Internal Dissolution and Pore Structural Evolution of Oolitic Dolomite

doi:10.19657/j.geoscience.1000-8527.2019.06.04

Abstract

Abstract:

Secondary pores formed by dissolution under surface and burial conditions are very important sites for carbonate reservoirs. To explore the pore erosion mechanism and its controlling factors, the dissolution experiments of oolitic dolomite in 0.2% acetic acid environment were carried out with one oolitic column sample. The experimental temperature and pressure were of 40-160 ℃ and 10-50 MPa, respectively. Continuous sampling was conducted to measure the Ca²⁺ and Mg²⁺ contents in the solution. Pore images before and after the dissolution reactions were obtained by CT imaging. Fractal and multifractal methods were used to quantify the pore heterogeneity in two-dimensional (2D) and three-dimensional (3D) space. The results show that the [Ca²⁺+ Mg²⁺] concentration of oolitic dolomite was the lowest at the beginning of the dissolution experiment, and then increased steadily and reached the maximum at 120 ℃ and 40 MPa, and subsequently decreased slowly with the dissolution window from 70 ℃ to 120 ℃. The 2D/3D spatial distribution of micro-pore structure has multifractal characteristics. This implies that the dissolution reduced the irregularity and singularity of the pores. These results are of great theoretical and practical significance to understand the dissolution effect on the pore structures, and the pore dynamic evolution in carbonate reservoirs under near-surface or deep-burial conditions.

Key words: internal dissolution, multifractal, pore structure, heterogeneity

CLC Number:

TE122.2

XIE Shuyun, LEI Lei, JIAO Cunli, HE Zhiliang, BAO Zhengyu, MA Jiayi, ZHANG Dianwei, PENG Shoutao. Internal Dissolution and Pore Structural Evolution of Oolitic Dolomite[J]. Geoscience, 2019, 33(06): 1174-1187.

Figures/Tables 14

Fig.1 Characteristics of the oolitic dolomite sample under the optical microscope

Fig.2 Sketch diagram of the dissolution kinetics simulator

Table 1 Concentrations of Ca2+ and Mg2+ in the solution and the corresponding reaction conditions

序号	温度/ ℃	压力/ bar	深度/ m	Ca²⁺/ (10^-3 mol/L)	Mg²⁺/ (10^-3 mol/L)	(Ca²⁺+Mg²⁺)/ (10^-3 mol/L)
1	40	100	1 333	4.28	4.33	8.61
2	40	100	1 333	4.35	4.38	8.73
3	70	200	2 333	6.28	6.38	12.65
4	70	200	2 333	6.25	6.33	12.58
5	90	300	3 000	6.45	6.58	13.03
6	90	300	3 000	6.33	6.46	12.78
7	120	400	4 000	6.55	6.63	13.18
8	120	400	4 000	6.48	6.58	13.06
9	140	500	4 667	6.25	6.33	12.58
10	140	500	4 667	6.23	6.33	12.56
11	160	500	5 333	5.95	6.04	11.99
12	160	500	5 333	5.90	6.04	11.94

Fig.3 Variation of Ca2+ and Mg2+ contents in the solution and the corresponding Ca2+/Mg2+ values (the temperature and pressure changing simultaneously during the reaction process)

Fig.4 Typical grey-scale CT images with 2.5 cm sampling diameter

Fig.5 Scatter plot of fractal and multifractal analysis parameters versus porosity value by 2D-imaging (blue and red dots represent the relationships before and after the dissolution experiment, respectively)

Fig.6 Dendrogram of cluster analysis based on fractal and multifractal analysis parameters: (a) before and (b) after the dissolution experiment

Fig.7 Scatter plot of Δα versus Δf by 2D-imaging (blue and red dots represent the relationships before and after the dissolution experiment, respectively)

Fig.8 3D-visualization of pores before (a) and after (b) the dissolution experiment

Fig.9 Histograms of pore volumes (blue and red dots represent the relationships before and after the dissolution experiment, respectively, (b) is a zoom-in of (a))

Table 2 Equivalent sizes and classification cumulative frequency of the oolitic dolomite samples

条件		累计概率/%
条件		5	25	50	75	97.5	99.5	100
等效面积/ μm²	反应前	255.97	511.94	1 535.82	5 119.41	54 777.73	525 852.06	3.52×10⁸
等效面积/ μm²	反应后	255.97	255.97	511.94	1 791.80	19 453.78	73 609.78	5.76×10⁸

Table 3 Cumulative frequency of pore space equivalent area classification for the oolitic dolomite samples

样品 (NSP-4)	累计概率/%
样品 (NSP-4)	a级	b级	c级	d级	e级	f级	g级
反应前	0	0	0	22.23	52.00	22.96	2.577
反应后	0	0	0	39.00	50.91	9.27	1.009

Table 4 Fractal and multifractal parameters of 3D pore spaces

	渗透率/10^-3μm²	孔隙度/%	D_b	Δα	Δf	Δα_L	Δα_R	Δf_L	Δf_R	R
反应前	0.189 6	3.48	2.76	4.09	1.39	1.55	2.53	1.64	3.03	-0.24
反应后	35 207	6.67	2.98	2.35	2.12	0.62	1.73	0.56	2.68	-0.48

Fig.10 Fractal and multifractal spectrum sketch maps of the pore spaces before and after the dissolution experiment

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