Limitations of Conventional Geothermal Resources

Geothermal energy is a proven “green” energy technology in areas where high temperature hydrothermal sources exist.  A hydrothermal source contains three key parameters:

  1. heat,
  2. a natural water source, and;
  3. sufficient permeability.

These conditions exist primarily at tectonic plate boundaries and are an important resource for large industrial power and direct use applications. There are currently hydrothermal power plants operating in over 30 countries. These plants are mainly along the “Pacific Ring of Fire.”

The largest capacity existing in:

  • United States,
  • Indonesia,
  • Philippines, and;
  • New Zealand.

Iceland and Turkey also have significant hydrothermal developments.

However, these resources do not occur in many locations that require baseload power or large amounts of hot water for industrial or district heating purposes. As a result, their impact on the global energy need is limited. While heat occurs at depth globally, in most locations there is no naturally occurring water source.  Additionally, in sedimentary reservoirs there may be permeability, but in hard rock, flow only exists in naturally or artificially created fractures.

Enhanced Geothermal Systems

Geothermal resources that contain heat but do not contain a natural water source are generally termed “Hot Dry Rock” resources.  In these cases, water is injected into the resource through one or more injection wells, The water is then returned to surface through one or more production wells.

Sufficient reservoir volume and permeability is usually be created by stimulating the rock. This allows for a large volume of water to be heated by contact with the rock and produced back to surface in a stable manner for many years.

Once at surface, the hot water is typically passed through a heat exchanger to heat another fluid in an Organic Rankine Cycle process.  The water is then re-injected to repeat the cycle.  This type of geothermal system is called an Enhanced Geothermal System (EGS).

The EGS process has the potential to be developed anywhere in the world, since the earth is hot at depth.  Over 30 EGS pilot projects have operated with varying success in the last 50 years. The first EGS pilot was drilled at Fenton Hill, USA in 1974.  Despite the promise of EGS, the challenge of efficiently recovering the available heat has limited the commercial success of EGS to-date.

New Technologies for EGS

New technologies are continually being developed. The industry is also looking at technologies from other industries which show promise in increasing the effectiveness and reducing the upfront risk for EGS. As these technologies advance, it is expected that EGS will gain traction and become viewed as a reliable option for generating baseload power.  Some examples of these technologies include:

Drilling

While rotary drilling has been the norm for deep wells, air and water hammer drilling show promise for increasing rate of penetration (ROP) in hard rock. Other types of drilling technologies are also being investigated including laser drilling.

Stimulation

Most EGS applications to-date have relied on stimulating naturally existing fractures with water injection. A seismic map where these stimulated fractures exist is created, and then production well(s) are drilled into the stimulated region. In unconventional oil applications, multi-well horizontal fracture (MWHF) technologies have been developed to initiate and maintain multiple fracture networks. These technologies, in conjunction with deviated well trajectories, may allow for more effective geothermal reservoirs to be created in the rock. This is one focus of the US Department of Energy (USDOE) FORGE and COLLAB projects.

Production Management

Most EGS applications to date have used “open-hole” completions in the injection and production wells. In these cases, no production casing and cement is used. This does not allow for control over specific injection and production points along the well. Instead, the fluid will flow in the areas of least resistance (i.e. highest permeability). In some cases, this can lead to much of the fluid flowing in one or two dominant channels. This tends to remove heat from the rock around these primary flow channels but does not recover heat from other parts of the reservoir where flow may be minimal.

In thermal oil production applications where bitumen is produced with Steam Assisted Gravity Drainage (SAGD), technologies have been developed to control the fluid injection and production points in the long horizontal wells. These include high temperature isolation packers, inflow and outflow control devices (ICDs and OCDs). These technologies have proven to be effective in improving reservoir contact and in limiting “short circuiting” of fluid between the injector and producer in these operations. There is potential for these technologies to provide the same benefits to EGS.

Key Technical Challenges for EGS

These challenges have led to many projects being uneconomic due to capital cost increases, as well as flow, temperature, or safety profiles being less than designed for.

  • Insufficient reservoir volume being created in the rock between the injection and production wells leading to lack of fluid flow
  • Fluid Short-circuiting in one or more major channels between the injection and production wells leading to lack of heat transfer between the rock and fluid
  • Difficulty with drilling rate of penetration (ROP) and stimulation effectiveness in connecting the production and injection wells
  • Some instances of Induced seismicity during drilling or stimulating activities in active seismic areas leading to safety concerns
  • Long term reliability of well completions and production equipment

Three Ways C-FER is Helping to Advance EGS

C-FER has a long history in helping the thermal oil and gas with qualifying and implementing technologies that can improve the efficiency, reliability and safety of circulating liquids and steam, at depth, through rock for the SAGD process. These same services are now being used to adapt oilfield technologies to support the geothermal industry.

C-FER can help geothermal operators, industry and government organizations, and technology developers to support EGS developments by:

Techno-economic Assessments

There are several key parameters that effect the technical and economic potential of an EGS development, including the geothermal gradient, rock properties, fracture spacing, drilling and completion costs, natural gas price, and carbon levy projections.

C-FER has experience in providing full techno-economic feasibility assessment for your application. We can work with your economic assessment tools or create a custom analysis tool to analyze the cost and well production sensitivities to depth, flow rate, completion size, and thermal gradient.

We can also provide a cost/benefit analysis of implementing newer technologies for EGS developments such as different forms of Artificial Lift (e.g. Electric Submersible Pumps) and advanced wellbore technologies such as isolation and flow control devices.

Where applicable, we expand our team to include expertise from Universities and geothermal developers to analyze fracture mechanics, reservoir strategies, and development costs.

Structural and Thermo-Hydraulic Well Analysis

C-FER performs detailed computational fluid mechanics (CFD) and finite element analysis (FEA) on thermal and geothermal wells.

CFD is used to perform complex flow and heat transfer analysis including phase-change behavior.  This is important for advanced well completions for EGS applications, including:

  • Determining where to place ICDs and/or OCDs in your system
  • Determining heat transfer interaction between the hot rock and wellbores along the full length of the wells from reservoir to surface
  • Analyzing artificial lift challenges due to well deviation and multiphase fluid behavior

FEA is used to perform detailed analysis of EGS wells to ensure long-term reliability of completions and well technologies, including:

  • High temperature premium and semi-premium casing connection design
  • Structural design and analysis of high temperature well technologies such as packers

C-FER has also developed software to visualize and assess EGS well condition and integrity.  The Well Xplore software is used to convert multi-finger caliper data into a 3D visualization of the wellbore.

Equipment Testing

As the utilization of advanced well technologies for EGS advances, technologies from other thermal industries will need to be qualified and developed further for the specific EGS requirements.  For example, thermal oil technologies will need to be developed for higher flow rates and more severe corrosion conditions.

C-FER performs testing services to adapt these technologies for EGS, including full-scale testing of:

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