of 75 MW, below the 80 MW of the first model. This is probably because the fault model used a finer grid near the surface and along the fault. In our tests, finer grids sealed the field more completely and lowered its heat output. Rupture raised permeability on the fault at the surface a thousandfold, from 10⁻¹⁶ to 10⁻¹³ m², and field output rose to 125 MW. However, the line where the fault meets the surface is also where silica deposits fastest. Output returned to its pre-rupture value within 1,000 to 2,000 years, and the near-surface fault was resealed over 10,000 years.

At the extinct Ohakuri field, quartz was deposited to at least 150 m depth (Henneberger and Browne, 1988), comparable to our modelled cap. Henneberger and Browne (1988) proposed that activity there ended because of changes at depth. Our results are consistent with this, because in our model the cap left the supply of hot water from depth largely unchanged.

The Te Kopia field is located on the active Paeroa Fault, which slips at about 1.5 mm per year (Villamor and Berryman, 2001). If each earthquake produces 2 to 3 m of slip, this implies a rupture every 1,000 to 2,000 years. The present pattern of activity at Te Kopia has probably formed over many ruptures. Renewed flow along the fault may be the usual state of this field.

Limitations

Our assumption that water always holds its maximum silica overestimates deposition where flow is fast, such as in a newly ruptured fault (Martin and Lowell, 2000). The 10,000-year resealing is therefore a minimum. The model also omits boiling, the fracturing caused by fluid pressure, and dissolved salts. Sealing rates in our model depend in part on grid resolution.

Natural geothermal fields also change through their deep fluid pathways, fluid chemistry, heat sources, and repeated faulting. We therefore caution against reading a model of one process as a complete history of any one field. Simulations with repeated fault ruptures and variable permeability may give further insight into circulation at Te Kopia.

Glossary

The Taupō Volcanic Zone (TVZ) is the region of active volcanism and geothermal activity in New Zealand's central North Island.

Porosity is the fraction of a rock's volume that is open pore space.

Permeability is a measure of how easily fluid flows through a rock, in square metres (m²).

Silica is silicon dioxide, which dissolves in hot water and precipitates as quartz when the water cools.

The temperature gradient is the rate at which temperature changes with distance, pointing towards hotter rock.

A plume is the column of hot, buoyant water rising from depth beneath a geothermal field.

The cap is the layer of low-permeability, silicified rock that forms at the surface above the plume.

Self-sealing is the reduction of a geothermal field's near-surface permeability by its own mineral deposits.

Heat flow is the heat leaving the ground per unit area, in watts per square metre (W/m²).

Heat output is the total heat leaving a field's surface, in megawatts.

A megawatt (MW) is a million watts, a unit of power.

A model grid divides the rock into cells, and the equations of heat and water flow are solved for each cell.

Conduction is heat transfer through rock without fluid flow.

Abstract

Natural geothermal convection abounds within the Taupo Volcanic Zone (TVZ) of New Zealand's Central North Island. In many locations the highly porous eruptive products that blanket the landscape have been altered by the throughput of hydrothermal fluids and the consequent deposition of silica. We detail a numerical model that considers the evolution of a geothermal plume in the presence of silica deposition/dissolution that controls an evolving permeability distribution. Precipitation of silica occurs according to a gradient reaction regime, in which the dissolved silica concentration is controlled by the temperature dependent silica solubility. Over a period of 120 kyr, continuous geothermal circulation leads to the development of a low permeability cap‐zone, approximately 200 m thick, above the main geothermal upflow zone. The cap‐zone encourages lateral flow of rising fluids, increasing the area across which geothermal expression is observed. It also has an insulating effect on fluids below the cap, causing increases in temperature, enthalpy, and the reservoir potential of the field. A second model is constructed to consider the specific scenario of fault rupture through the impermeable cap‐zone. Coseismic increases in permeability along the fault plane produce vigorous, renewed flow through the center of the geothermal field, temporarily reducing lateral flows. However, resealing of near surface permeability is rapid, and the restoration of lateral flows and recovery of the geothermal reservoir occurs within ∼10 kyr. These effects are discussed in the context of two TVZ geothermal fields: the extinct Ohakuri field, and Te Kopia, which is situated on a major active normal fault.