An old dog with new tricks – applying deep hydrocarbon expertise to improve geothermal operations

An old dog with new tricks – applying deep hydrocarbon expertise to improve geothermal operations Welltec staff at work (source: company)
Alexander Richter 23 Nov 2020

With an increased interest by the oil and gas sector in geothermal energy operations, there are great opportunities to apply the experience from deep hydrocarbon operations to geothermal energy, as described in this article by Danish Welltec.

According to the International Energy Agency (IEA), geothermal electricity generation increased by an estimated three percent in 2019. The technology is still not on track to reach the IEA’s Sustainable Development Scenario (SDS) level, which would require a ten percent annual increase in generation between 2019-30. The IEA’s SDS outlines a major transformation of the global energy system, showing how the world can change course to deliver on the three main energy-related UN Sustainable Development Goals (SDGs) simultaneously. Policies tackling challenges associated with pre-development risks are needed to increase the deployment of geothermal resources for power generation.

Utilizing transferable technology to improve geothermal performance

Geothermal projects are typically fraught with difficulty, as the high pressure and high temperature nature of a well sets an incredibly high standard for achieving safe and reliable well integrity. With many similarities to the oil & gas industry, it is possible to draw on decades of experience to help manage these challenges in geothermal systems. Much like with oil & gas, geothermal wells require drilling in open hole, as well as a certain degree of cementing in order to achieve casing integrity.

The main challenge relating to geothermal wells is the temperature – which is often twice that of oil and gas wells – posing a serious challenge to the integrity of cement during setting, and therefore also the integrity of the well. Additionally, due to the approach of often targeting areas of the sub-surface with many formation faults, cement cannot be relied upon to provide full integrity and isolation. That means that a secondary element known as a packer is required to ensure that well integrity is maintained.

This is remarkably similar to an oil or gas well, although there is a difference in size – oil and gas wells are often constructed using a variety of borehole sizes, whereas geothermal systems rely on larger diameters to secure a reliable mass flow to drive the turbine efficiently.

From hydrocarbons to renewables

There are many similarities between the traditional oil & gas wellbore and those for geothermal energy. Oil wells can be extremely deep – to date, the deepest is at BP’s Tiber Field in the Gulf of Mexico at 10,668 meters vertical depth. When considering vertical and horizontal drilling (directional drilling) the Sakhalin well in Russia comes in at a lengthy 14,900 meters.

The depth of geothermal wells varies dramatically depending on their region. In the Philippines and Iceland, the wells tend to be fairly shallow, often around two kilometers, which is not very deep. When looking at places with no volcanic activity such as Finland – where the hot rocks are nowhere near the surface – drilling can be required as deep as six kilometers to reach sufficient heat.

Geothermal wells employ both vertical and directional drilling. The prime requirement for a vertical geothermal well is a hydrothermal system with sufficient water/steam and a hot subsurface.

In a country like Iceland, with its abundance of water and vigorous volcanic activity, drilling is done vertically. However, with a vertical well there is less contact with the rocks. In some of the recent geothermal activities in North America the heat from the rocks has been at the lower end of the required threshold, so a deviated well is needed to maintain contact – these wells have a lot in common with oil and gas operations. The optimal solution in such situations would be a horizontal well, but despite the obvious advantages and numerous efforts, horizontal wells have yet to become widely established in the geothermal setting.

What are the general challenges?

A divergence in similarity begins when considering the challenges in more detail. Having established the significance of temperature within the geothermal sector, this is also related to the challenge of thermal cycling. When a well starts producing hot water that is shut in the well, the temperature rises rapidly causing a cycling process. This can often cause casing to collapse, bulge, or crack, regardless of its condition, whether corroded and already damaged or brand new.

The second challenge is the isolation of any fault. This is a similar concern in oil & gas operations although the problem is reversed. In oil and gas there is a need to prevent water or gas entering the hydrocarbon flow, whereas for geothermal wells the requirement is to ensure that the fluid stays in the desired part of the formation.

To explore the full potential of geothermal energy, the use of improved systems at greater depths is required, as conventional geothermal systems are limited to a few geographical hotspots around the world. This has been contemplated for years in the form of Hot Rock or Enhanced Geothermal Systems (EGS). This is where completions experience from the oil & gas industry can play a pivotal role in the creation of horizontal systems with zonal isolation to fully take advantage of formation heat conductivity.

Advanced packer technology

Another facet of operation that all oil & gas operators are familiar with is regulation. Despite differences between regions, the global experience of the oil & gas industry can help prevent undesired migration of fluids or gases through the lithosphere in a geothermal context i.e. this is generally not permitted as a consequence of operations.

This is where packers come in. In the relining of a well, a packer is installed inside the casing and provides a base for cement to cover the previous casing that is damaged. These packers are specifically modified for geothermal application, first because it is vital that there be no trapped volume during packer relining, and second to mitigate for extreme thermal cycling over the life of a well.

The second application for the packers is that of cement assurance. This is when a packer is installed to isolate a loss zone or cold feed zone, again providing a base for the cement. This essentially creates a strata system at the point of the relevant zone to guarantee the integrity of the casing.

The best solution for such applications is an all-metal expandable packer. It is much stronger than a packer with durable rubber seals, which is widely used in the oil & gas industry. The all metal expendable packer is more robust and can be rotated to facilitate the installation process. The obvious advantage is that unlike rubber packers, a metal packer is ideal for geothermal application as it does not lose strength nor melt, as has often been the experienced in the geothermal industry.

Building on hydrocarbon experience – a case story

In the Philippines, a geothermal well operator had to shut in one of its most prolific producers due to the failure of the 9 5/8” casing and the loss of the 7” slotted production liner – the result of acid corrosion. The solution was a Welltec® Annular Barrier (WAB®) that provided the foundation for pressure support to the column of cement slurry placed above, preventing the curing cement from becoming contaminated. The 812WAB would also offer anchoring support to the casing string in the event of thermal contraction or expansion during production or shut-down operations.

For this operation, a standard 812WAB was adapted to incorporate non-elastomeric isolation valves and burst discs to facilitate bleed-off in the event of high-pressure steam build-up behind the casing. The 812WAB was run just above the high CRA 7” casing tailpipe and set inside the existing 9 5/8” casing. Following installation, second stage cementing was performed above the installed 812WAB.

During deployment, water flooded the well to lower the temperature, full production temperatures would be 285°Celsius. The adapted WAB supported the cement column during curing and prevented free water or acid from contaminating the cement or reaching the surface. This was the second time a WAB had been used to revive a geothermal well.

The operating company estimates that a new well would have cost upwards of $9mm, and this initial success has led to the purchase of an additional 1214 and 812WAB for deployment on upcoming wells to address the same issue.

Source: Company release