Orazzini, Simone (ENEL Italy) | Kasirin, Regillio Sarijo (Smith Bits) | Ferrari, Giampaolo (Smith Bits, A Schlumberger Company) | Bertini, Alessandro (Smith Bits, A Schlumberger Company) | Bizzocchi, Isabella (Schlumberger Italiana SPA) | Ford, Robert J. (Schlumberger) | Li, Qingxiu (Smith Bits, A Schlumberger Company) | Zhang, Ming (Smith)
Geothermal energy has been use for centuries to satisfy general heating requirements. The modern geothermal plant is powered by production wells drilled to a source rock to produce steam at the surface. Depending on the location and depth, source formation temperatures vary.
In Italy, the operator must penetrate very hard and abrasive sediment and metamorphic formations to access steam in the granite basement formation. Historically, this was accomplished with a tungsten carbide insert (TCI) roller cone bit (RC). Standard geothermal bits and components, including grease and elastomer seals, are adequate for temperatures up to 150°C (302°F). Beyond these temperatures, the bit's internal components and lubricating material can degrade causing bearing failure limiting on-bottom drilling hours. In the application, the bottom hole temperature is approximately 180°C (350°F) and in some instances it can exceed 280°C (536°F). The extreme heat reduces on-bottom drilling hours leading to multiple bit runs/trips that drive up development costs. The operator required new roller cone technology that would endure the downhole environment.
To solve this challenge, a series of tests were conducted with temperature resistant elastomers and grease compounds in a controlled laboratory environment. The experiments resulted in a new line of roller cone bits equipped with an innovative bearing system that includes new proprietary composite elastomer seals with Kevlar® fabric and a proprietary high temperature grease formula. These innovations increased seal life, lubricity and load capacity at elevated temperatures for HT/HP applications.
The new geothermal bit technology has been run in the Italian application with outstanding results. Compared to standard roller cone products, the high-temperature bits have greatly increased on-bottom drilling hours while reducing total bit consumption and costly tripping for bit change out. Since successful development of the geothermal project is tied to reducing drilling costs, the new bit technology has significantly improved project economics. The authors will discuss development of the high temperature seal and grease compounds for drilling the granite basement source rock. They will also outline changes to the TCI cutting structure, field application, dull grades and bit performance data.
The Larderello area of central Italy (Figure 1) is geologically active and known for its geothermal productivity.1 The first evidence of organized use of the geothermal resource dates back to the 3rd century BC when the Romans used its hot sulfur springs for bathing. In 1817 a group of entrepreneurs led by Francois de Larderel used steam heated cauldrons to extract boric acid (H3BO3) from volcanic mud. The Grand Duke of Tuscany (Leopold II) was a supporter of Larderel's technique and in 1827 built a town for the factory workers named Larderello in honor of Larderel's contribution to the area.2
In 1904 an experiment using steam emerging from surface vents was used to run a rudimentary generator that produced enough electricity to power five light bulbs. It was the first ever practical demonstration of geothermal power. In 1913 the region's first geothermal power plant went into operation and by 1944 five geothermal generating stations were up and running with a combined capacity of 127 MWe.
A recent LWD density log in an exploration well showed excessive abrasive metal loss on the density measurement stabilizer. Towards the end of the drilling run it was noticed that the bottom quadrant density correction (delta rho) was slowly moving from values normalized on zero to a more positive number of about 0.15 g/cm3. Measurements of the density stabilizer diameters performed after the logging run showed the diameter had been reduced by abrasion by approximately 0.2 inch along the entire length of the stabilizer. Therefore, the compensated density measurement was logically questioned.
A post-job calibration showed a significant difference from the pre-job calibration, as expected. What was unexpected was that the compensated density computed from the pre- and post-job calibrations compared favorably at the end of the well, but not at the beginning of the well. This implies that the density correction algorithms derived during characterization will compensate for metal loss but not for metal gain. Monte Carlo N-Particle (MCNP) modeling is used to review this finding and investigate a method to define the amount of metal loss that can be tolerated before compensated density measurement inaccuracies exceed specifications.
In order to compute an accurate photoelectric effect (PEF) and caliper that are derived from the individual short and long detector densities, the pre- and post-job calibrations need to be utilized for processing the data. A new methodology of blending the pre- and post-job calibrations as a function of metal loss was developed to accurately reprocess the density count rate data over the entire drilled interval. The final compensated density measurement from this reprocessing compared favorably to the original compensated density measurement (with only the pre drilling calibration in effect). This blending process resulted in valid single detector and compensated density data over the entire interval confirmed by independent measurements.