Energy losses can be significantly reduced if thermally insulating cement is used for energy storage and recovery. The thermal conductivity (TC) of the currently used cement is between 1 and 1.2 W/mK. This paper reports development and evaluation of three different highly hydrophobic thermally insulating cement systems resistant to thermal shock (TS) conditions common in geothermal wells. To decrease cement TC lightweight formulations were prepared with pozzolan-based thermally insulating hollow aggregates. To prevent alkali degradation reactions (AR) in alkali cement slurries at high geothermal temperatures and loss of the insulating gasses from the insulating aggregates, three different strategies were used. In the first one shells of fly ash cenospheres (FCS) were treated with superhydrophobic polymethylhydrosiloxane (PMHS). During the treatment the dehydrogenation reactions of hydroxylated Al and Si groups on surfaces of FCS with ≡Si-H group within PMHS, M (Al and Si)-OH + H-Si≡ → M-O-Si≡ + H2 ↑ took place making the FCS surfaces hydrophobic. The highly hydrophobic calcium aluminate cement (CAC) containing PMHS-ceramic microsphere displayed the four major characteristics compared with PMHS-free cement; 1) great water repellency, 2) improved compressive toughness, 3) thermal conductivity (TC) 0.4-0.5 W/mK under water saturated conditions, and 4) excellent resistance to TS. In the second strategy, a low pH calcium-phosphate (CAP) cement was used to minimize alkaline degradation of insulating particles. The TC in these formulations was further decreased to 0.2-0.3 W/mK by using hydrophobic silica aerogel (SAG) possessing very low 0.01-0.02 W/mK TC. SAG surface was treated with hexamethyldisilazane (HMDS) to make aerogel surface hydrophobic. During the treatment of silica aerogel the de-ammonia reactions of amine (=NH) group of HMDS with silanol Si-OH group of aerogel took place; 2≡SiOH + (CH3)3≡Si–NH–Si≡(CH3)3 (HMDS) →2≡Si–O–Si≡(CH3)3 + NH3↑. The optimized material had the following properties after 3 cycles TS tests (one cycle for 100o and 250oC-autoclaved samples was 175o and 250oC-24hr-heating → 25oC water): 1) TC of 0.35 and 0.28 W/mK for 100o and 250oC cured cements, respectively; 2) compressive strength greater than 800 psi; and 3) sheath shear bond strength to casing greater than 40 psi. Finally, OPC-based lightweight hydrophobic cement was developed by combing OPC with FCS and using styrene butadiene latex rubber to render it hydrophobic. The TC below 0.23-0.34 W/mK in combination with the compressive strength of 1500 psi and good TS resistance was achieved for these formulations.

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