Evaluation of Acid Fracturing Treatments in Shale Formation

Independent project analysis, inc. USA. Abstract: Hydraulic fracturing by stimulated reservoir volume (SRV) is a necessity to realize commercial devel...
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Evaluation of Acid Fracturing Treatments in Shale Formation Tiankui Guo,† Yanchao Li,‡ Yong Ding,§ Zhanqing Qu,† Naicheng Gai,† and Zhenhua Rui*,∥ †

College of Petroleum Engineering, China University of Petroleum, Huadong 266580, China CNPC Chuanqing Drilling Engineering Company Limited, Chengdu 610051, China § CNPC Changqing Oil and Gas Technology Research Institute, Xi’an 710018, China ∥ Independent Project Analysis, Inc., Ashburn, Virginia 20147, United States ‡

ABSTRACT: Hydraulic fracturing by stimulated reservoir volume (SRV) is a necessity to realize commercial development of shale gas, and its stimulation mechanism still needs further study, and the effectiveness of supplementary stimulation measures needs further exploration. The shale always contains some carbonate minerals. This paper tests the permeability of acid-etched fracture in shale to explore the influence of carbonate mineral content, acid fluid types and concentration, fracture plane roughness, proppant, and confining pressure on the acid-etched effects in shale, and uses CT scanning to conduct research on variation of microscopic pore-throat texture in shale before and after acid-etching. The test shows that the roughness of the fracture plane perpendicular to the bedding plane is higher than the roughness of that paralleled to the bedding plane, and the roughness in both fracture planes perpendicular to and paralleled to the bedding plane increases as the carbonate minerals content increases. In same group of shale samples, the permeability of self-propped fracture before and after acid-etching respectively is positively correlated with the fractal dimension of the fracture plane before and after acid-etching, and the variation of permeability of self-propped fracture before and after acid-etching is also positively correlated with the variation of fractal dimension of fracture plane before and after acid-etching, which is not shown in different groups of shale samples. When the content of carbonate minerals in shale is between 10% and 30%, the relation between optimum HCL concentration and carbonate mineral content is expressed as Y(OptimumHCLconcentration) = −0.5X(Carbonatemineralcontent) + 0.15. If the shale has a high carbonate mineral content (>30%), the effect of acid-etching is not easily controlled, so the technique of acid fracturing should be carried out cautiously. The permeability of single-layer proppant and self-propped fracture after acid-etching conforms to Walsh theory within certain pressure, and variation and migration of curve slope reflects unstable arrangement, imbedding, and crush of proppant, and nonreactive filled impurity of clay and quartz desquamated and migrated, which coincides well with constant variation of permeability. Applying proper acid fluids and optimum concentration in shale with varying carbonate contents will increase pore size, fracture width, and fracture number. For the shale with abundant calcite-cemented fractures, the optimum acid fluid concentration should be increased properly. Due to ultralow permeability of the matrix in shale, even high concentration acid fluid could not penetrate the core with barren natural fractures or calcium-filled fractures. The results of research provide valuable information for design of acid fracturing in shale play. higher conductivity with a more rough plane.11−14 Acid fracturing in the shale with carbonate will increase roughness and pore size in the fracture plane with carbonate minerals and not change the pore and texture with clay, quartz, and organic matter. The acid fluids dissolve the calcite and dolomite, and the residual clay, quartz, etc. act as proppant to prop the fractures under closure pressure. The experiment of acidizing in shale by Cash et al.15 showed that acidizing with 28% HCL produces a high initial conductivity, which reduces sharply in the latter period, and treatment with 15% HCL produces a constantly high conductivity. It is thought that a higher content of carbonatite in shale results in a larger dissolution space, and high conductivity is likely to generate. However, the relation between content of carbonatite and increment of conductivity and the optimum acid fluid formulation for shale with varying carbonate content were not illustrated in their research. The research by Wu et al.16 showed that acidizing corrodes

1. INTRODUCTION In shale with a tight matrix and ultralow permeability, development of shale gas and commercial production are hardly realized, though some natural fractures are developed, which improves local permeability to some extent.1−4 The effective measure for developing shale gas is hydraulic fracturing by SRV, which produces massive self-propped fractures and a small amount of proppant-filled fractures and constructs a fracture network,5,6 improving initial production and ultimate recovery factor of shale reservoir. At present, the stimulation mechanism of hydraulic fracturing by SRV in shale still needs further study, and the effectiveness of various supplementary stimulation measures needs further exploration.7 It is proved that acid fracturing could effectively improve production in carbonate reservoir.8−10 Generally, there are some carbonate minerals in shale; thus, stimulation in a shale reservoir with acid fracturing possibly further improves production of shale gas. The result of acid fracturing in a carbonate reservoir shows that the conductivity of fracture is dependent on the geometry and roughness of fracture plane, and the closed fractures have © XXXX American Chemical Society

Received: May 15, 2017 Revised: August 15, 2017 Published: September 1, 2017 A

DOI: 10.1021/acs.energyfuels.7b01398 Energy Fuels XXXX, XXX, XXX−XXX

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to bedding is referred to as the fracture parallel to bedding) and four pieces of samples perpendicular to bedding (similarly, the core sample perpendicular to bedding is referred to as the fracture perpendicular to bedding), respectively in Wufeng Fm, Xujiahe Fm, Longmaxi Fm, and Lujiaping Fm. The mineral compositions of the core samples are as follows: Wufeng Fm: clay content 14.5% and carbonate content 2%; Xujiahe Fm: clay content 40.6% and carbonate content 10%; Longmaxi Fm: clay content 31% and carbonate content 15%; Lujiaping Fm: clay content 22.5% and carbonate content 25%. The artificial fractures were created in the core samples by shear split (Figure 1). Because the shale bedding is very developed, the failure

the carbonate minerals in shale, but the quartz and organic matter still remain stable, which produces the channel, pores, and cavity, forming a roughened plane and improving fracture conductivity. However, acidizing will decreases hardness of fracture plane by 30%−70%, which makes the proppant embed into the rough closed fractures, reducing fracture conductivity. Mou et al.17,18 developed a 3D medium-scale model of acid fracturing and verified the simulation result with experiment of fracture conductivity in acid fracturing. It is thought that the conductivity of fracture is dependent on corrosion mode of fracture plane, which is also dependent on permeability and distribution of minerals, and the more rough fracture plane produces higher conductivity, which is due to deeper channel produced within the same acid residence time. Pournik et al.19 used 15% HCL to conduct the experiment of dipping the core sample in acids and infiltrating acids into fractures of varing width propped by proppant, which showed that the acid fluids obviously corrode the core sample and increase the pore network. Except for dipping the core sample in acids under pressure, other tests show that the conductivity of acidized fractures decreases, which is possibly due to a decrease of rock strength and proppant embedded into the fracture plane which were observed in the tests. Morsy et al.20−22 split the fractures perpendicular to and parallel to the bedding, and they tested the porosity before and after acidizing and recovery during spontaneous imbibition for fractures of varying direction. The results showed that acidizing increases the porosity by 1.3−3.5 times and the recovery by 2.15−13.25 times. The recovery in fractures perpendicular to bedding is about 9.5 times that in fractures parallel to bedding, and Young modulus of rock exposed in 1%−3% HCL decreases by 25%−82%. Acidizing the matrix in the shale reservoir will improve the conductivity of microfractures at a distance along the bore hole, and injecting a low-concentration of HCL into the formation near and far away from borehole will permanently improve the conductivity of hydraulic fractures, and application of low pH fracturing fluids and optimization of fracture direction will improve recovery of unconventional shale gas. Many scholars have put effort into research on the influence of fracture displacement, fracture roughness, rock mechanical properties, and closure pressure on conductivity of fractures without proppant,23−27 and the research on treatment of acidizing in shale is centralized in the influence of acids on shale microscopic texture and rock physical and mechanic parameters, less in optimization of acid types and formulation in acidizing in various shale reservoirs. There are a few reports about the influence of acids on different kinds of fractures. Thus, systematic research is needed to further identify the mechanism of stimulation in acid fracturing in a shale reservoir, aiming at improving effectiveness and economy of acid fracturing. Based on characteristics of acid fracturing in a shale reservoir, this paper conducts research in artificially fractured shale cores to explore the influence of carbonate mineral content, acid fluid types and concentration, fracture plane roughness, proppant, and confining pressure on permeability of acid-etching shearing fracture in shale and uses CT scanning to conduct research on the variation of microscopic pore-throat texture in shale before and after acid-etching, which provides reference for effective acid fracturing treatments in shale play.

Figure 1. Different types of fractures: (a) displaced fracture and (b) proppant-filled fractures. planes of the core samples sheared along the direction parallel to the bedding are almost bedding planes (or stylolites).28 Since the cementation of the bedding surface (or stylolite) is very weak and relatively flat, the applied shear force is very low, and the success rate of available core samples is close to 95%. When the core samples are sheared along the direction perpendicular to the bedding, as there is almost no natural fractures or stylolite in the direction perpendicular to the bedding for experimental shale cores, the failure planes are almost forced to crack by applying very high pressure, and because the artificial fracture surface is perpendicular to abundant bedding planes, core samples are broken very seriously. The success rate of core samples is less than 30%. After some experiments, we finally adopted a way that cores were wrapped in a transparent tape to improve the success rate. 2.2. Experimental Method. The liquid permeability tester was used for self-propped fracture (displaced) and proppant-filled fracture samples. Because their permeabilities were usually higher than 0.01 md, the steady-state measurement was more appropriate. The test fluid is kerosene, and the confining pressure is 15 MPa. For acid etching permeability, a core acidification flooding experimental apparatus was utilized with confining pressure of 15 MPa. Testing fluids are different acids. First, the core samples were scanned in laser profilometer, used for calculation of the roughness of fracture surface. Then, displaced fracture permeability testing before and after acidizing was carried out, respectively, and then the core samples after acidizing were scanned again in laser profilometer. Based on the results of the acidetched fracture permeability, the permeability of proppant-filled fractures before and after acid etching was carried out. Finally, a few nonshear core samples were selected to carry out CT scanning. 2.3. Characterization of Surface Roughness. The core samples were scanned in laser profilometer, and the surface appearance is drawn with Surfer software based on scanning data (Figure 2). The roughness of the fracture plane is characterized by surface fractal dimension,29 which is calculated with the cubic covering method.30 The modified cubic covering method31 was programmed in Matlab to calculate the surface fractal dimension of the fracture plane in 16 pieces of the core sample,32 which showed various morphologies of the fracture plane with a fractal dimension between 2.0351 and 2.1091. The fractal dimensions of fractures parallel to and perpendicular to bedding were averaged respectively to plot relation between fractal

2. EXPERIMENTAL SECTION 2.1. Fractured Cores. Eight pieces of core samples were cut, i.e., four pieces of samples parallel to the bedding (the core sample parallel B

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the permeability changes significantly, and when the injection time reached 75 min and the injection volume reached 150 mL, the permeability was basically stabilized. The permeability of self-propped fractures (k0) is between 13.08 and 59.10 mD, and the permeability of the acid-etched fracture (k) was between 7.86 and 58.60 mD. Core samples from Wufeng Fm have a carbonate content around 2%, which is lowest among the four groups of samples. Tests of acidizing were first conducted with 15% HCL, which showed that the permeability of no.1 fracture parallel to the bedding basically keeps invariable, and that of the no. 1 fracture perpendicular to the bedding increases slightly. Analysis showed that core samples from Wufeng Fm have excessively low carbonate content, which reacted weakly with HCL and had little influence on permeability. Then, 15% HCL was changed to mud acid of 10% HCL + 8% HF to continue the test, which showed that, as the interaction between rock and mud acid continued, the permeability of both samples gradually decreased to 20% of the original value, indicating excessive interaction between mud acid and the core sample, resulting in nonreactive filled impurity of clay and quartz desquamated from the fracture plane, which blocked the fracture and reduced the permeability. Core samples from Xujiahe Fm have a carbonate content around 10%. Tests of acidizing were first conducted with 15% HCL, which showed that the permeability of no.1 fracture parallel to the bedding gradually decreased to 40% of the original value and that of no. 1 fracture perpendicular to the bedding increased first and then decreased, with the ultimate permeability close to the original value. Analysis showed excessive interaction between 15% HCL and no. 1 fracture paralleled to bedding, resulting in nonreactive filled impurity of clay and quartz desquamated, which blocked fracture and reduced permeability. In the early stage, no. 1 fracture perpendicular to bedding reacted rapidly with 15% HCL, improving permeability, but excessive interaction also made nonreactive filled impurity of clay and quartz desquamated, which blocked fracture and reduced permeability. Then, 15% HCL was changed to 5% HCL to continue the test, which showed slower interaction between 5% HCL and no.2 fracture parallel to bedding and no.2 fracture perpendicular to bedding, which increased permeability slightly. Thus, due to an excessively low concentration, 5% HCL did not fully react with the core sample, which contributed little to improvement of permeability. Core samples from Longmaxi Fm have carbonate content around 15%. Tests of acidizing were first conducted with 15% HCL, which still showed that excessive interaction between acid and rock shortly increased permeability, but the permeability decreased rapidly again. The permeability of both samples gradually decreased to 25% of the original value. Then, 15% HCL was changed to 5% HCL to continue the test, which increased permeability slightly. Core samples from Lujiaping Fm have carbonate content around 25%. Test of acidizing was first conducted with 15% HCL, which still showed that the permeability of no. 1 fracture parallel to bedding gradually decreases to 20% of the original value. In the interaction between 15% HCL and no. 1 fracture perpendicular to bedding, the permeability fluctuated obviously. The maximum permeability was 1.44 times the original value, and it decreased gradually to 18% of the original value. Thus, there was still excessive interaction. The HCL concentration was changed to 5% HCL, and the test results showed that the permeability of both core samples fluctuated largely, and it was

Figure 2. Initial surface topography of shale fractures paralleled to bedding from (a) Wufeng Fm, (b) Xujiahe Fm, (c) Longmaxi Fm, and (d) Lujiaping Fm and shale fractures perpendicular to bedding (e) Wufeng Fm, (f) Xujiahe Fm, (g) Longmaxi Fm, and (h) Lujiaping Fm.

Figure 3. Relationship between fractal dimension and carbonate content in shale. dimension and carbonate content in Figure 3, which shows that, in each group of core samples (with similar mineral compositions), the roughness of the fracture plane perpendicular to bedding is higher than that of the fracture plane parallel to bedding, due to the different rupture ways between the directions perpendicular and parallel to bedding, there is a certain correlation between the roughness of the fracture plane and its orientation to the bedding plane. For both fractures, the roughness of the fracture plane basically increases as the carbonate content increases. Analysis showed that due to a larger size of grain crystal and smaller sphericity, when the content of carbonate minerals increases, the local concentrated cementation will increase the roughness of the fracture plane.

3. EXPERIMENTAL RESULTS AND ANALYSIS 3.1. Acid-Etched Fracture Permeability for Displaced Fractures. Volume fracturing in shale produces massive shearing displaced “self-propped fractures”, which largely contributes to production of shale gas. Fracture displacement (0.2 mm) was generated by placing a copper foil spacer on the opposite fracture plane (Figure 1), the permeability of selfpropped fractures before and after acid etching through the method of steady liquid permeability test under confining pressure of 15 MPa (Figure 4), and acid flow rate of 2 mL/min. It was shown that, in the early stage of the acid-rock reaction, C

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Figure 4. Acid-etched permeability variation curve of displaced fractures from (a) Wufeng Fm, (b) Xujiahe Fm, (c) Longmaxi Fm, and (d) Lujiaping Fm.

Figure 5. Relationship between displaced fracture permeability and fractal dimension before and after acidizing (APB, BPB, CPB, and DPB represent the fractures parallel to bedding from Wufeng Fm, Xujiahe Fm, Longmaxi Fm, and Lujiaping Fm, respectively; AVB, BVB, CVB, and DVB represent the fractures perpendicular to bedding from Wufeng Fm, Xujiahe Fm, Longmaxi Fm, and Lujiaping Fm, respectively).

correlated with the fractal dimension of the fracture plane, and the permeability of the acid-etched self-propped fracture is positively correlated with the fractal dimension of the acid-etched fracture plane. The higher fractal dimension of the acid-etched fracture plane corresponds to higher permeability. The variation of permeability of self-propped fracture before and after acid etching is positively correlated with the

stabilized at 90% of the original value, indicating that 5% is slightly higher than the optimum concentration of acid etching in Lujiaping Fm. The relation between permeability of self-propped fractures and fracture plane fractal dimension before and after acid etching in Figure 5 shows that, in the same group of shale samples, the permeability of self-propped fractures is positively D

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Energy & Fuels variation of fractal dimension of the fracture plane before and after acid etching. Increased roughness of the acid-etched fracture plane corresponds to an increment of permeability, and decreased roughness of the acid-etched fracture plane corresponds to a decrease of permeability, which is not shown in different groups of shale samples. Moreover, for the same group of core samples, there is no regular variation of permeability of acidized fractures parallel to and perpendicular to bedding. 3.2. Acid-Etched Fracture Permeability for ProppantFilled Fractures. The single layer of 40/70 mesh ceramsite was evenly laid out in self-propped fractures with a concentration of 0.25 kg/m2. The variation of permeability of fractures filled by a single layer proppant before and after acid etching under confining pressure of 15 MPa is shown in Figure 6. According to an acid flow rate of 2 mL/min and a fracture width of 0.3 mm, the injection rate of acid is 26 cm/min. When the injection time reaches 75 min, the permeability is stabilized. The permeability of fractures filled by a single-layer proppant is between 11.43 and 650.89 mD, and the permeability of acid-etched fractures is between 10.07 and 710.65 mD. In the test of acid etching no. 1 fracture paralleled to bedding with 20% HCL, it was found that the permeability increased slightly, and the mud acid should be chosen as an optimum formulation in combination with previous tests. Acid etching no. 2 fracture parallel to bedding with 15% HCL + 3% HF showed a weak interaction between acid and rock, which contributed little to the improvement of the permeability. Moreover, according to the median method, 12.5% HCL + 5.5% HF was chosen to conduct the test of acid etching no. 1 fracture perpendicular to bedding, which showed obvious improvement of permeability, and 11.25% HCL + 6.75% HF

was chosen to conduct the test of acid etching no. 2 fracture perpendicular to bedding, which showed the best results, with permeability increased to 1.31 times from 468.20 to 611.21 mD. Thus, a favorable result was achieved in acid etching Wufeng Fm rock with mud acid formulation of 11.25% HCL + 6.75% HF. The previous test showed that Xujiahe Fm shale hardly reacted with 5% HCL and excessively reacted with 15% HCL. Thus, according to the median method, 7.5% HCL, 10% HCL, and 12.5% HCL were respectively chosen to conduct the test of acid etching no. 1 and no. 2 fractures parallel to bedding and no. 1 fracture perpendicular to bedding, which showed that 12.5% HCL strongly reacted with the shale sample, where the permeability fluctuated largely and decreased ultimately. 10% HCL significantly improved the permeability of the shale sample, and 7.5% HCL improved the permeability to some extent, with the result less than that of 10% HCL. Moreover, according to the median method, 11.5% HCL was chosen to conduct the test of the acid etching shale sample, which showed 11.5% HCL possibly reduced permeability due to excessively high concentration. 10% HCL increased the permeability to 1.82 times from 41.84 to 76.32 mD. Thus, 10% HCL is the optimum formulation for acid etching the Xujiahe Fm shale sample. The previous test showed that Longmaxi Fm shale hardly reacted with 5% HCL and excessively reacted with 15% HCL. Similarly, 7.5% HCL, 10% HCL, and 12.5% HCL were chosen to conduct the test of acid etching no. 1 and no. 2 fractures parallel to bedding and no. 1 fracture perpendicular to bedding, which showed that 12.5% HCL obviously reduced the permeability, 10% HCL reduced the permeability less, and 7.5% HCL obviously improved the permeability. Moreover, according to

Figure 6. Acid-etched permeability variation curve of proppant-filled fractures from (a) Wufeng Fm, (b) Xujiahe Fm, (c) Longmaxi Fm, and (d) Lujiaping Fm. E

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Energy & Fuels the median method, 6.25% HCL was chosen to conduct the test of acid etching no. 2 fractures perpendicular to bedding, which showed 6.25% HCL improved the permeability less than 7.5% HCL did. 7.5% HCL increased the permeability to 1.54 times from 111.38 to 171.06 mD. Thus, 7.5% HCL is the optimum formulation for acid etching the Longmaxi Fm shale sample. In the test of shearing and acid etching the Lujiaping Fm shale sample, 15% HCL significantly reduced the permeability, and the permeability fluctuates largely in the test with 5% HCL and ultimately approximated the original value. Similarly, the same HCL formulations were chosen in the test of acid etching the same shale samples, which showed a significant reduction of permeability caused by 12.5% HCL, which was followed by 10% HCL and 7.5% HCL. Moreover, 2.5% HCL was chosen to conduct the test of acid etching no. 2 fractures perpendicular to bedding, which showed the permeability increased to 1.28 times from 505.16 to 647.32 mD. Thus, 2.5% HCL is the optimum formulation for acid etching the Lujiaping Fm shale sample. According to the relation between carbonate content in shale and the optimum acid concentration shown in Figure 7,

Figure 8. Observation of the fracture plane after acid etching: (a) proppants were wrapped and gathered by clay and (b) some proppants were broken.

roughness was focused on by Witherspoon and Tsang. The flow rate decreases as the shear displacement increases in the core with a large-size wavy surface if the shear displacement reaches 1/2 of the large wavelength. It is thought by Gangi35 that the dependence of fractures on the pressure difference could be reflected by a “2D comb function model”. In the paper, theWalsh model36 was used to interpret the relation between fracture permeability and pressure, and it was the simplest model describing the relation between permeability and confining pressure, as shown in Formula 1, where the roughness is described as the root-mean-square of roughness height h. ⎡ 1 − b(P − P0) ⎤ K /K 0 = [1 − ( 2 h/a0)ln(P /P0)]3 ⎢ ⎥ ⎣ 1 + b(P − P0) ⎦ (1)

where K0 is the initial fracture permeability, h is the roughness height, a0 is the initial fracture width, P0 is the initial pressure, and b is the variable of the ratio of contact area to fracture area as the pressure changes. b = 0 means that the roughness hardly changes as the pressure changes. Thus, eq 1 is simplified as eq 2: ⎛ K ⎞1/3 ⎛ 2h⎞ ⎛ P ⎞ ⎜ ⎟ =1−⎜ ⎟ln⎜ ⎟ ⎝ K0 ⎠ ⎝ a0 ⎠ ⎝ P0 ⎠

Figure 7. Relationship between optimal acid concentration and carbonate content in shale.

if the content of carbonate minerals in shale is between 10% and 30%, the relation between optimum HCL concentration and carbonate mineral content is expressed as Y(OptimumHCLconcentration) = −0.5X(Carbonatemineralcontent) + 0.15, and if the content of carbonate minerals in shale is more than 30%, acidizing has poor results. Observation of the core fracture plane after test of acid etching (Figure 8) showed that in some area proppant grains were wrapped and gathered by clay, which blocks the fracture channel and reduces permeability, and some proppants were broken, which results in insufficient support and contributes little to improvement of the permeability. 3.3. Pressure Dependence of Fracture Permeability. The characteristics of fracture permeability were described in previous research. Kranz et al.33 made research on the influence of effective stress and roughness on fracture permeability of intact and fractured granite core samples, and the result shows that the fractured granite is more sensitive to stress than the total rock of granite, and the slab model could not accurately describe the fracture plane. According to the model proposed by Tsang and Witherspoon,34 the displaced fractures relieve the nonlinearity of the stress-fracture closure. The importance of

(2) 1/3

Equation 2 shows that the relation between

( )

is expressed with a straight line with slope

2h . a0

K K0

()

and ln

P P0

If the fracture

width remains invariable, the variation of slope corresponds to the variation of the surface roughness, which occurs in contact with the salient point under pressure. 3.3.1. Displaced Fracture. After acid-etching the selfpropped fractures of Wufeng Fm core samples, both no. 1 fracture parallel to bedding and no. 1 fracture perpendicular to bedding showed linear variation, and the roughness of the fracture plane and permeability basically remained invariable, and no. 2 fracture parallel to bedding and no. 2 fracture perpendicular to bedding showed linear variation under confining pressure between 3 and 10 MPa (Figure 9). Analysis shows that, with a confining pressure less than 10 MPa, the surface salient point is not broken, and the roughness basically remains invariable, and with a confining pressure more than 10 MPa, the surface salient point is broken, and the roughness of the fracture plane is changed, corresponding to the variation of the permeability. After acid etching the self-propped fracture in the Xujiahe Fm shale sample, the slope 2 h in core with no. 1 a0

fracture parallel to bedding constantly changed, indicating F

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Figure 9. Relationship between displaced fracture and pressure: (a) Wufeng Fm, (b) Xujiahe Fm, (c) Longmaxi Fm, and (d) Lujiaping Fm.

Figure 10. Relationship between proppant-filled fracture and pressure: (a) Wufeng Fm, (b) Xujiahe Fm, (c) Longmaxi Fm, and (d) Lujiaping Fm.

of the fracture plane basically remained invariable, and no. 1 fracture perpendicular to bedding showed linear variation under a confining pressure between 3 and 10 MPa, indicating that,

constantly changed roughness and corresponding permeability. Both no. 2 fracture parallel to bedding and no. 2 fracture perpendicular to bedding showed linear variation, and the roughness G

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Energy & Fuels with a confining pressure less than 10 MPa, the surface salient point is not broken and, with a confining pressure more than 10 MPa, the surface salient point is broken, and the roughness of the fracture plane is changed, corresponding to the variation of the permeability. After acid etching the self-propped fracture, the Lujiaping Fm shale sample showed linear variation under a confining pressure between 3 and 10 MPa. Analysis shows that, with a confining pressure less than 10 MPa, the surface salient point is not broken, and the roughness basically remains invariable and, with a confining pressure more than 10 MPa, the surface salient point is broken, and the roughness of the fracture plane is changed, corresponding to the variation of the permeability. 3.3.2. Displaced Fracture and Proppant. After acid-etching fractures filled by a single layer of proppant in Wufeng Fm and Lujiaping Fm core sample, the slope 2 h changes as the con-

Figure 11. CT images of Wufeng Fm shale: (a) before acidizing and (b) after acidizing.

a0

fining pressure changes, indicating a constantly large change of fracture plane roughness (shape) and corresponding fracture permeability (Figure 10). For the Xujiahe Fm core sample, the fracture was propped by a single layer proppant, and no. 1 and no. 2 fractures perpendicular to bedding all showed a liner shape under confining pressure between 3 and 10 MPa. Analysis shows that proppants are embedded and some grains are broken under a high confining pressure, which changes the roughness of the fracture plane and corresponding permeability. After acid-etching fractures propped by a single layer of propping, the Longmaxi Fm core sample basically showed linear variation with a confining pressure between 5 and 20 MPa. Analysis showed that, with a confining pressure of more than 5 MPa, the proppants inside the fractures are stabilized and not migrated, and the roughness of the fracture plane remains invariable. After acid etching and with a confining pressure between 3 and 15 MPa, no. 1 fracture parallel to bedding showed linear variation, but no. 2 fracture parallel to bedding basically remained invariable. 3.4. CT Scanning Experiment. CT scanning and imaging of the shale core before and after acid displacement was used to conduct research on the variation of the microscopic texture of fractures and pore-throat of shale with varying carbonate content after being acidized by different acid fluids, which helps to understand the effect of acid fracturing shale with varying carbonate content. By using an image reconstruction method, X-ray projection drawing of the core sample was used to generate the gray image reflecting the true texture of the core, and the 3D digital image is stacked. 3.4.1. Wufeng Fm Shale. The Wufeng Fm core sample was acid etched with mud acid of 11.25% HCL + 6.75% HF, and observation of the appearance of the acid-etched core showed that the fine particles were generated on the top and bottom of the core sample, which is due to the desquamated nonreactive filling impurity particles of acid-etched clay and quartz. CT scanning images (Figure 11) showed that a fracture fully penetrated through the core sample, indicating further widened fracture after acid etching. 3.4.2. Longmaxi Fm Shale. The Longmaxi Fm core sample was acid etched with 7.5% HCL, and observation of the appearance of the acid-etched core showed that the fracture width was increased and some filling white substance reacted with HCL. The white substance was judged to be calcite cementation. CT scanning images before acid etching showed some pyrite in the core sample. Moreover, comparison of CT

Figure 12. CT images of Longmaxi Fm shale: (a) before acidizing and (b) after acidizing.

scanning images (Figure 12) showed that both fracture planes and conductivity were increased significantly after acid etching. 3.4.3. No. 1 Shale Core from Lujiaping Fm. The appearance of the core sample showed an unpenetrated white cemented fracture, where the acid fluids were impossibly displaced. Thus, a small sample with diameter of 2 mm and length of 15 mm was cut from core section for nanometer CT scanning, as shown in Figure 13a. It can be seen that little micronano pores were sparsely distributed; thus, there is no production capacity without fracturing. HCL with concentrations of 2.5%, 5%, 10%, 15%, 20%, and 25% and mud acid of 15% HCL + 8% HF were respectively used to acid etch the core sample. The inlet pressure was increased rapidly, but acid fluids were not infiltrated through the core sample. Some white cementation substance inside the fracture reacted with 2.5% HCL; thus, the substance is judged as calcite cementation. Analysis shows that, due to excessively low permeability of the matrix in shale, even a high concentration acid fluids could not penetrate through the core sample without opening natural fractures or soluble calcium filling fractures. 3.4.4. No. 2 Shale Core from Lujiaping Fm. Observation of the appearance of the core sample showed that a white cemented substance in the middle of the fracture penetrated the whole core sample. The CT scanning image before acid etching showed that calcite cementation of the fracture in the middle of the core sample penetrated the whole core sample, and it was filled by mineral composition carried by hydrothermal solution, and there were massive isolated holes, which were separated by a substance with a density higher than that of the matrix (Figure 14). 2.5% HCL was first used to acid etch the core sample, and the confining pressure was increased rapidly and exceeded the confining pressure of 5 MPa. H

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Figure 13. CT images of no. 1 shale core from Lujiaping Fm: (a) nanometer CT scanning of the small sample with diameter of 2 mm and length of 15 mm and (b and c) pore distribution of the sample with a diameter of 2.5 cm at different angles.

Figure 14. CT images of no. 2 shale core from Lujiaping Fm: (a) before acidizing and (b) after acidizing.

Figure 15. CT images of the carbonate sample: (a) before acidizing and (b) after acidizing.

The confining pressure was increased to 10, 15, and 20 MPa successively, and the inlet pressure was increased rapidly, indicating that 2.5% HCL did not penetrate through the core sample. Then, observation of the core sample showed that the white cementation substance in the inlet has totally reacted with HCL; thus, the white cementation substance was judged as calcite cementation. Then, 2.5% HCL was changed to 5% HCL and 10% HCL, which still could not penetrate the core sample. Next, 5% HCL and 10% HCL were changed to 15% HCL, and the acid fluid flowed from the outlet, indicating that 15% HCL penetrated through the core sample. CT scanning of the acid etched core sample showed an obvious fracture plane. The fracture width was increased, and fractures were favorably connected, and the density of the matrix near fracture plane was decreased slightly (gray level in CT section increases). 3.4.5. Carbonate Core. In order to further clarify the influence of carbonate content on acid etching, the test of acid etching and displacement was conducted in a carbonate core sample (Figure 15). The acid etching wormholes were generated after displacing acid fluids. The acid etching wormholes were developed in the area with larger pores in the core inlet and propagated along the favorably connected large pore-throat. Most of the acid fluids flow through wormholes, and little acid fluids pass through other parts of the core, indicating unstable and competitive development and propagation of wormholes, which has two sides. On the one hand, the acid fluids only dissolve some rock to form the wormholes, penetrating through the pollution zone and removing pollution with high efficiency. On the other hand, due to strong heterogeneity in carbonate rock, the acid fluids always flow through the high-permeability

zone, not the low permeability area in a thick-layer and multilayer reservoir, which goes against even acid distribution.37,38

4. CONCLUSIONS AND SUGGESTIONS (1) The fracture plane showed various morphologies, and the roughness of the fracture plane is characterized by the fractal dimension, which is between 2.0351 and 2.1091. In core samples with similar mineral compositions, the roughness of the fracture plane perpendicular to bedding plane is higher than the roughness of that parallel to the latter, and the roughness in both fracture planes perpendicular to and parallel to the bedding plane increases as the carbonate mineral content increases. (2) The permeability of self-propped fractures is between 13.08 and 59.10 mD, and the permeability of the acid-etched fracture is between 7.86 and 58.60 mD. In same group of shale samples, the permeability of self-propped fractures is positively correlated with the fractal dimension of the fracture plane, and there is no significant difference between the fracture plane perpendicular to bedding plane and that parallel to the latter; the fractal dimension of acid-etched fracture planes is between 2.0356 and 2.0854. In same group of core samples, the permeability of the acid-etched self-propped fracture is positively correlated with the fractal dimension of the acid-etched fracture plane; the variation of the permeability of the self-propped fracture before and after acid etching is positively correlated with the variation of the fractal dimension of the fracture plane before and after acid etching. (3) The permeability of fractures filled by single-layer proppant is between 11.43 and 650.89 mD, and the permeability of acid-etched fractures is between 10.07 and 710.65 mD; if the I

DOI: 10.1021/acs.energyfuels.7b01398 Energy Fuels XXXX, XXX, XXX−XXX

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content of carbonate minerals in shale is between 10% and 30%, the relation between optimum HCL concentration and carbonate mineral content is expressed as Y(OptimumHCLconcentration) = −0.5X(Carbonatemineralcontent) + 0.15

The permeability of acid-etched fractures is about 1.5 times that of the untreated. This effect is not too ideal. If the shale with high carbonate mineral content (>30%), the effect of acidetching is not easily controlled, so the technique of acid fracturing should be carried out cautiously. The permeability of acid-etched fracture in all shale samples is stabilized within 75 min with an acid flow rate of 26 cm/min. (4) The permeability of single-layer proppant and selfpropped fracture after acid-etching conforms to Walsh theory within certain pressures, and variation and migration of the curve slope reflects an unstable arrangement, imbedding, and crush of proppant, and nonreactive filled impurity of clay and quartz desquamated and migrated, which coincides well with constant variation of the permeability. (5) According to the different content of carbonate in shale, the appropriate acid type and acid concentration can increase the pore size, the crack width, and the number of fractures. If there are abundant natural fractures filled with calcareous in shale, the effect of acid fracturing treatments is very good and the optimal acid concentration should be increased. The permeability of shale matrix is extremely low, in the absence of opening natural fracture or fracture filling with soluble calcareous, and even high concentrations of acid can not drive through the core.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tiankui Guo: 0000-0003-1942-9627 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 51404288), the Fundamental Research Funds for the Central Universities (Grant No. 17CX02077), and the Applied Basic Research Project for Qingdao (Grant No. 17-1-1-20-jch).

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DOI: 10.1021/acs.energyfuels.7b01398 Energy Fuels XXXX, XXX, XXX−XXX