Xiaoi Li ,Jiano Wang ,Wei Zhou ,Jiantong Liu ,Li Qu

a Petroleum Institute,China University of Petroleum-Beijing at Karamay,Karamay 834000,China

b Research Institute of Experiment and Detection of Xinjiang Oilfield Company,Karamay 834000,China

c Unconventional oil and Gas Technology Research Institute,China University of Petroleum,Beijing 102249,China

d Kela Oil and Gas Development Department of PetroChina Tarim Oilfield Company,Korla 841000,China

Keywords:Shale Mineral composition Frictional slip Friction coefficient Roughness

ABSTRACT Natural fractures are widely distributed in shale reservoirs.During the hydraulic fracturing process,frictional slip occurring on natural fractures can increase the reservoir permeability and is of great significance to improve the efficiency of reservoir stimulation.Shale contains a large amount of clay and organic matter,and its frictional behavior is different from that of other previously studied lithologies.In this paper,the frictional behavior of shale is analyzed,and the results show that the frictional behavior is controlled by the content of clay and organic matter.As the content of clay and organic matter increases,the micro support type transforms from the particle support mode by hard quartz mineral to matrix support mode by plastic clay and organic matter.Accordingly,the shear strength and friction coefficient of shale both decrease,and the shear type transforms from brittle to plastic.When the content of clay and organic matter is low,the asperity of friction surfaces will break in a brittle manner and the wear degree of surfaces is low.Therefore,fractures are still featured by moderate apertures after friction.The lower the content of clay and organic matter is,the easier the asperity of crack surfaces supports themselves,and the higher the fracture residual permeability is.Thus,promoting shear slip is the main measure of reservoir stimulation.However,when the content of clay and organic matter is high,the remaining post-slip fracture aperture is small.It is difficult to increase reservoir permeability through the frictional slip of natural fractures,and in this situation,the proppant support efficiency needs to be improved.

1.Introduction

Substantial evidence indicates that activating the wide-spread natural fractures in shale reservoirs is an important measure for increasing the stimulation volume (East L.et al.,2011;King,2010;Meyer and Bazan,2011;Rafiee et al.,2012;Soliman et al.,2010).In the process of hydraulic fracturing,the injected fluid breaks the original in-situ stress balance,resulting in the opening or shear slip of natural fractures(Azad M.et al.,2017).In addition,micro-seismic data collected from fields and numerical simulation results show that activating natural fractures may account for a significant portion of stimulated volume and thus is the main controlling factor for increasing stimulation volume (Zhang X.et al.,2007;Zhang X.et al.,2010;Warpinski N.R.et al.,2009;Dahi--Taleghani A.et al.,2011;Ellsworth W.L.et al.,2013;Johri M.et al.,2013;Warpinski N.R.et al.,2013;Chuprakov D.et al.,2014;Elsworth D.et al.,2016;Li Y.et al.,2017).However,not all natural fractures in shale reservoirs are self-supported after shear slip.If the content of clay and organic matter is high,the post-slip residual permeability of the fracture is low(Wu W.et al.,2017).Therefore,detailed investigations regarding the shear mechanical properties of natural fractures in shale reservoirs are still needed.Focusing on the fracture shear slip mechanics in rock-like materials,numerous previous studies indicate that the main influencing factors on shear slip include surface roughness,normal stress,mineral composition and water saturation(Patton F.D.,1966;Ladanyi B.et al.,1969;Barton N.R.et al.,1972;Kwafniewski M.A.et al.,1997;Johnston I.W.et al.,1989;Seidel J.P.et al.,2002;Huang T.H.et al.,2002;Zandarin M.T.et al.,2013;Yang Z.Y.et al.,2000;Han W.M.et al.,2013).Among these factors,the content of low strength minerals,such as clay and organic matter,is an important one which affects shale shear slip.According to the study of the influence of sand-clay ratio on the shear slip of rocks,shear mechanical properties of shale are sensitive to clay content(Dafalla M.A.,2013;Zhang L.et al.,2016).The influence of the content of clay minerals,such as chlorite,montmorillonite and illite,on the friction coefficient has been extensively studied,and the shear slip has been classified into different types according to the clay content(Morrow C.et al.,1992;Lupini J.F.et al.,1981;Horn H.M.et al.,1962).In addition,the effect of organic matter on shear slip is similar to that of clay.Therefore,the influence of clay and organic matter content on shear slip is also studied in great detail(Kohli A.H.et al.,2013;Sone H.et al.,2010;Orellana L.F.et al.,2018).Although these studies are based on the influence of clay and organic matter content on fault shear slip,conclusion can still be drawn that clay and organic matter in shale may exert some controlling effects on the shear slip of reservoir fractures.

Fig.1.Scheme of the packed sample.

In this paper,the effect of clay and organic matter content on the shear slip of shale is studied.Considering that the clay and organic matter content controls the shear slip of shale,this paper aims to analyze the influence of clay content,organic matter content and normal stress on the shear slip of fractures in shale.Furthermore,direct shear tests under constant normal stress are conducted on four shale rocks with different contents of clay and organic matter.

2.Experimental sample and apparatus

These four experimental samples are collected from three formations of four blocks,namely the Wufeng formation of Yibin(YW),the Lujiaping formation of Chengkou(CL)and the Yanchang formation of Yan'an(YY-1 and YY-2).The former two formations deposit marine shale,while the last one deposits continental shale.

X-ray diffraction test is conducted to obtain the mineral composition and TOC content of each sample,as shown in Table 1.The mineral types of samples are mainly quartz,feldspar,carbonate minerals,pyrite and clay.Continental shale has higher clay content than marine shale,while both continental and marine shale have similar TOC contents.Marine shale has a total clay and TOC content between 19.1–65.4%.

Table 1 Mineral composition and organic matter content of experimental samples.

Shale samples need to be reinforced before experiment as they are easy to fracture.First,samples are cut into circular thin slices with a diameter of about 20 mm and a variable thickness.Then,the slices are put into a grinding tool into which epoxy resin was poured.After solidification,they are taken out and polished on the lathe until the inner shale is exposed (as shown in Fig.1) This process reinforces shale samples,preventing them from rupturing in the experiment.Biran O.'s method is used for reference to polish the shear slip surfaces (Biran O.et al.,2009).To be more specific,the surfaces are polished with emery of 120 mesh so that they can retain their roughness for simulating the fracture surfaces of shale.After the polishing process,the surface debris is removed by airgun blowing.Finally,the surface is dried at 100°C for 24 hr before tests.

The instrument used for shear test is UMT-Tribolab multi-functional shear testing machine from BRUKER Corporation,as shown in Fig.2(a).The device has many modules.In this study,the low-speed linear module is used,whose schematic diagram is shown in Fig.2 (b).The load resolution and range of the testing machine are 100 mN and 0.1 mN～2 kN,respectively,and the displacement resolution and range are 0.25 μm and 0–120 mm,respectively.The loading speed range is 0.002–10 mm/s,and the vertical displacement resolution is 0.5 μm.The shear and normal stress are recorded by the stress sensor at the top of the instrument.The sample is fixed in the sample slot by screws at its upper and lower surfaces.Driven by the motor,the ball seat slider moves to the right,pushing and moving the lower half sample,thus realizing direct shearing,as shown in Fig.2 (c).The shear displacement,shear stress,normal stress,normal displacement,shear velocity and other relative parameters are recorded and stored synchronously during the experiment.

In this experiment,the direct shear between fracture surfaces requires high sample processing accuracy,which is difficult to realize.By rotating the fine-tune knob of the leveling nut shown in Fig.2 (b),the ball head would rotate along the inner wall of the ball seat slider.After the surfaces of the upper and lower half samples touch,the leveling nut is locked.Fig.2(d)shows the sample surfaces before and after the shear slip.It is obvious that post-slip wear marks distribute all over the sample surface,indicating that a good surface-to-surface shear slip is formed.

Fig.2.Experimental apparatus(a)UMT-TriboLab machine used in this study;(b)Diagram of the core components of the low-speed linear module of the shear testing machine;(c) Scheme of loading method;(d) Surfaces of the sample before and after shear slip (red arrow denotes the shear direction).

3.Experimental results

3.1. Influence of mineral composition and normal stress on frictional behavior

In general,as the content of clay and mineral content increases,the slope of linear segment,peak shear stress and post-peak curve fluctuation all decrease.However,different groups of curves have different shapes and the curves can be divided into two categories.Fig.3 (a) and (b) show that the friction force fluctuates significantly,decreases sharply at the peak point,and fluctuates greatly after the peak point.This kind of shear is defined by Goodman et al.as brittle shear (Goodman,1976).The friction force curves of the latter two groups of samples only fluctuate slightly and increase smoothly to the peak value.There is no obvious fluctuation and peak points in these curves,and the post-peak parts of curves are relatively smooth without obvious fluctuation.Goodman et al.define this kind of shear as plastic shear.These phenomena indicate that the content of clay and organic matter controls the frictional behavior of shale.As the content increases,the shear strength and fiction coefficient both decrease.Meanwhile,the shear type transforms from brittle to plastic.The experimental results provide consistent rules even under different normal stress conditions.

The normal stress and friction force show approximately linear positive correlation,and the slope is the friction coefficient (see Fig.4).The friction coefficient is negatively correlated with clay and organic matter content,suggesting that the higher the content of clay and organic matter,the lower the friction coefficient.When the content of clay and organic matter increases from 19.21%to 65.43%,the friction coefficient decreases from 0.70 to 0.51(see Fig.5).

3.2.Surface topography after friction slip

The rock surface topography before and after shearing can reflect the wear state of asperities on rock surfaces.There are a number of methods to measure the surface topography,among which white light interference method is frequently used in recent years due to its high efficiency and non-contact feature.In this test,we use the optical surface contour measurement system in BRUKER GT-X,which has an amplification factor of 0.5–230 times and a vertical resolution of 0.1 nm–10 mm.Fig.6 (a)shows the three-dimensional surface topography of the 5 × 5 mm area before friction,and Fig.6 (b) is the corresponding two-dimensional topography.The black dashed line in Fig.6 (b) indicates the position shown in Fig.6 (c).The surface fluctuates between -30–30 μm.The asperities distribute randomly on the surfaces,preventing the relative sliding between fracture surfaces.

Fig.3.Friction force-displacement curves of four groups of samples under 100 N,200 N and 300N normal stresses,(a),(b),(c) and (d) correspond to YW,CL,YY-1 and YY-2,respectively.

Fig.4.Relationship between friction force and normal stress.

Fig.5.Relationship between the content of clay and organic matter and friction coefficient.

Fig.6.Surface topography measurements (a) 3-D surface topography,in which the color represents the height of the surface;(b) Corresponding plane;(c) Surface topography of the black dotted line position in Fig.6 (b).

Fig.7 shows the surface topographies after friction under different normal stresses.The shear direction is approximately horizontal and the scale in Fig.7 is consistent with that in Fig.6(b).The comparison among different rows in Fig.7 shows that the higher the content of clay and organic matter,the more obvious the surfaces scratch,and the more severely the asperities are worn.The comparison among different columns in Fig.7 shows that the higher the normal stress is,the more severely the surfaces wear and the larger the residual asperity area after friction is.Therefore,it can be concluded that when the content of clay and organic matter is low,asperities of the friction surface will undergo brittle failure and wear less,after which fractures can retain a certain degree of opening.When the content of clay and organic matter is high,asperities of the friction surface are seriously worn and flattened to conical shapes,after which fractures are almost closed.

3.3.Microstructure

Fig.8 shows the backscatter electron images of the experimental samples,all of which are magnified 5000 times.As shown in the microstructure,hard and brittle minerals such as quartz,feldspar and calcite are in the form of blocks,with the size between 5–20 μm.Clay is stratified or flaky.To facilitate analysis,the images are arranged in the order of clay and organic matter content.When the content of clay and organic matter is low,the asperities of hard and brittle minerals contact each other in a convex-concave form(such as YW,etc.),forming an inlay cementation mode dominated by particle support mode.When the content of clay and organic matter is high,the hard and brittle minerals embed into clay and organic matter,forming a matrix support cementation mode(such as YY-2).There is also a mixed support pattern(such as CL),in which brittle minerals,clay and organic matter are cemented together.

4.Discussion

The shear slip behavior of tight and loose sandstones is investigated by Dafalla M.A.et al.(2013)and briefly summarized below.Considering that clay has low cementing strength while quartz,feldspar and calcite are characterized by high cementing strength,they propose that hard mineral particles are tightly packed in tight sandstone,with clay and organic matter filling the void,and vice versa.Organic matter has similar mechanical properties to clay due to its low mechanical strength and irregular shape (amorphous particle state).Fig.9 shows the mineral distribution of three representative samples,in which different colors represent different minerals.YW group(19.21%clay and organic matter content) is mainly composed of quartz,calcite and dolomite.The hard components contact each other in a concave-convex manner,with clay and organic matter distributed in voids,forming a typical particle support pattern.The composition of CL group (clay and organic matter content is 35.26%)is mainly quartz,calcite,dolomite and clay.The hard components contact each other in a point-to-point manner,with the soft components continuously distributed in the pores between hard components,forming a mixed support mode.In YY-2 group(clay and organic matter content is 65.43%),the mineral composition is mainly quartz and clay,and the hard components(mainly quartz)are embedded in clay and distributed discretely.The supporting mode in this situation is matrix support controlled by clay.As shown in Fig.9,the increase in the content of clay and organic matter leads to changes in the support pattern.Shale with particle-support pattern has brittle shear-slip characteristics which are controlled by hard minerals.On the other hand,shale with matrix-support pattern has plastic shear-slip characteristics which are controlled by soft minerals.

Johnston (1989) proposes a micro model which describes the controlling effect of asperities on shear strength.In this model,if the shear strength of the fracture is high enough,asperities would slide upwards along the inclined surface until they are cut by sufficient shear stress.As shown in Fig.10,when the normal stress on fracture surface is lower than the uniaxial compressive strength of rock,the lengthzof the asperity shear section can be calculated with the following equation:

Wherezis the length of the shear section of a single asperity,L0is the wavelength of a single asperity,quis the uniaxial compressive strength,σneis the normal stress on the fracture surface.As shown in Fig.7,the higher the content of clay and organic matter is,the wider the residual asperity is,and the largerzis.The higher the normal stress is,the wider the residual asperity is.The higher the content of clay and organic matter is,the wider the residual asperity is,and the lowerquis.In some extreme cases,asperities are cut out completely.

Fig.7.Surface topographies after shear slip under different normal stresses(Shear slip direction is almost horizontal,and the scale in the figure is the same as that in Fig.6 (b)).

Fig.8.SEM images of four groups of samples,(a),(b),(c),(d) correspond to YW,CL,YY-1 and YY-2，respectively.

To sum up,the content of clay and organic matter exerts a great controlling effect on the frictional behavior of shale.The higher the content of clay and organic matter,the stronger the plastic characteristics of the frictional behavior of the shale,the greater the possibility of the surface asperity tips being cut out,and the smaller the fracture residual aperture after friction.At the same time,the greater the normal stress,the greater the friction damage on asperities.As a result,the fracture surface will be easier to close after shear slip,which is not conducive to the improvement of reservoir conductivity by hydraulic fracturing.

5.Conclusions

By studying the frictional behavior of shale with different mineral compositions,organic matter contents and normal stresses at the microscale,the friction mechanism is discussed and the conclusions are listed below:

(1) The content of clay and organic matter controls the frictional behavior of shale.As the content increases,the shear strength and friction coefficient decrease.At the same time,the shear type changes from brittle to plastic.This rule is consistent under different normal stress conditions.

(2) When the content of clay and organic matter is low,the micro asperities on friction surfaces break in a brittle manner and the wear degree of surfaces is low.In this case,fractures are still characterized by moderate apertures after shear slip.When the content of clay and organic matter is high,the micro asperity of friction surfaces is worn severely into conical shape.In this situation,fractures are almost closed after shear slip.

(3) It can be observed under electron microscope that with the increase of clay and organic matter content,the microscopic support pattern transforms from the particle support mode by hard minerals such as quartz to the matrix support mode by clay and organic matter.These two support patterns correspond to brittle and plastic shear as well as different friction surface failure situations.

(4) The lower the clay and organic matter content,the smaller the damage degree of asperities,which can increase fracture permeability through the self-support feature of asperities.However,the shear strength and friction coefficient are both large under this situation.Therefore,lubricating fracture surfaces with fracturing fluid and increasing fracture net pressure are the main measures to promote shear slip and reservoir stimulation.Conversely,when the content of clay and organic matter is high,asperities on fracture surfaces are easy to fail,resulting in the closure of fractures.In this situation,it is difficult to stimulate reservoirs through activating natural fractures;thus,the support efficiency of proppant needs to be improved.

Fig.9.(a)Schematic diagram of the friction failure of asperities,(b)Enlarged schematic diagram of(a)showing the friction failure of asperities as the content of clay and organic matter increases.

Fig.10.Schematic diagram of frictional failure of a single asperity.

Acknowledgement

This study is sponsored by the Natural Science Youth Project of University Scientific Research Plan in Xinjiang (XJEDU2021Y053),the Talent introduction research project of China University of Petroleum–Beijing at Karamay (XQSQ20200056),the Strategic Cooperation Technology Projects of CNPC and CUPB(Grant No.ZLZX2020-01).