Increase Geothermal Production with Deeper Casing Depths
Byline by: Sam Abraham, GLOBAL TECHNICAL ADVISOR – LCS – GEOTHERMAL
Summary: Ideal conditions for geothermal power can create some of the biggest cementing challenges for wells. Where traditional cementing methods fail, tailored cement formulations and processes can help run casing deeper and lower geothermal well costs.
Prospect exploration uncovered a rich geothermal resource at Blue Mountain in Northern Nevada. In 2010, under the Financial Institution Partnership Program (FIPP), the Department of Energy issued a $98.5 million partial loan guarantee to finance a geothermal power plant. Today, the plant taps into an underground geothermal reservoir to harness renewable energy.
Operators discovered ideal conditions for geothermal power production as they drilled wells, including high temperatures and high permeability. Ideal geothermal reservoirs often create non-ideal circumstances for casing and cement jobs. Operators must consider new geothermal well construction methods to unlock full production potential in these conditions, such as non-traditional cement formulations, placement techniques, and deeper casing depths.
At Blue Mountain, the wells’ geology contained fractures and cavernous zones that could cause partial to total lost circulation. These conditions require cement plugs while drilling to make the wells viable for geothermal production. To ensure well integrity, setting casing deeper in certain wells improves zonal isolation and lighter cement solutions can reduce hydrostatic pressure on the geological formation during the placement of cement.
Ideal Geothermal Conditions Create Cementing and Casing Challenges
Although conditions for geothermal production were ideal, the high temperature and permeability of the formation posed several major cementing challenges. The goal of every cement job is to help prevent fluid migration into the well with zonal isolation and filling the annulus with cement. Cement jobs must also protect the casing from mechanical shock due to thermal cycling and corrosion, so the well casings do not collapse.
High temperature and high permeability in wellbores can affect the rate at which Portland cement sets. To help prevent premature dehydration, operators often pump cold water into the well before pumping cement to lower temperatures. They may also slow the cement thickening process.
Another issue with high permeability is lost circulation when drilling fluid may seep into pores or fractures in the rock formation. Low-density cement formulated with additives helps address lost circulation due to reduced hydrostatic pressure on the wellbore. The tradeoff in a high temperature environment is a cement mixture that may not hold up as well under pressure.
Traditional Cementing Methods Fall Short, Increase Costs
Halliburton and other geothermal service providers often use high-strength microspheres (HSM) to create a lightweight cement design. This results in a low-density cement that can stand up to the high-pressures and high-temperatures in geothermal environments. The disadvantage of HSM is its high cost, and when used in high concentrations, it can require special processes to blend and mix the cement to ensure consistency.
Most of the Blue Mountain reservoirs have the potential to use lightweight lead cement with HSM, combined with a standard geothermal tail cement design. Some wells required large top jobs, which raised costs due to non-productive time (NPT). Due to costs, the use of HSM-enhanced cement was unsustainable, and reverse circulation methods to minimize casing losses only increased the risk of trapped fluid behind the casing and decreased the ability to predict where the cement was placed. Halliburton searched for alternatives to solve the Blue Mountain cement challenge and foamed cement was identified as a potential solution.
Foamed Cement Increases Geothermal Production Success Rate
The geothermal operators identified two wells ideal for foamed cement jobs and deeper intermediate casings. Deeper casing depths might isolate zones better and allow for more accurate wellbore tests. Meanwhile, foamed cement can counteract the problems Blue Mountain operators have experienced throughout previous cementing jobs.
Foamed cement is made of a combination of cement slurry, foaming agents, and a gas, often nitrogen. The mixture contains tiny, discrete bubbles that are stabilized to avoid coalescence and migration, unlike HSM. This creates a low-density cement with low permeability and high strength compared to other mixtures and is better suited for high-temperature and high-pressure geothermal formations.
Foamed cement has the potential to eliminate top jobs and control fluid migration behind casing. It can withstand higher wellbore pressures than traditional cement and offers a resilient cement sheath to help absorb the cyclic temperature and pressure loads that occur throughout the life of a geothermal well. Foamed cement provides insulation two to ten times higher than conventional cement, which can reduce the heat loss during production and helps provide protection for casing strings.
Blue Mountain Geothermal Success Runs Deep
At Blue Mountain, Halliburton pumped foamed cement and cemented 13 3/8-in. casing after the completion of lab tests and temperature simulations to determine the correct formula for the cement. One well had casing set shallower at 300 ft, while the other surface casing extended to 830 ft. The deeper casing outperformed its shallower counterpart. Lower back side pressures coupled with the casing depth likely prevented formation breakdown near the surface casing shoe. In contrast, the shallower casing needed a top job and showed evidence of formation breakdown near the casing shoe.
When operators analyzed costs, they found that foamed cement had a lower cost per barrel than an HSM lead cement design. The Blue Mountain project proved that foamed cement job costs were comparable to other specialized cement formulations.
The results also showed that foamed cement becomes the first choice for primary geothermal well cementing when conventional techniques fail and when zonal isolation is critical. Halliburton now recommends foamed cement for geothermal applications because it provides better insulation than traditional cement, and it can help enable successful geothermal well cementing in low fracture-gradient zones for the life of the well.
To learn more about Halliburton’s geothermal services and technology solutions, visit www.halliburton.com/geothermal.