DOI 10.1007/s10553-017-0766-x Chemistry and Technology of Fuels and Oils, Vol. 52, No. 6, January, 2017 (Russian Original No.6, November– December, 2016)

HIGH-TEMPERATURE SLURRY SYSTEM FOR DRY HOT ROCK

Yang Hao

Dry hot rock (DHR) is a kind of rock without water or steam lying 2-6 km underground with a temperature of 150-650°C. Domestic DHR at 3.5-7.5 km depth had a temperature of 150-250°C with a total energy of 6.3 · 10 24 J, which was 1320 times the total energy consumption of China in 2010 assuming 2% of available exploration volume. Mixing of H 2S, H 2SO 3, etc. in the steam and hot water of a DHR well can easily corrode the pipe materials and the well-stabilization slurry. A high-temperature sulfur-corrosion-resistant slurry system containing HTR2000 (retarder) (density 1.86 g/cm 3, initial consistency 12 Bc, consistency time 252 min, water loss 23 mL in 30 min (API), bleeding rate 1.20%, 1-d strength 34.4 MPa, 28-d strength 25.6 MPa) that met all DHR well-stabilization requirements was developed. Keywords: dry hot rock, well stabilization, slurry. Dry hot rock (DHR) is a variety without water and steam lying 2-6 km underground with a temperature of 150-650°C. The most common types of DHR consist of biotite gneiss, granite, granodiorite, etc. Currently, DHR is used to produce electrical energy. High-pressure water is injected into a well at a depth of 2-6 km, penetrates into cracks and pores of the hot rock while being heated to their temperature, and then is extracted as superheated steam (~150-200°C) through another well located 200-600 m from the injection well. The steam is fed into heat exchangers in order to produce energy. A closed circulation system allows the cooled water to be recycled. ____________________________________________________________________________________________________ University of Geosciences, Beijing, China. E-mail: [email protected]. Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 6, pp. 68 – 70, November – December, 2016. 732

0009-3092/16/5206–0731 2017 Springer Science+Business Media New York

Geothermal sources in mainland China are found mainly in the Himalayas, which were formed during the Cenozoic Era at the junction of the Eurasian and Indian plates, although low-temperature sources are located in other regions of China. Geothermal sources are highly promising due to their distribution and the high heat capacity of the rock [1]. The total energy of DHR lying at 3-10 km is 25 · 1025 J, which is equivalent to 8.6 · 1014 tons of coal or 5200 times greater than the current energy demand. DHR lying at 3.5-7.5 km with a temperature of 150-250°C has even greater reserves. The total energy in these rocks is 6.3 · 10 6 J, which is 1320 times greater than the total energy demand in China in 2010. The potential of exploration in this direction is also tremendous. A high-quality slurry system is required to stabilize exploration wells due to their high temperatures. Several novel slurries including a high-density system, anti-channeling system, and flexible and special systems were proposed [2, 3]. Current slurry systems are stable to an average of 160°C with several of them reaching 200°C [4-7]. Steel pipes are corroded and the slurry itself is damaged by H 2S and H 2SO 3 that are usually present in the steam. In this instance, anti-sulfur cement is used in laboratory tests to choose the optimum slurry formulation and to study the well-wall stability. The slurry composition comprises the baseline component (Grade G high anti-sulfur cement), active additives, setting retardants, anti-water-loss agents, drag-reduction agents, anti-foam agents, and water itself. Active additives. Analytical results from the literature and strength tests showed that the addition of 25-40% silica fume (passing through a 200-mesh sieve) was recommended for improving the strength and avoiding its reduction by hot steam. It was found that the cement became very strong and impermeable after adding silica fume (passing through a 1000-mesh sieve). However, it became obvious after preliminary tests that flash-setting occurred after adding silica fume (passing through a >400-mesh sieve). Anti-setting agents are some of the most important components for ensuring successful stabilization because they increase the time before the cement starts setting at the high temperatures and pressures. Retarder materials are typically selected from calcium lignosulfonate, tartaric acid, boric acid, citric acid, phosphoric acid, bis-phosphonic acid tetrasodium salt, P30-H5 (used by Sinopec at high temperatures), HTR200 (foreign retarder), etc. The reagent was selected by comparing setting times (at 200°C constant temperature) and compression strength (curing at 260°C and 21 MPa). Testing established that the optimum reagent for the anti-setting agent was tartaric acid mixed with HTR200 (1:1). Anti-water-loss agents are used to decrease water loss from the slurry and to increase the cement strength. Silica fume can be used to fill effectively gaps between cement particles, thereby decreasing water loss. A decision was made to use 4% anti-water-loss agent. Bentonite can be used in addition to silica fume. Drag-reduction agents decrease hydraulic resistance, allow the water content to be reduced, and increase the slurry mobility during pumping, which makes slurry pumping more efficient. Tests compared sulfonated tannin and lignite solution and led to the selection of sulfonated tannin at 1-4 mass%. Water content. The pumpability of a solution is enhanced by adding large amounts of water. However, it also leads to high water loss and a cement aggregate of low strength. The pumpability is worse with lower amounts of water. A water:solids ratio of 0.4-0.45 was selected based on the results. Anti-foam agents are used to prevent foam formation while the slurry is mixed. They reduce the cement aggregate strength. The selected agent was n-caprylic alcohol (0.08 wt. %). Thus, the Grade G baseline slurry contained (wt. %):

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Consistency

Pressure, MPa

Temperature, °C

Time Fig. 1. Time dependences of temperature (1), pressure (2), and consistency (3) of slurry solution. Silica fume (200 mesh) ........................................... 50.00 Tartaric acid and HTR200 (1:1) ............................... 4.00 Sulfonated tannin ................................................... 4.00 Silica fume (anti-water-loss agent) ......................... 3.00 n-Caprylic alcohol ................................................... 0,08 Technical water ....................................................... 45.0 Figure 1 shows the test results for this composition. It can be seen that the initial consistency (12 Bc) was stable for 252 min. Test results for the cement after setting were: Days 1

Compression strength, MPa 34,4

3 7

35,5 30,8

14 21

27,3 26,6

28

25,6

Fluid losses at 220°C according to API requirements for this solution were 32 mL in 30 min; water losses, 3 mL of H2O from 250 mL of slurry (1.2%). Thus, the obtained composition satisfied all requirements for slurries.

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ACKNOWLEDGMENTS The research was supported by NSFC project “Study on the evolution mechanism of composite materials with high temperature-high pressure-low elasticity modulus,” No. 51474192; and Basic research funding of central universities, No. 2652015067. REFERENCES 1. 2.

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