Hydraulics And Geomechanics Parameters For Hydraulic .
al of PeturnroleJovi& En ronmenhiotec nologylBtaumISSN: 2157-7463OPENJournal of Petroleum &Environmental BiotechnologyACCESS Freely available onlineReview ArticleHydraulics and Geomechanics Parameters for Hydraulic FracturingOptimization in Production's Developments of Shale Gas/ Shale Oil inNorth AmericaBastos Fernandes Fernando1,2,3*, Campos Wellington1, Martins Barbosa Braga1, Marcelo Otaviano AlvesBotelho2, Alessandra Conde de Freitas3, Luiz Felipe de Queiroz Ferreira Braga1 and Rewbenio Araújo Frota2Pontifical Catholic University of Rio de Janeiro (PUC-Rio), R. Marques de Sao Vicente, 225 - Gavea, Rio de Janeiro - RJ, 22451-900, Brazil;Petróleo Brasileiro S.A. (Petrobras), Rio de Janeiro, RJ - 20031-912 Brazil; 3Federal University of Rio de Janeiro (UFRJ), RJ, 21941-901, Brazil12ABSTRACTUnconventional reservoirs (NCR) have been responsible for an important revolution in the volume and profile ofgas production in the USA and are now arousing interest of other countries e.g. China, Canada, Argentina andAustralia. In most of the NCR, the development of production has been made possible through horizontal wellswith multiple transverse fractures. In order to optimize the well hydraulic fracturing design for exploration andproduction’s development of shale gas/shale oil, is required to understand the key parameters that influence inthe complex fractures network. In terms of reservoir stimulation through horizontal wells, the practice by smalleroperators came before the theory, generating nice results and promoting theoretical development. The recentjoin of big operators into the unconventional reservoirs environment raised about the huge volumes of fluids andpropping agents used in the complex fractures network, have generated some questions about shale gas/shale oilwell stimulation. This work presents the main parameters that have influence on the complex fractures networkbuilt in shales, aiming to understand their effects in shale rock in order to avoid problems and optimize thehydraulic fracturing design.Keywords: Shales reservoirs; Organic matter; Source rocks; Conventional reservoirs; Non-conventional reservoirsINTRODUCTIONShale gas/shale oil reservoirs have a significant amount of worldhydrocarbon reserves and therefore has a huge importance for theglobal energetic matrix supply. The generation of petroleum occursin sedimentary basins such as lakes, oceans, rivers and marshes,where sedimentary rocks with large amounts of ne grains, clayand silt are deposited over millions of years and carrying a certainvolume of organic matter accumulated in the interior of their pores,[1] Such rocks are called source rocks, [2] After the maturation ofthe organic matter present in the source rock, a natural fractureoccurs in the same one, due to the conditions of high pressure andtemperature providing an ideal generation window and generatingthe primary migration of the hydrocarbon for its later imprisonmentin a rock with high permo-porous properties, denominated rockreservoir, [3] The next step of the conventional petroleum systemgenerating mechanism is to surround the reservoir by means oftraps and a sealant rock, i.e., with low permeability, e.g. evaporite(salt) or shale itself, thereby preventing the hydrocarbon be drainedto other layers [3].The shale gas/shale oil system is different from the conventionalreservoirs (CR) mentioned above, since shales are part of a groupknown in the literature as non-conventional reservoirs (NCR),because the primary migration has not yet occurred and also,because the shales have low permo-porous properties. Thus, thedrilling operation is done directly on the source rock, so the shalesare classified as source-reservoir rocks (SRR), [4] The ultra-lowpermeability and various gas accumulation features render theshale gas reservoirs difficult to be developed without hydraulicfracturing treatment, except a few with highly developed naturalfracture networks [5] With the global commercial developmentof unconventional hydrocarbon reservoirs with low porosityand permeability, hydraulic fracturing has been one of commonpractices and major methods for reservoir stimulation [6].The research of natural fractures shows that there are not manyopen NFs in shale reservoirs [7] because most of the pre-existingnatural fractures are sealed by cementing materials during thediagenetic process. However, as the weak parts, these fractures canbe reactivated to increase the effectiveness of hydraulic fracturingtreatment.Correspondence to: Bastos Fernandes Fernando, Pontifical Catholic University of Rio de Janeiro (PUC-Rio), R. Marques de Sao Vicente, 225 - Gavea,Rio de Janeiro - RJ, 22451-900, Brazil, Tel: 5521983682385; E-mail: fbf.cefet@gmail.comReceived: January 09, 2020, Accepted: January 24, 2020, Published: January 31, 2020Citation: Fernando BF, Wellington C, Braga MB, Alves Botelho MO, Conde de Freitas A, Queiroz Ferreira Braga LF, et al. (2020) Hydraulics andGeomechanics Parameters for Hydraulic Fracturing Optimization in Production's Developments of Shale Gas/ Shale Oil in North America. J Pet EnvironBiotechnol. 11:399. doi: 10.35248/2157-7463.20.11.399Copyright: 2020 Fernando BF, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.J Pet Environ Biotechnol, Vol. 11 Iss. 1 No: 3991
Fernando BF, et al.OPENACCESS Freely available onlineLITERATURE REVIEW Presence of natural fractures [20]NFs provide the accumulation space for natural gas. Hydraulicfracturing stimulates also NFs. Some NFs are opened and theconductivity in these fractures is greatly increased. If injectedproppant reaches into the reactivated natural fractures, gasproduction will be greatly enhanced [8]. By around 2012, morethan 55,000 wells have been drilled in shale gas reservoirs in theUSA and as many wells are drilled for the development of an area,reducing the cost of well construction is an important aspect tobe considered in a project of exploration and development of theproduction of shale gas/oil reservoirs, especially when the sellingprice of the gas/oil falls [9]. Permeability between nanodarcy and microdarcy [3]Normally, routinized and optimized operations are adopted andoften improved, based on studies in specific areas. These studiesare usually done through partnerships between operators andsuppliers [10].Shales proprietiesThe structure of the shales is formed by clastic or detritic rocks. Itsformation is by transport by water or air. Such rocks are normallycomposed of silica (e.g. quartz), combined with other commonminerals such as feldspar and clay minerals e.g. quartz, limestoneand feldspar, which have low particle size, making it difficult toexploit unused shale gas/shale oil of stimulation techniques.Another characteristic of shales is their high total organic contentTOC [11]. A shale-gas reservoir primarily consists in three maintypes of porous media: organic matter, inorganic matter and naturalfractures; where a fourth type called hydraulic/induced fracturesoccurs during hydraulic stimulation. It must be mentioned that,gas desorption and diffusion are the dominant physics in theorganic matter or kerogen, where the organic content is expressedin terms of TOC [12]The shale reservoir properties are critical factors determiningfracture network generation, e.g. mineral composition, rockmechanical properties, horizontal stress difference and naturalfracture properties. Brittle mineral-rich shale is beneficial inmaximizing fracture complexity, whereas high clay mineral contentis harmful in stimulating the reservoir [13,14] listed four conditionsthat made fracture complexity in the Barnett shale a desired andpossible outcome: extremely low matrix permeability, low Poisson’sratio, low horizontal stress difference, and orthogonal regionaltensile fractures. The importance of natural fractures in shale forhydraulic fracturing treatments has been particularly studied byGale et al. [15].The main challenge of petroleum engineering for the explorationand production of shale gas/oil reserves is that their geomechanicalproperties are very different from conventional reservoirs.Shales developmentsThe stimulation of a given formation by means of the hydraulicfracturing technique consists in the pumping of a high pressurefracturing fluid and controlled flow in a producing zone thathas low permo-porous properties, in order to exceed its limitporoelastic and to generate artificial fractures which aim to increasethe productivity index of the well by increasing the conductivityand permeability of the zone of interest [21]. In conventionalreservoirs, HF propagates in a two-wing shape, however, in shalegas/shale oil reservoirs, [22,23] due to some geo-mechanical,hydraulic and design parameters, the activation of natural fracturesthrough the HFs occurs, building a network complex fractures, asshown in Figure 1.Parameters that influence the construction of the complexfractures network in shale gas/shale oil reservoirsRock parameters: In a hydraulic fracturing design in shale gas/shale oil reservoirs, the mechanical properties and mineralogicalcomposition of the rocks are of great importance, since in order toestimate fracture pressure it is necessary to know NFs orientationand distribution, overburden gradient, horizontal stresses,modulus of elasticity and Poisson's ratio. Such parameters, relatedto mineralogical composition, provide the rock fragility coefficient.NFs orientation and distribution: According to previousconsiderations, during the hydraulic fracturing process in shalegas/shale oil reservoirs, NFs are activated in order to allow the gasflow trapped in the shale pores to the well and to optimize thestimulation operation [15,24] simulated the interaction betweenHF and NF in shale rocks and realized the behaviors shown inFigure 2a, Field tests have found that any HF in a naturallyfractured medium is influenced by the approach angles of the NFs[25,26] and generates a network of complex fractures, as shown inFigure 2b, Warpinski and Teufel [25], Guo et al. [26] found thatthe amount and pattern of fracture orientation directly influencedthe shape of the complex fracture network. Figure 3 shows thenetwork for a vertical well with Rn 5 for cases Figures 3a and 3b,and Rn 2 in case Figure 3c. For Figure 3a, NFs are parallel to insitu HF direction, for Figure 3b and 3c, NFs are tilted 5 clockwisefrom HF direction [27].NF cementing strength: Many researchers, geologists andpetroleum engineers working in the Barnett shale believedthat cemented NFs enhanced the effectiveness of hydraulicfracturing treatments because these characteristics could act asIn general, the main characteristics of shales are: Large variations in mineralogy [16] High anisotropy and heterogeneity, due to the laminationsof the depositional process [9] Porosity between 2% and 15% [17] Presence of high volume of organic matter [18] High degree of fragility [19]J Pet Environ Biotechnol, Vol. 11 Iss. 1 No: 399Figure 1: Presence of NFs and HFs in a shale rock and Types of HFsconstructed by hydraulic fracturing operation in conventional reservoirsand shale gas/shale oil reservoirs [24].2
Fernando BF, et al.zones of weakness that serve to deflect the growing HFs [7]. Inorder to understand how the cementing strength of NFs affectsthe building of complex fractures network, [26] investigated thefracture morphology for various NFs cementing strengths and wasdefined three NF cohesive strength grades as strong, medium, andweak according to various magnitudes of cohesive zone parametersto represent NFs. Figure 4 shows fracture lengths and widths fordifferent cementing strengths when the approaching angle is 75 .Direction of beddings: Beddings are flat sedimentary structuresgenerated by the deposition of overlapping lithological layers. Inrocks such as shale, which have different types of granulometry andmineralogical composition, the answers of the fundamentals aremore evident [28].Gomaa et al. [29] experimentally verified that the direction inwhich the foundations are accommodated, directly influencingthe fracturing pressure and no pumping time, where parallelfoundations use rock fracture with lower pressures and shorterpumping time.Sun et al. [30] made tests with two kinds of shale samples: Test 1: Shale beddings were vertical to the minimum in situOPENACCESS Freely available onlinestress direction and the HFs propagates along the beddingsplane. Test 2: Shale bedding is parallel to the minimum in situstress direction, and the HFs propagates along the beddingplane.In test 1, HFs were initiated at the eighth second and 9.87 MPa.The instantaneous initiation pressure declines and then rebounds.With the propagation of HFs, the injection pressure stabilizes at6.5 - 7.0 MPa. At the thirty-second second, fractures penetrate theentire sample, the injection pressure sharply declines, and liquidoutflows from exit end of the sample.In Test 2, HFs were initiated at the tenth second and 14.8 MPa.Since the size in the bedding direction is small, HFs propagate onlywithin a short time, under the pressure of 11 - 12 MPa. Figure 5ashows the variation of injection pressure along with time. Figure5b shows the HF area and opening degree in different beddingdirections. It is indicated that, given a constant displacement, asthe angle increases, the pressure required for HFs propagationincreases, the HFs propagation rate decreases. Within the sametime, the HFs area decreases, and the opening degree increases.Figure 2: (a) HF-NF different types of interaction (b) Numerical and experimental comparison studies about HF-NF interaction for different anglesof NFs [25,26].Figure 3: Hydraulic fracture network for a vertical well with Rn 5 for cases (a) and (b), and Rn 2 in case (c). For (a): NFs are parallel to in situ HFdirection. For (b) and (c): NFs are tilted 5 clockwise from HF direction [27].J Pet Environ Biotechnol, Vol. 11 Iss. 1 No: 3993
Fernando BF, et al.OPENACCESS Freely available onlineFigure 4: The fracture geometries for 75 approaching angle for various cohesive strengths: (a) HF. (b) NF.Figure 5: Influence of beddings in complex fractures network construction: (a) Correlation between beddings direction on pressure breakdownof the shale rock and pumping time (b) Correlation between beddings angle in HFs area and opening degree [30].Horizontal stress field: Ren et al. [16] experimentally found, bymeans of a triaxial test in a naturally fractured environment thatHF propagates along the NF for low horizontal stresses differencevalues and crosses natural fractures for high stress differencevalues, generating changes in the behavior of the complexfracture network. Li et al. [31] used the cohesive zone model andverified that an increase in horizontal stress difference tends toincrease the probability of NFs crossing under lower values of theapproaching angle. However, the stress shadow resulting frommulti-fractures can change this phenomenon in multiple-clusterhydraulic fracturing. The results show that HFs tend to cross theNFs and form a less complex fracture network as the horizontalstress difference increases, which agrees well with the findings ofZou et al. [32]. The simulations showed too that in the isotropiccase of stress difference about 2 MPa, the HFs that initiate fromthe two perforation clusters mainly open and propagate alongthe NFs, resulting in a highly complex fracture network near thehorizontal wellbore. However, the geometry of HFs changes withthe propagation process owing to stress interference. When thestress difference increases, the complexity of the fracture networkreduces, and fractures propagate in a straighter manner within thesame injection process [33].The stress shadowing effect: Interaction among multiplepropagating hydraulic fractures or the so-called stress shadowingeffect, especially its influence on fracture propagation path, hasbeen previously studied for fractures initiated and growing inJ Pet Environ Biotechnol, Vol. 11 Iss. 1 No: 399approximately the same formation layers [34]. The stress shadowingeffect can influence the height growth for fractures propagating inthe same layer or in the different layers in depth. Roussel et al.[35] used a poroelastic model of non-orthogonal 3-D fractures toperform numerical simulation on the effects of fracturing sequencein this stress field reorientation in Barnett shale and proposedtwo sequence-based fracture methods of HFs construction: Theconsecutive fracture method and the alternate fracture method.Alternate fracturing has been proposed in some USA shale gasreservoirs to reduce fracture spacing and increase the complexityof the fracture network [36,37] This technique consists of placingthe 2nd fracture where the 3rd fracture site would be. Roussel andSharma [36], Weng [38] found that the presence of an NF closeto the HF or interactions between HFs close to each other alteredthe stress field in the domain considered and thus influences thegeometry of the HF and the way it spreads. This effect is calledstress shadowing effect and can change the trajectory of the entirecomplex fracture network, reaching unwanted layers during thehydraulic fracturing operation. For a shale gas/shale oil reservoirstimulation operation to be successful, two design parameters areextremely important: Multi-fracture operational sequence [36]. Fracture spacing, as it has a direct correlation with thebehavior of the HF being generated (Figure 6) [39,40].Roussel and Sharma [36], Fisher et al. [39] and Mayerhofer et al.4
Fernando BF, et al.[40] realized that the fractures spacing influences in reorientationof the stress field of the shale rock. This way, the fracturing designmust considerate this parameter in order to optimize the complexnetwork extension and SRV. Figure 7a show the in situ stress stateperturbation due to the presence of an HF and the in situ stressreorientation angles, respectively, in the Barnett shale [36]. Figure7b shows the influence of secondary fracture length in relationshipto the HF spacing with the angle of deviation of the orthogonalpath to the axial axis.Internal friction angle of NFs: By setting the internal frictionangle of natural fractures as 10 , 20 , 30 and 40 , and the hydraulicinjection time at 20 minutes, the fracture geometries are simulated.Figure 8a indicates that smaller internal friction angle can lead tomore complicated fracture patterns, higher fracture propagationpressure as well as more shear fractures. Figure 8b illustrates thatthe smaller the internal friction angle, the larger the SRA of naturalfractures can be achieved, manifesting that the internal frictionangle is a key factor in influencing the SRV. When the half fracturelength is the larger, the fracture width is the smaller. It indicatesthat there is a great relationship between the number of shearfractures and internal friction angles of the natural fractures.Young’s modulus: Simulation results at the injection time of 50minutes with Young’s modulus of 20, 30, 40, and 50 GPa, areshown in Figures 9a and 9b. These figures show that, the higherthe Young modulus, the more complicated the fracture networksaround the wellbore, and the more shear failure of natural fracturesattained. Figures 10a and 10b present that, the larger the Young’sOPENACCESS Freely available onlinemodulus, the more obvious the shear failure of NF and the lessdominant role the HF plays. When Young’s modulus is relativelysmall, fracture width decreases a little while fracture length slowlyincreases. When the Young’s modulus surpasses 30 GPa, the shearfailure of natural fractures dominates, accompanied by a decreasein hydraulic fracture length and an increase in fracture width.Poisson’s ratio: Simulation results at the injection time of 20minutes with different Poisson’s ratio of
Geomechanics Parameters for Hydraulic Fracturing Optimization in Production's Developments of Shale Gas/ Shale Oil in North America. J Pet Envi
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