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Oct 31, 2017 - School of Petroleum Engineering, China University of Petroleum, Qingdao, Shandong 266580, People,s Republic of China. ‡. Petroleum En...
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Optimum Fluid Injection Rate in Carbonate Acidizing Based on Acid Dissolution Morphology Analysis Ning Qi,*,† Boyang Li,† Guobin Chen,† Mingjun Fang,‡ Xiaqing Li,§ and Chong Liang∥ †

School of Petroleum Engineering, China University of Petroleum, Qingdao, Shandong 266580, People’s Republic of China Petroleum Engineering Department, Panjin Vocational and Technical College, Panjin, Liaoning 124000, People’s Republic of China § Petroleum Engineering Technology Research Institute of Shengli Oilfield Company, Sinopec, Dongying, Shandong 257000, People’s Republic of China ∥ Langfang Branch of Research Institute of Petroleum Exploration and Development, Langfang, Hebei 065007, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Successful carbonate rock matrix acidizing should contribute to the formation of several main acid dissolution holes (wormholes), which provide favorable oil and gas seepage channels through the contaminated zone near the wellbore. The distribution and characteristics of wormholes play a critical role in deciding the acid penetration distance and acidification effect, while the development of wormholes is directly influenced by the acid fluid injection rate. In this study, carbonate rock cores are selected for displacement experiments, during which effects of different injection rates on wormhole development are compared. The breakthrough volume is a key parameter in determining the optimum acidizing fluid injection rate, while the pressure drop curve is used to analyze the development process of wormholes. Meanwhile, nuclear magnetic resonance imaging technology is employed to analyze the core end surface and internal structure after acidification to identify acid dissolution types and further determine the corresponding boundary of fluid injection rates for different acid dissolution types. The acid breakthrough volumes with different injection rates are calculated and compared when the hydrochloric acid concentration is 20%, which comes to the conclusion that the injection rate of 2 mL/min initiates the formation of wormholes and the injection rate of 3−4 mL/min results in the optimum acidification effect.

1. INTRODUCTION In the process of carbonate reservoir matrix acidizing, most acids tend to flow through large interconnected pores. The rapid reaction between hydrochloric acid and rocks leads to quick consumption of acid in the pores and corresponding pore diameter enlargement. Because more acid absorption takes place in larger pores, the larger pores will expand at a much faster rate than the smaller pores, which will result in several large acid dissolution holes in the reservoir, referred to as wormholes. Diameters of these wormholes are much larger than those of natural pores, and therefore, wormholes act as the main flow channel as a result of their high seepage capacities. Because wormholes are crucial in the acidification process, their effects should be well-considered in the simulation of carbonate reservoir acidification. The possibly least acid consumption is expected in the acidification by generating wormholes to improve the reservoir heterogeneity. It is gradually found in the detrital carbonate acidizing dissolution displacement process that, with the presence of wormholes, there is a certain relationship between the breakthrough volume and injection rate, namely, the existence of the optimal acid injection rate. Then, much attention has been paid to this phenomenon, and numerical modeling begins to be widely applied to quantitatively characterize the effects of wormholes. Among the achievements, the semi-empirical wormhole extension rule proposed by Buijse and Glasbergen and the capillary model elaborating the relationship between © XXXX American Chemical Society

the breakthrough volume and injection rate by Daccord are the most representative.1,2 With the continuous development of computer technology, finer mesh generation becomes possible. Zhang and co-workers put forward a new two-scale model, which is based on the two-scale two-dimensional displacement model by Panga et al. and polar coordinate transformation by Kalia and Balakotaiah.3−6 They have studied the conventional acid dissolution model and put forward the transition area model of the pollution zone. Maheshwari et al. extended the two-scale model to three-dimensional space,7,8 which made contributions to a fine wormhole description. Although large amounts of works have been performed on the wormhole development rule, they are mostly empirical or semi-empirical.2,4−11 The descriptions of wormholes are mainly through a series of dimensionless parameters, such as the Damköhler number and Péclet number.2,12 However, these theoretical studies cannot be directly applied to guide the acidizing process. Moreover, the carbonate mineral composition and structure, the acid rock reaction, and the complex and random acidification process commonly make the conventional method fail to well describe the wormhole development. Numerical modeling has been widely used to explore the relationship between the acid breakthrough volume and Received: September 8, 2017 Revised: October 30, 2017 Published: October 31, 2017 A

DOI: 10.1021/acs.energyfuels.7b02674 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

a SHB-III circulating-water-type multipurpose vacuum pump, manufactured by Zhengzhou Great Wall Industrial and Trade Co., Ltd. 3.2. Experimental Method. 3.2.1. Experimental Scheme. Experimental conditions are described in Table 1. Under the room-

injection rate. However, physical simulation models have not been reported to be introduced into this issue. Previous works simply classify the dissolution morphology into face dissolution, wormhole dissolution, and uniform dissolution but fail to clarify the boundary of acid injection rates. In this study, acidizing physical simulation is integrated with pressure drop curve analysis and core pore structure characterization to identify the optimum injection rate in the wormhole acidification process, which can guide the design and optimization of the acidification process.

Table 1. Experimental Conditions acid type hydrochloric acid

2. WORMHOLE FORMATION AND DEVELOPMENT MECHANISM 2.1. Wormhole Formation Mechanism. Carbonate rocks are featured by strong heterogeneity, and the rapid acid−rock reaction can lead to uneven dissolution in the formation. Moreover, the amounts of acids in different pores vary with the pore size and shapes. Because acids enter into the pore mainly by convection, zones with smaller resistance (such as large pores, natural fractures, and caves) have more acids in them, which promote the acid−rock reaction in these highpermeability zones, further enhance the permeability, and finally form the acid-dissolution-related wormholes.13 The rates of two processes in the acidification, H+ mass transfer and acid−rock surface reaction, commonly determine the acid dissolution pattern. When the acid−rock surface reaction rate is far higher than the H+ mass transfer rate, there is a little amount of H+ and the acid−rock reaction only takes place on the surface, leading to the face dissolution. When the acid−rock surface reaction rate is far lower than the H+ mass transfer rate, uniform dissolution takes place because there is plenty of H+ concentrating on the core inlet, which reacts with the core surface in the high-permeability zone and finally forms several branches. Only when the two rates are roughly equivalent can individual wormholes form.20 2.2. Factors Influencing Wormhole Development. There are many factors influencing the wormhole development, such as types of acid and rock, injection rate, surface/volume ratio, temperature, pressure, etc.21,22 However, the main factor is the acid injection rate for a fixed acid and rock type. It has been generally acknowledged that surface dissolution is formed under a low injection rate, uniform dissolution is formed under a high velocity injection, and a wormhole is formed at a medium injection rate.17−19 One of the purposes of this paper is to find the injection velocity boundaries between the three dissolution patterns.

acid concentration (%)

injection rate (mL min−1)

confining pressure (MPa)

temperature (°C)

20

2.0−5.0

5

25

temperature conditions, the core saturated with deionized water is placed in the core gripper displacement device. The 20% mass concentration hydrochloric acid is prepared with deionized water and restored in the middle container for later use. The advection pump operates with the injection rate of 2 mL/min for acid displacement. Real-time pressures at both the inlet and outlet are recorded by the software linked to the built-in pressure sensor. The experiment is terminated when the pressure difference on both ends is close to zero, and then data are exported. When the acid concentration is kept as a constant, the experiments are repeated in other cores with respective injection rates of 2.5, 3, 3.5, 4, 4.5, and 5 mL/min. 3.2.2. Evaluation Method. With the increasing injection rate, different dissolution morphologies dominate in different rate ranges, in the order of face dissolution, wormhole dissolution, and uniform dissolution. The injection rate limit can be roughly estimated through the observation of the core inlet end dissolution morphology. To accurately identify the limit, the acid breakthrough volume, pressure drop curve, and core pore structure characterization are combined comprehensively. 3.2.2.1. Acid Breakthrough Volume. The acid breakthrough volume is defined as the ratio of acid assumption when breakthrough takes place over the total pore volume.17 The acid injection rate corresponding to the smallest breakthrough volume is referred to as the optimum injection rate in this acid concentration.14 By calculation of each experimental breakthrough volume, the optimal injection rate can be gained when the acid concentration is 20%. 3.2.2.2. Pressure Drop Curve. The pressure drop curve can reflect the development process of the wormhole, and the accurate breakthrough time of wormholes can be described and obtained through the interpretation of pressure difference curves. 3.2.2.3. Core Pore Structure Characterization. NMR can be used to analyze the core internal structure.15,16 As shown in Supplementary Figure 1 of the Supporting Information, the brightness of the core area indicates the water content, and to be specific, high brightness means high water saturation. For the acidized core, the continuous water zone is the wormhole area. It should be noted that there are some highlighted areas surrounding the core, and they are the water in the container where cores are put. A total of 16 NMR images at different locations along the core axis are modeled, from which the threedimensional NMR figure for wormhole parameters can be obtained. A comparison of the figure before and after the experiment can help us better understand the core internal situation and more intuitively and fully explain the wormhole development rule.

3. EXPERIMENTAL SECTION 3.1. Materials and Equipment. Experimental materials include 36−38% hydrochloric acid (analytical grade, from Sinopharm Chemical Reagent Co., Ltd.), deionized water, and artificial core (from outcrops in Xinjiang, China, with a calcium carbonate content of 75%). Among them, the artificial core is cemented using an inorganic cementing agent and shows hydrophilic characteristics, with the diameter of 2.50 cm, the average length of 5.65 cm, and the permeability of about 10 × 10−3 μm2. The experimental apparatus includes a multi-functional acid flow and acidification evaluation system (core displacement device), manufactured by Chinese Hai’an County Petroleum Scientific Instruments Co., Ltd., nuclear magnetic resonance (NMR) equipment, manufactured by Suzhou Niumai Analytical Instruments Co., Ltd., a 101-2A electrothermal blowing drybox, manufactured by Wuhan Yahua Electric Furnace Co., Ltd., a BSA423S precision electronic balance, manufactured by Sartorius Scientific Instrument Co., Ltd., and

4. RESULTS AND DISCUSSION 4.1. Breakthrough Volume Analysis. The breakthrough volumes calculated according to eq 1 are recorded in Table 2. The relationship between the breakthrough volume and injection rate is as shown in Figure 1 tQ /60 Vwh = (M 2 − M1)/ρ (1) where Vwh is the wormhole breakthrough volume, t is the breakthrough time (s), Q is the injection rate (cm3/min), M1 is the core weight after drying (g), M2 is the weight of the core B

DOI: 10.1021/acs.energyfuels.7b02674 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 2. Breakthrough Volumes injection rate (mL min−1) breakthrough volume

2.0 7.630

2.5 7.767

3.0 6.795

3.5 6.309

4.0 7.156

4.5 7.156

5.0 4.458

Figure 1. Relationship between the acid breakthrough volume and injection rate.

saturated with distilled water (g), and ρ is the density of distilled water (1 g/mL). It can be seen from Figure 1 that the injection rate corresponding to the smallest breakthrough volume is 3−4 mL/min, which means that the optimal injection rate should be 3−4 mL/min when the acid concentration is 20%, which can achieve the least amount of acid consumption. 4.2. Pressure Drop Curve Analysis. The pressure difference between the core inlet and outlet vary with the time, which is shown in Figure 2. The pressure difference generally first gradually increases and then sharply decreases, which is consistent with the results by Tardy et al.23−25 We find that the increase of the pressure difference corresponds to the wormhole development, during which the collapse of acid dissolution channels and the migration of particles can clog pores, lowering the core permeability. Meanwhile, generated gas enters into the pore, leading to Jamin effects and hindering acid flow. These entire factors make the acid flow difficult, and thus, acid is concentrated on the surface, which increases the inlet pressure constantly. The sharp pressure difference drop indicates the pressure release, which means that the emergence of wormholes adjusts the pressures at the inlet and outlet, and thus, the pressure difference decreases from the maximum to zero. As shown in panels a and c of Figure 2, there are some fluctuations in the pressure difference curves, which indicate the incomplete pressure release. The generated wormholes are not linearly distributed, and they can break through to the core wall and then return to the core internal. Such explanation can be demonstrated by the NMR images of cores after acidification (Figure 3). Figure 3 shows one cross section of the threedimensional NMR figure, which is a composite of 16 different NMR images along the axial direction of the core (Supplementary Figure 2 of the Supporting Information), of the acidized core with the injection rate of 3 mL/min. The rectangle bright area is the core, and the brightness is positively related to the water content. The continuous water zones are the connected wormholes. We can observe that the wormhole development trajectory is not a straight line but a curve. Moreover, it turns to the core wall twice. The possible reason is that the artificial core is heterogeneous (same as carbonate rocks), and thus, the wormhole always tends to grow along the

Figure 2. Pressure drop curves under different injection rates.

Figure 3. One cross section of the three-dimensional NMR figure.

high-permeability zone. In this context, the curved development is quite normal. Another explanation is that the high confining pressure is inhibited as a result of the insufficient artificial core strength, which causes the breakthrough of the wormhole to the core wall. C

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Energy & Fuels 4.3. Core Side Analysis. Figure 4 shows the images of the core inlet side after displacement with injection rates of 2, 4, and 5 mL/min.

than 2 mL/min, there develops face dissolution, and when it is higher than 2 mL/min, there develops wormhole dissolution. 4.3.2. Acid Dissolution with the Injection Rate of 5 mL/ min. It is found that there are many densely distributed small dissolution holes around the wormhole in the core inlet side when the injection rate is 5 mL/min. However, if we cut the core from the middle, we can find that there is only one wormhole (Figure 7). NMR imaging also demonstrates that

Figure 4. Core inlet side after displacement.

According to previous analyses, we assume the wormhole caused by the injection rate of 4 mL/min as the optimal acid dissolution result, that caused by the injection rate of 2 mL/min is coarser, and that by 5 mL/min is featured by many densely distributed small holes. These situations will be discussed in the following parts. 4.3.1. Acid Dissolution with the Injection Rate of 2 mL/ min. As shown in Figure 4a, a large area of the core inlet side is dissolved and there develops a large wormhole with the injection rate of 2 mL/min. It is thought that, in the beginning of the experiment, acid keeps dissolving the inlet side of the core. With the collapse of the core in the dissolution process, the acid−rock reaction only takes place on the inlet side. However, as the acid keeps being injected, the pressure at the inlet side will enable the acid flow through pores and finally form the wormhole. Along with the axial development, the wormhole also expands radially as a result of the spread of H+ to the inner wormhole wall, which is demonstrated in Figures 5

Figure 7. Comparison of the core at the inlet and middle after acidification.

Figure 8. Cross section of the three-dimensional NMR figure with the injection rate of 5 mL/min.

(Figure 8). Therefore, we cannot attribute this to effects of uniform dissolution because these small holes are only distributed on the inlet side. This could be caused as a result of the high injection rate that results in abundant H+. However, these small holes fail to finally form the wormhole. Once a main channel emerges, acids will concentrate in the high-permeability channel, which evolves into the exclusive wormhole. Therefore, we can infer that, with the constantly increasing injection rate, the dissolution will completely transform from the wormhole into uniform dissolution, and we can regard these small holes as transitional products. Limited by the bearing capacity of experimental equipment, the injection rate cannot be increased further, and thus, we fail to know the injection rate limit when uniform dissolution dominates. In view of the limitation that the physical model experiment cannot be driven at a high injection rate, the method of solving the two-scale model is adopted to simulate the uniform dissolution.26 The injection rate is controlled by adjusting the Damköhler number (Da) that a parameter negatively correlated with the injection rate, and the result is shown in Figures 9 and 10. In this paper, Da varies from 1 to 50, corresponding to a change in the injection rate from high to low. It can be seen from Figures 9 and 10 that, with the increase of the Da value, the dissolution pattern changes from uniform dissolution to a wormhole. When Da = 1, from the entrance to the outlet of the core, the porosity increases slightly. The profile of the concentration field is vertical to the acid-advancing aspect.

Figure 5. Core inlet side NMR image after acidification.

Figure 6. Cross section of the three-dimensional NMR figure with the injection rate of 2 mL/min.

and 6. In Figure 5, there is a large bright area in the inlet side, indicating significant face dissolution. Figure 6 is a cross section of the three-dimensional NMR figure, and the coarse wormhole in the core can be clearly seen. When the injection rate is increased to 2.5 mL/min, a normal wormhole appears. Therefore, when the injection rate is lower D

DOI: 10.1021/acs.energyfuels.7b02674 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 9. Porosity distribution under different Da values.

Figure 10. Acid concentration distribution with varied Da.

Along the direction of injection, the gradient of concentration is small, which means that the core is dissolved uniformly and the concentration of acid is able to be kept high in the deep formation. In this case, the permeability of the formation cannot be improved effectively. Besides, the treatment range of acidizing includes the whole formation, leading to the low utilization of acid. Therefore, this condition should be avoided in acidizing.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 86-15898855079. Fax: 86-532-86981303. E-mail: [email protected].

5. CONCLUSION A wormhole will finally form under all injection speeds that the experiment designed. When the injection rate is low, face dissolution first takes place and a wormhole appears subsequently. However, when the injection rate is high, uniform dissolution at the inlet side will take place. The wormhole in the displacement test extends nonlinearly and turns to the core wall, which might be due to the restriction of the core size and confining pressure. When the acid concentration is 20%, the optimum injection rate is 3−4 mL/ min. In this condition, the acid breakthrough volume is the smallest as well as the acid consumption. When the acid concentration is 20%, the injection rate lower than 2 mL/min will lead to face dissolution, while that higher than 2 mL/min will result in wormhole dissolution.

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NMR image and brightness scale (Supplementary Figure 1) and 16 NMR images at different locations along the core axis (Supplementary Figure 2) (PDF)

ORCID

Ning Qi: 0000-0002-7602-8179 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key Technologies Research and Development Program of China during the 13th Five-Year Plan Period (Project 2017ZX005030005), the Shandong Provincial Natural Science Foundation, China (ZR2017MEE073), and the Fundamental Research Funds for the Central Universities (14CX05019A). Their sponsorship is gratefully acknowledged.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b02674. E

DOI: 10.1021/acs.energyfuels.7b02674 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.7b02674 Energy Fuels XXXX, XXX, XXX−XXX