Fracture Characterization in Enhanced Geothermal Systems by Wellbore and Reservoir Analysis (2009-2012)

Project Technology Type: EGS Component R&D › Reservoir Characterization
Awardee: Stanford University
Partners: none
Location: Stanford, CA

Objectives

Characterizing the size, shape, and conductivity of fractures is crucial for successful development of Enhanced Geothermal Systems. Hydraulic stimulation of fractures is the primary means of creating functional geothermal reservoirs at sites where the permeability of the rock is too limited to allow heat recovery at economic rates. The extraction rate of energy will be dependent on the creation of sufficient fracture area within a sufficient volume of hot rock. However, currently neither the tools nor the analysis approaches are adequate, especially at high temperature and at greater depth. Hence, the objectives of this project are to develop new tools and approaches for fracture characterization. The main research will focus on development of reservoir engineering approaches includingnanotechnology, resistivity tomography (RCT) method, and nonparameteric regression. The project will develop the preliminary RCT approaches studied earlier for porous materials, and extend their application to fractured media. The project will also extend the nonparameteric regression method and other approaches to identify the fracture features (including fracture direction, shape, and size) using production data.
In summary, this project will develop systematic techniques and tools to characterize the entire fracture network in Enhanced Geothermal Systems, both in the wellbore and in the reservoir. A complete characterization of fractures in the region both near the wellbore and in the interwell reservoir regions is central to maximizing the efficiency of energy extraction from Enhanced Geothermal Systems. By allowing for the optimal design of the recovery process, the results of this research will permit the energy extraction from a given area of enhanced fractures to be maximized. Given the significant cost of producing an enhanced fracture system in the field, the improvement of energy recovery is a key to making this energy source viable economically.

Results and Accomplishments

1. Nanosensor Task

Silicon dioxide nanoparticles were transported successfullyboth through a Berea sandstone core as well as through a 10 meterelong slim tube packed with sand. The slim tube experiment wasinitiated to test the feasibility of passing nanoparticles throughlonger pore networks than have previously been tested. The injectednanoparticles were transported 10 meters through the sand-packed slim tube and were detected in the effluent. Scanning Electron Microscopy (SEM) imaging demonstrated unambiguously that the nanoparticles hadbeen transported through the pore spaces within the sand-packed tube.

The objective of this experiment was to investigate thetransport and recovery of nanoparticles through a longer flow pathapproaching actual field distances such as in interwell tracer testing.The silicon dioxide nanoparticles were detected at the effluent,confirming their transport (Figure 1). The permeability was unaltered during and after the injection of the nanofluid.

In the next step, Hematite (Fe2O3) nanoparticles, informallycalled “nanorice” due to their spindle shape, were synthesized tocontinue to investigate the transport of nonspherical particles throughporous media. The idea is to use change of shape as an indicatorof the temperature experienced during transport of the nanoparticlesthrough the reservoir. Testing of this concept is now in progressusing the nanorice we have fabricated.

2. Interwell Correlation Task

A fast and simple method for characterizing flow through afracture network has been developed. The method is based on theassumption of steady-state, incompressible single-phase flow, whichshould be a relatively good approximation for most EGS and othersingle-phase geothermal systems already in production. This methodlends itself well to analysis by graph theory which has seen rapiddevelopment in the Computer Science industry in recent years (e.g.for analyzing road maps, internet searches, social networks etc.).Prototype algorithms to compute the fastest path, most transmissiblepath and largest flow path between wells were developed.

Nodal analysis of the fracture network led to a formulationwhich allowed explicit calculation of the flow rate along each fracturesegment. Based on this information, the flow rate contribution wascomputed for each path connecting any two wells. This led to a method for analyzing the contribution of the fracture network to thedispersion seen in a tracer arrival profile. Being able to solve this problem without any numerical dispersion effects (which was one ofthe major goals stated in the last quarterly report) has brought us a step closer to characterizing the effects of fracture networks onproduction data. An example network and flow solution is shown in Figure 2 below.

Figure 2: Modeled tracer and thermal signatures of sparse and dense fracture distributions.

3. Resisitivity Modeling

In this task, the goal is to find ways to use Electrical Resistivity Tomography (ERT) to characterize fractures in geothermal reservoirs. ERT is a technique for imaging the resistivity of a subsurface from electrical measurements. Typically, electrical current is injected into the subsurface throughconducting electrodes and the resulting electrical potentials are measured. Due to the large contrast in resistivity between water and rock, the resistivity measurements could be efficiently used to indicate fracture locations.

Resistivity measurements have been widely used in the medical industry to image the internal conductivity of the human body, for example to monitor epilepsy, strokes and lung functions. In Iceland, electrical resistivity tomography methods have been used to map geothermal reservoirs. Different resistivity measurements have been used there to locate high temperature fields by using electrodes located on the ground’s surface. Stacey et al. (2009) investigated the feasibility of using resistivity to measure geothermal core saturation. A direct current pulse was applied through electrodes attached in rings around a sandstone core and it resulted in data that could be used to infer the resistivity distribution and thereby the saturation distribution in the core.

In the approach considered in this project, electrodes would be placed inside geothermal wells and the resistivity anomalies studied between them to locate fractures and infer their properties by resistivity modeling. Due to the sparsity of measurement points, i.e. limited number of test wells, we will endeavor to find ways toimprove the process of characterizing fractures from limited resistivity data. To enhance the contrast in resistivity between the rock and fracture zones, the possibility of using conductive fluid will be explored. Furthermore, the influences of temperatures and fluid stream on resistivity measurements will be studied. The effects of mineralization in the fractures will also be examined, as fractures containing minerals may be easier (or more difficult) to distinguish from the surrounding rocks. A resistivity model has been made to calculate a potential field due to point sources of excitation. Currently, we are attempting to use that model to characterize different fractures patterns. An example of the generated electrical field is shown in Figure 3 below.

Figure 3: Example electric field distribution caused by a pair of fractures.

Funding Opportunity Announcement

Enhanced Geothermal Systems Research, Development, and Demonstration; Funding Opportunity Announcement Number: DE-PS36-08GO98008

Funding Source: DOE
DOE Funding Level*: Total Award: $967,541
Total Project Cost: $1,209,426
Principal Investigator(s): Roland N. Horne

Description of Technical Approach

One task of the project is the development of possibleapproaches to characterizing the fracture systems by using nanoparticles as temperature sensors. The second task is extending thenonparameteric regression method and other approaches to identify the fracture features (including fracture direction, shape, and size)using production data such as tracer and thermal breakthrough.Thirdly, the project is investigating the enhancement of theresistivity computer tomography (RCT) approaches we developed earlier for porous rocks, to make them applicable to analysis of fracturedrock.

Project Management, Targets/Milestones

• Phase I (first year): Explore the possibility tocharacterize the fractures created in EGS using nanotechnology toestimate connectivity and temperature. (completed)
• Phase II (second year): extend the nonparameteric regressionmethod and other approaches to identify the fracture features(including fracture direction, shape, and size) using productiondata. (in progress)
• Phase III (third year): develop the approaches tocharacterizing the fracture systems by using nanotechnology orresistivity tomography (RCT). (future)

Future Directions

The nanoparticles have been shown to pass successfully through rock materials, for distances as long as 10 meters. It isalso appears promising that we can fabricate temperature-sensitivenanoparticles as a means of carrying information about the thermalconditions inside the fracture network of an EGS reservoir. Thenonparametric approach of relating injection well inputs toproduction well outputs appears promising as another way ofcharacterizing the EGS fracture network. Thirdly, resistivitymapping may provide a way of indicating which fractures are actuallycarrying the fluid.

Awards/Recognition for Project or Researchers

ABC television news, June 2009. http://abclocal.go.com/kgo/video?id=6889459