Evaluation of NaCl and MgCl2 heat exchange fluids in a deep binary geothermal system in a sedimentary halite formation

Moore, Kayla R.; Holländer, Hartmut M.

doi:10.1186/s40517-021-00190-3

Geothermal Energy

Science – Society – Technology

Table 5 A NaCl + MgCl₂-brine heat exchange fluid composition and mineral saturation indices (SI) during heating and cooling processes as determined using PHREEQC 3 and Pitzer.dat

From: Evaluation of NaCl and MgCl₂ heat exchange fluids in a deep binary geothermal system in a sedimentary halite formation

Mineral	Formula		Heating (injection)				Cooling (extraction)
Mineral	Formula		10 °C	40 °C	70 °C	120 °C	100 °C	70 °C	10 °C
Anhydrite	CaSO₄	SI	0.00	− 0.43	− 0.94	− 1.91	− 1.50	− 0.76	0.01
Bischofite	MgCl₂ * 6H₂0	SI	0.00	− 0.21	− 0.42	− 0.76	− 0.62	− 0.34	0.01
Brucite	Mg(OH)₂	SI	− 0.07	− 0.41	− 0.83	− 1.64	− 1.30	− 0.69	− 0.08
Carnallite	KMgCl₃:6H₂O	SI	− 0.69	− 1.44	− 2.06	− 2.85	− 2.56	− 1.86	− 0.68
Glauberite	Na₂Ca(SO₄)₂	SI	− 3.49	− 4.09	− 4.76	− 6.06	− 5.51	− 4.52	− 3.49
Goergeyite	K₂Ca₅(SO₄)6H₂O	SI	− 0.41	− 6.38	− 11.5	− 19.0	− 16.1	− 9.86	− 0.37
Gypsum	CaSO₄:2H₂O	SI	− 0.44	− 1.16	− 1.90	− 3.20	− 2.66	− 1.64	− 0.43
Halite	NaCl	SI	0.00*	0.00*	0.00*	0.00*	0.00*	0.00*	0.00*
Kieserite	MgSO₄: H₂O	SI	− 0.49	− 0.94	− 1.25	− 1.62	− 1.48	− 1.15	− 0.48
Polyhalite	K₂MgCa₂(SO₄)₄ * 2H₂O	SI	− 5.42	− 8.28	− 11.6	− 17.9	− 15.3	− 10.4	− 3.5
Sylvite	KCl	SI	− 1.84	− 2.23	− 2.53	− 2.92	− 2.77	− 2.43	− 1.43
pH			6.7	5.9	5.4	4.8	5.1	5.6	6.7
Ca	Ca²⁺	mol kg⁻¹	5.76 × 10^–3	5.76 × 10^–3	5.76 × 10^–3	5.76 × 10^–3	5.76 × 10^–3	5.76 × 10^–3	5.76 × 10^–3
Cl	Cl⁻	mol kg⁻¹	11.4	11.5	11.6	11.8	11.7	11.6	11.5
Fe	Fe³⁺	mol kg⁻¹	3.00 × 10^–7	3.00 × 10^–7	3.00 × 10^–7	3.00 × 10^–7	3.00 × 10^–7	3.00 × 10^–7	3.00 × 10^–7
K	K⁺	mol kg⁻¹	1.93 × 10^–3	1.93 × 10^–3	1.93 × 10^–3	1.93 × 10^–3	1.93 × 10^–3	1.93 × 10^–3	1.93 × 10^–3
Mg	Mg²⁺	mol kg⁻¹	5.61	5.61	5.61	5.61	5.61	5.61	5.61
Na	Na⁺	mol kg⁻¹	7.53 × 10^–2	1.20 × 10^–1	1.92 × 10^–1	3.81 × 10^–1	2.94 × 10^–1	1.64 × 10^–1	7.48 × 10^–2
SO₄	SO₄²⁻	mol kg⁻¹	6.95 × 10^–3	6.95 × 10^–3	6.95 × 10^–3	6.95 × 10^–3	6.95 × 10^–3	6.95 × 10^–3	6.95 × 10^–3
ΔHalite	Incremental	mol L⁻¹	0	4.47 × 10^–2	7.21 × 10^–2	1.89 × 10^–1	− 8.75 × 10^–2	− 1.30 × 10^–1	− 8.92 × 10^–2
ΔHalite	Total	mol L⁻¹	0	4.47 × 10^–2	1.17 × 10^–1	3.06 × 10^–1	2.19 × 10^–1	8.87 × 10^–2	− 5.00 × 10^–4

Italics numbers indicate saturation or supersaturation
*Indicates equilibrium phase

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