The term chemical stimulation refers to the injection of fluids with chemical additives into the geothermal target formation. It is performed to overcome formation damage in the rock matrix as well as inside existing fractures and fissures leading to dissolution of certain minerals and thus increasing the hydraulic pathways in the rock and enhance the permeability.
In contrast to other stimulation techniques, chemical stimulation affects mostly the near borehole volume up to several meters from the well; fast reaction kinetics associated to the high reservoir temperature are leading to rapid acid degradation. However, in fractured rocks or karst formations acid solution can react further through existing flow paths.
Generally, three basic types of chemical stimulation can be distinguished: Acid Washing, Matrix Acidizing and Fracture Acidizing. They are differentiated by pumping pressure and therewith penetration depth. Acid Washing aims at cleaning the wellbore and dissolving scales at the casing or drilling related damage at the formation face.
For thermal stimulation of a well, cold water is injected below the fracturing pressure over a certain time period, generally a few days to weeks. Due to the low temperature of the water compared to the temperature of the rock, the stress in the rock changes, leading to stimulation of natural fracture networks or initiation of new fractures. Thus, the effect of thermal stimulation is very similar to the effect of hydraulic stimulation.
Thermal stimulation or thermal fracturing occurs in many cold-water injection wells and often an increase in injectivity with decreasing temperature of the injected water is observed. This is probably due to initiation and growth of thermal fractures near the well bore and higher weight of the fluid column.
Multi-stage and cyclic stimulation concepts
Horizontal multi-stage hydraulic fracturing is the process by which multiple fractures are created along a horizontal section of the wellbore in a series of consecutive operations. It is a well-established practice in the oil and gas industry and can be seen as a key technology enabling the development of enhanced geothermal systems. It provides better access to reservoirs, since horizontal wellbores allow for much greater exposure to a formation to develop a downhole heat exchanger with an appropriate size for extraction of heat. The individual stimulation is carried out stepwise in isolated intervals. This can be achieved by perforating the intervals of interest. One crucial aspect is the right distance between the perforation intervals to avoid any interference. This depends on the rock properties and the state of stress and needs to be simulated in advance. In geothermal, multi-stage shearing through the pre-existing fractures is the main mechanism and swellable packers are considered to be used instead of perforations.
Furthermore, suitable stimulation schemes may include cyclic stimulations with varying flow rates (so called soft/fatigue stimulation) to reduce the risk of unwanted seismic events with large magnitudes.
All scenarios have to be evaluated regarding the site-specific geological requirements to optimize the performance of the reservoir for geothermal heat extraction and simultaneously reduce the risk of unwanted seismic nuisance and environmental impact.