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 3 NaCl 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)			Max	Cooling (extraction)
Mineral	Formula		10 °C	40 °C	70 °C	120 °C	100 °C	70 °C	10 °C
Anhydrite	CaSO₄	SI	− 0.46	− 0.14	0.10	0.33	0.26	0.10	− 0.46
Bischofite	MgCl₂ * 6H₂0	SI	− 6.58	− 6.38	− 6.20	− 6.00	− 6.07	− 6.20	− 6.58
Brucite	Mg(OH)₂	SI	− 6.51	− 6.48	− 6.19	− 5.84	− 5.94	− 6.19	− 6.50
Carnallite	KMgCl₃:6H₂O	SI	− 7.41	− 7.63	− 7.74	− 7.80	− 7.79	− 7.74	− 7.41
Glauberite	Na₂Ca(SO₄)₂	SI	− 1.02	− 0.71	− 0.48	− 0.27	− 0.32	− 0.48	− 1.03
Goergeyite	K₂Ca₅(SO₄)6H₂O	SI	0.32	− 1.54	− 2.59	− 3.47	− 3.21	− 2.60	0.32
Gypsum	CaSO₄:2H₂O	SI	− 0.16	− 0.22	− 0.30	− 0.51	− 0.41	− 0.30	− 0.16
Halite	NaCl	SI	0.00*	0.00*	0.00*	0.00*	0.00*	0.00*	0.00*
Kieserite	MgSO₄: H₂O	SI	− 5.99	− 5.64	− 5.19	− 4.42	− 4.72	− 5.16	− 5.99
Polyhalite	K₂MgCa₂(SO₄)₄ * 2H₂O	SI	− 8.82	− 9.01	− 9.73	− 11.9	− 10.9	− 9.7	− 8.82
Sylvite	KCl	SI	− 1.98	− 2.24	− 2.42	− 2.63	− 2.56	− 2.42	− 1.98
pH			7.0	6.3	5.9	5.7	5.8	6.0	7.0
Ca	Ca²⁺	mol kg⁻¹	2.59 × 10^–2	2.59 × 10^–2	2.59 × 10^–2	2.59 × 10^–2	2.59 × 10^–2	2.59 × 10^–2	2.59 × 10^–2
Cl	Cl⁻	mol kg⁻¹	6.74	6.85	7.09	7.57	7.37	7.01	6.74
Fe	Fe³⁺	mol kg⁻¹	2.76 × 10^–6	2.76 × 10^–6	2.76 × 10^–6	2.76 × 10^–6	2.76 × 10^–6	2.76 × 10^–6	2.76 × 10^–6
K	K⁺	mol kg⁻¹	1.78 × 10^–2	1.78 × 10^–2	1.78 × 10^–2	1.78 × 10^–2	1.78 × 10^–2	1.78 × 10^–2	1.78 × 10^–2
Mg	Mg²⁺	mol kg⁻¹	5.02 × 10^–4	5.02 × 10^–4	5.02 × 10^–4	5.02 × 10^–4	5.02 × 10^–4	5.02 × 10^–4	5.02 × 10^–4
Na	Na⁺	mol kg⁻¹	5.47	5.58	5.81	6.29	6.10	5.82	5.47
S	SO₄²⁻	mol kg⁻¹	3.69 × 10^–2	3.69 × 10^–2	3.69 × 10^–2	3.69 × 10^–2	2.856 × 10^–2	3.69 × 10^–2	3.69 × 10^–2
ΔHalite	Incremental	mol kg⁻¹	0	1.11 × 10^–1	2.39 × 10^–1	4.79 × 10^–1	− 1.96 × 10^–1	− 2.82 × 10^–1	− 3.48 × 10^–1
ΔHalite	Total	mol kg⁻¹	0	1.11 × 10^–1	3.50 × 10^–1	8.29 × 10^–1	6.33 × 10^–1	3.51 × 10^–1	3.33 × 10^–3

Italics numbers indicate saturation or supersaturation. Solutions is considered at equilibrium with halite.

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