Some hot and permeable liquid-dominated geothermal wells do not naturally self-discharge their reservoir fluid after they are drilled or shut in for some time. Non-self-discharge wells are common in conventional liquid-dominated geothermal fields.
Here are several reasons why some geothermal wells are unable to flow:
- Low reservoir pressure.
- Formation damage caused by drilling.
- Build up of water column in the well after a long shut in period.
- The long water column in the well after drilling or workover.
- Poor reservoir permeability.
- High-elevation terrain.
- Small production casing.
After a geothermal well is drilled and completed, as a standard practice, it will be tested to determine its potential.
Before it is tested, engineers will assess whether the well will flow. For new wells, the two common analysis methods used are the Af/Ac ratio method and the water column length method.
A well is considered a non-self-discharge when the ratio of Af/Ac is less than 0.7, while a well with an Af/Ac ratio of more than 0.85 has an excellent chance to self-discharge.
Based on a study of geothermal wells with more than 200 degrees C in Indonesia, a well is likely capable to self-discharge when the length of the water column in the well is less than 600 meters, whereas if it is more than 600 m, the well may not self-discharge. The water column length is the vertical distance between the water level in the well and the depth of the feed zone.
How do you stimulate a non-self-discharge geothermal well to make it flow?
Once it is established or predicted that the newly drilled geothermal well will not or is unlikely to self-discharge, engineers will apply a stimulation technique to make the well flow.
When the well reservoir pressure is not sufficient to lift the column of water that has accumulated in the well, a well discharge stimulation technique will be applied to jump-start the well. There are five stimulation techniques to discharge the water. They are:
- Air compression
- Well-to-well stimulation
- Nitrogen injection
- Air injection
- Injection of steam from a portable boiler.
If a well is unable to flow due to formation damage or low permeability, other stimulation techniques such as fracturing or acidizing may be considered.
Air Compression Stimulation Technique
Air compression stimulation is the simplest and cheapest method with a proven high success rate compared with other methods. This method does not need complicated facilities, mobilization, or installation.
One disadvantage of this method is the well casing may crack due to sudden thermal shock from the flow of hot fluids during well discharge. It should be considered with caution for wells with temperatures above 300 °C.
In this technique, an air compressor is connected to one of the wellhead side valves to inject pressurized air into the well to depress the water level and push all the cold water from the wellbore into the hot formation. This will make the hydrostatic pressure of the water column lower than the reservoir pressure. After pushing the water level below the casing shoe and allowing enough time for the cold water to heat up by the hot reservoir rock, the wellhead valve is opened quickly to create a sudden effect of buoyancy.
Here is an example of a successful application of the air compression method to stimulate a geothermal well.
Well A Case
The company wanted to test Well A. The Af/Ac ratio method was used to predict whether Well A would self-discharge. The initial Af/Ac ratio of Well A is zero and thus it was categorized as a non-self-discharge well. To stimulate the well, the air compression method was chosen.
To stimulate the well, engineers injected air into the well at a pressure of 50 Bars using a double booster compressor for 24 hours. The compressed air pushed down the water level inside the well by 500 m resulting in a final Af/Ac ratio of 2.371.
The application was successful as the well flowed by itself when the well was reopened.
This article is adapted based on the following sources:
- Post by Dr. Mohamad Husni Mubarok on LinkedIn – Determining the minimum air compression pressure to stimulate the non-self-discharge geothermal well.
- “Stimulation program of air compression and nitrogen injection in geothermal well” by E. Budirianto, W.A. Nugroho, B.N. Jayanto, A.H. Lukmana, D.R. Ratnaningsih
- Article by Mohamad Husni Mubarok and Sadiq J. Zarrouk – Discharge Stimulation of Geothermal Wells: Overview and Analysis.