Geothermal Energy has been listed as one of the cleanest energy forms in the world. Extracted directly through a well or hot spring, geothermal energy has been identified as a renewable and sustainable energy form. While its application is in numerous fields such as space heating, raising plants in greenhouses, drying crops and others, Geothermal Energy is more appropriate for industrial purposes. Though its production is minuscule in comparison to oil and gas, total geothermal energy power generation capacity towards the end of 2018 was 14,600 MW. A lot of contributions to the global power generation capacity came from Turkey, which has added over 1300 MW in the last 10 years.
Geothermal drilling has benefited most from the ongoing industry developments. Air/Foam Balanced Drilling, Horizontal Drilling, Reverse Circulation Cementing, Logging While Drilling, tool modifications made for high- temperature drilling applications and advanced downhole tools like thrusters are some of the noteworthy examples of these improvements.
In this article, let us talk about the technological advancements confronting different challenges in the production of geothermal energy and some of the major cost drivers involved with geothermal drilling.
Cougar Mechanical Thruster™: For years, operators in Turkey were facing challenges in the form of continuous abrasion on the drill string and great shock-vibration downhole. Cougar’s Mechanical Thruster™ was identified as a unique solution to this problem. The Mechanical Thruster™ improved drilling efficiency by counterbalancing for the irregular motion of the drill string. With more consistent weight-on-bit (WOB), stall events were reduced, thereby enhancing motor and bit life. Thruster absorbs axial vibrations, therefore, reduces overall vibration, which in turn results in Bottom Hole Assembly (BHA) being exposed to less lateral vibration and whirl as well.
Drilling with Casing (DWC): Another important innovation in this field is the Drilling with Casing (DWC) technology. There are numerous advantages to this technology, including reduced costs, time, and problems related to tripping drill string, decrease in lost circulation problems, improvement in safety and others. Especially in the case of lost circulation, DWC technology has proved to be very effective. DWC systems can carry on drilling when lost circulation is discovered. The process results in rock cuttings getting washed away into the fractures or permeable zones, effectively acting as lost circulation material. The fluid flow rates will also be lowered due to a relatively narrow annulus.
Expandable Tubulars: With the innovative Expandable Tubulars, an operator can send a string of casing with normal clearances and then increase the diameter of the inner string so that the clearance between the two strings is minor. The expandable tubulars require way less clearance between the successive casing strings. This allows the usage of smaller upper casings for a given production zone diameter that could be possible with the conventional casing.
Major Cost Drivers
Geothermal formations are much harder and abrasive than oil and gas formations, which results in a tougher drilling environment. Typical challenges faced by operators in geothermal drilling are increased wear on the drill string and high downhole vibration, up to 120 G’s. These challenges pose drilling problems such as premature downhole motor/rotary steerable system (RSS)/MWD and bit failures, which then further leads to non-productive time and higher drilling costs. Challenges also arise from the presence of dissolved or free carbon dioxide (CO2) and hydrogen sulfide (H2S) gases, which results in corrosion. Particularly, H2S restricts the type of materials that can be used for drilling equipment and for the casing to the lower strength steels. This comes from the failure of higher strength steels due to sulfide stress cracking.
Geothermal drilling alone accounts for 30%-50% of the cost involved in a geothermal electricity project and more than half of the total cost of Enhanced Geothermal Systems (EGS).
Geothermal drilling tends to be costlier than the onshore oil and gas drilling (in terms of cost per depth) for three major reasons:
- Lost Circulation and Reservoir Damage: This is perhaps the most common problem faced by operators during drilling. Lost circulation is sometimes massive, wherein the loss of returns is in figures of hundreds of barrels per hour. Time and materials for handling lost circulation can account for 15% of the well cost. The under-pressured formation worsens differential sticking, delivering a major impact on drilling cost. Geothermal wells are often abandoned due to the inability to drill through a loss zone.
- Technical Challenges: A lot of capital is invested in acquiring tools and deploying techniques to overcome the harsh downhole conditions described above.
- Large Diameters: Since the produced fluid (hot water or steam) is of low value, flow rates have to be high which means large holes and casings are required. In particular cases, more casing strings will also become a necessity to attain a given depth in a geothermal well than in an oil well to the same depth.
It has to be mentioned here that geothermal wells and oil and gas wells have a lot of differences (even in the same field), which renders the learning curve from experience less helpful.
Now that we know the challenges arising in the production of geothermal energy, let us understand in detail what new technological advancements have benefited the overall process!
Geothermal Exploration: What’s on the cards
With the degree of research and development going on in the field of geothermal exploration and production, it wouldn’t be an exaggeration to say that the future is eventful for this renewable energy form. Especially, the geothermal exploration currently underway in Iceland has shown promising results. In January 2017, the world’s deepest geothermal well was drilled at a depth of 4,659 metres under the Iceland Deep Drilling Project (IDDP-2). As was in the case of the IDDP-1, the bottom of the well reached fluids at supercritical conditions. The project operators are still probing the potential utilization of the well, results from the investigation are expected in early 2019. If IDDP-2 results prove to be fruitful, similar power output will be available from drilling fewer wells.
Nonetheless, with advances like Cougar Mechanical ThrusterTM, geothermal energy production will be easier and more efficient than before!
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