Expansion of fracture network in granites via chemical stimulation: a laboratory study

doi:10.13745/j.esf.sf.2019.12.2

Abstract

Abstract:

Enhanced geothermal system (EGS) has been used to extract heat from deep hot dry rock with low permeability by creating an artificial geothermal reservoir. Hydraulic fracturing, along with chemical stimulation, was usually adopted to improve the permeability of a target EGS reservoir. In this study, we collected granodiorite samples from the Gonghe basin of Qinghai Province and subjected them to a systematic chemical stimulation test by use of three chemical agents (sodium hydroxide, hydrochloric acid and mud acid) at three different injection rates. The results show that the permeability of the rock samples increased upon injection of hydrochloric acid or mud acid, with the latter bringing larger increases; whereas the application of sodium hydroxide solution reduced the permeability. Among the three chemical agents, mud acid exhibited the strongest ability of dissolving feldspar minerals in granodiorite; in contrast, quartz was eroded most severely by sodium hydroxide solution. Nevertheless, during the chemical stimulation test using sodium hydroxide solution, we saw amorphous silica or aluminosilicates precipitation due to excess dissolution of the primary minerals on the fracture surface; and precipitation resulted in micro-fracture filling which definitely had a negative effect on improving permeability. Thus, in general, mud acid is the best chemical agent for the target hot dry rock in this study. At a moderate injection rate (3 mL·min^-1), mud acid can most effectively erode granodiorite samples and remarkably enhance sample permeability. Lowering injection rates, however, could cause secondary mineral precipitation and fracture filling therefore lowering sample permeability.

Key words: hot dry rock, enhanced geothermal system, chemical stimulation, mud acid, permeability

CLC Number:

GUO Qinghai, HE Tong, ZHUANG Yaqin, LUO Jin, ZHANG Canhai. Expansion of fracture network in granites via chemical stimulation: a laboratory study[J]. Earth Science Frontiers, 2020, 27(1): 159-169.

Figures/Tables 12

Fig.1 Photograph (Left) and schematic description (Right) of the core flow apparatus used in this study

Fig.2 Permeability change of rock samples upon blank solution injection at three injection rates

Fig.3 Permeability change of rock samples upon hydrochloric acid injection at three injection

Fig.4 Permeability change of rock samples upon blank solution injection at three injection rates

Fig.5 Permeability change of rock samples upon injection of sodium hydroxide solution at three injection rates

Table 1 Permeability change of rock samples in different stages of chemical stimulation test

实验条件			岩心裂隙渗透率/(10^-4μm²)
			注前置液		注中置液			注后置液
			k_1-avg		k_2-avg / k_2-max			k_3-avg
	1	2.60		3.85		/	4.65		4.37	1.68
10% HCl	3	7.61		11.09		/	14.53		9.51	1.25
	5	3.01		4.13		/	5.56		4.76	1.56
	1	0.48		1.76		/	3.31		3.41	7.15
10% HCl+0.5% HF	3	1.67		21.7		/	90.95		18.93	11.34
	5	2.98		16.61		/	27.47		21.16	7.11
	1	5.87		3.55		/	4.90		1.24	0.21
10% NaOH	3	6.18		1.74		/	2.82		0.30	0.05
	5	9.76		0.84		/	1.59		0.08	0.01

Fig.6 SEM images of the fracture surfaces of rock samples treated by various chemical agents, showing the overall erosion. (a) After hydrochloric acid injection; (b) After mud acid injection; and (c) After injection of sodium hydroxide solution.

Fig.7 XRD patterns of the minerals on the fracture surfaces of rock samples treated by various chemical agents

Fig.8 Erosion of quartz in the rock samples treated by various chemical agents according to the SEM-EDX analysis

Fig.9 Erosion of feldspar minerals in the rock samples treated by various chemical agents according to SEM-EDX analysis

Table2 PHs and major constituent concentrations of the liquid phase after chemical stimulation test

实验条件	注入流速 /(mL·min^-1)	pH	化学组分质量浓度/(mg·L^-1)
实验条件	注入流速 /(mL·min^-1)	pH	K	Na	Ca	Mg	Al	Fe	Si	Cl	F
	1	-0.42	220.3	384.3	334.4	265.4	265.4	603.9	57.8	106.5	n.d.
10% HCl	3	-0.43	179.5	358.7	291.3	194.1	182.3	404.9	47.4	107.2	n.d.
	5	-0.42	98.5	352.2	255.2	127.6	103.2	457.8	46.1	105.1	n.d.
	1	-0.37	195.7	614.8	434.6	122.3	396.1	277.1	373.7	103.5	5.1
10% HCl+0.5% HF	3	-0.37	277.1	665.4	464.1	172.5	414.7	308.0	770.0	102.8	4.9
	5	-0.37	266.4	654.2	393.2	156.3	445.8	303.5	438.6	103.2	5.0
	1	14.04	144.7	1 217.6	127.5	13.7	13.0	n.d.	47.4	n.d.	n.d.
10% NaOH	3	14.04	143.9	742.7	103.8	13.8	3.5	n.d.	31.1	n.d.	n.d.
	5	14.05	154.0	1 216.0	195.9	25.4	4.2	n.d.	52.9	n.d.	n.d.

Fig.10 Concentration variations of liquid phase constituents after chemical stimulation test

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