VOL. 14, NO. 18, SEPTEMBER 2019 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences ©2006-2019 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com NEW MODEL OF BRITTLENESS INDEX TO LOCATE THE SWEET SPOTS FOR HYDRAULIC FRACTURING IN UNCONVENTIONAL RESERVOIRS Omer Iqbal1, Maqsood Ahmad1 and Eswaran Padmanabhan2 1Department of Petroleum Engineering, Institute of Hydrocarbon recovery (IHR), Universiti Teknologi PETRONAS, Malaysia 2Department of Petroleum Geoscience, Institute of Hydrocarbon recovery (IHR), Universiti Teknologi PETRONAS, Malaysia E-Mail: [email protected] ABSTRACT Rock characterization in term of brittleness is necessary for successful stimulation of shale gas reservoirs. High brittleness is required to prevent healing of natural and induced hydraulic fractures and also to decrease the breakdown pressure for fracture initiation and propagation. Several definitions of brittleness and methods for its estimation has been reviewed in this study in order to come up with most applicable and promising conclusion. The brittleness in term of brittleness index (BI) can be quantified from laboratory on core samples, geophysical methods and from well logs. There are many limitations in lab-based estimation of BI on core samples but still consider benchmark for calibration with other methods. The estimation of brittleness from mineralogy and dynamic elastic parameters like Young’s modulus, Poison’s ratio is common in field application. The new model of brittleness index is proposed based on mineral contents and geomechanical properties, which could be used to classify rock into brittle and ductile layers. The importance of mechanical behavior in term of brittle and ductile in shale gas fracturing were also reviewed because shale with high brittleness index (BI) or brittle shale exist natural fractures that are closed before stimulation and can provide fracture network or avenues through stimulation. The brittle shale also has low breakdown pressure and no fracture healing as compared to ductile shale. The integration of laboratory and geophysical methods (determination of P and S waves from well logs) are recommended for accurate estimation of brittleness index (BI) for shale gas reservoirs. Keywords: new model of brittleness index, hydraulic fracturing, shale gas reservoirs. 1. INTRODUCTION shale can be fractured easily as compared to ductile shale The increasing demand of energy urged the where proppants healing usually happen. Therefore, there exploitation of unconventional hydrocarbon reservoirs is need to have correct estimation of brittleness in order to especially in shale gas. The shale gas has ultra-low maximize the production. permeability that need enhancement of production rate through hydraulic fracturing stimulation. The successful 1.1 Concept of Brittleness stimulation is needed with low proppant embedment and It is mechanical property of rock and due to non- high stimulated reservoir volume (Wang and Gale, 2009). uniform definition different researchers define and use The rock mechanical properties and mineralogy are brittleness for their respective practical use i.e. Brittleness playing crucial role for efficient stimulation. is lack of ductility [47], in brittle behavior there is no The mechanical behavior of rock is divided into plastic deformation [48], a rock will show brittle behavior brittle and ductile [1] [2] [3]. Brittle shale contains more when its internal cohesion will exceed [47] brittle rock natural fractures and easily fractured by hydraulic make fracture with little or no plastic flow [49], [50]. The stimulation. While ductile rock cause fracture healing [4], brittleness can be measured from stress strain curve [3]; [5]. The mechanical response of source rock is obtained by triaxial compressive tests i.e. ratio of elastic described by term brittleness or fracability [3], [6] [7] [8]. strain to total strain at failure. The most acceptable Unfortunate, there is no uniform definition exist for definition proposed by [9] that fractures in brittle rock brittleness. According to current studies [9] [10] [1] [11] initiated at or slightly beyond yield stress and they further [4] [12] [8] brittle rock has following characteristics i.e. observed following characteristics in brittle rock i.e. low value of elongation upon loading, fracture failure, fracture failure, higher ratio of compressive to tensile formation of fine particles, higher ratio of compressive to strength, low values of elongation of grains higher tensile strength, higher internal friction angle, greater toughness; higher angle of internal friction, formation of percentage of brittle minerals, higher value of Young’s cracks in indentation. Brittleness can be measured from modulus, lower value of Poison’s ratio, strength reduction mineralogy, static and dynamic mechanical properties and occur with failure, and brittle rock fail in an intensive way. expressed in index [13], [14] [15], [50], [11]. [16][7] The estimation of brittleness is important in many fields including shale gas recovery through stimulation. 2. ESTIMATION OF BRITTLENESS INDEX During hydraulic fracturing stimulation, a pressure fluid is injected to create fractures or avenues through which gas 2.1 Brittleness based on Mineralogy can flow to wellbore easily. However, the ability of shale Brittle minerals in shale reservoir are responsible to make fractures is described by brittleness because brittle for fracture initiation or network [11]. Brittleness is now 3271 VOL. 14, NO. 18, SEPTEMBER 2019 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences ©2006-2019 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com calculated by term known as brittleness index ([14], [15]). > 60% considered highly brittle [22]. Brittleness based on Brittle minerals play the key role in stimulations, mineralogy may have some limitations because to get increasing interface between reservoir and the well bore mineralogy of deep shale is complex and expensive [23] ([11]). However, there is no standard criteria exist for but mineralogy from well logs is quite economical as brittle minerals. The quartz, feldspar, and carbonates are compared to laboratory. [23] reviewed the literature and considered brittle while clay minerals are ductile that found that weight percentage of minerals may not give decrease brittleness index [50] [17] [18]. The most brittle precise description of brittleness because two rock mass mineral is quartz that contribute to high brittleness as with same mineralogy can have different porosity and shown in Figure-1. The productive well in Barnett shale density due to different compaction history and based on were located in pay zone with quartz (45%) and clays this explanation the strength and failure process may be (27%) and the brittleness can be quantified on bases of different. According to [24] rock grain and cementation mineralogy using equation 1 given below [11]: Some also affect the brittle behavior of rock and also researchers pointed out that quartz is more brittle than compressive stresses effect the brittleness [3] [7] [25]. carbonates, so carbonate minerals should be treated only Therefore, all the brittle minerals i.e. Silicates (quartz, partially brittle [55]. However, previous studies have feldspar and mica) and carbonates (limestone, dolomite, documented that both quartz and carbonate minerals are and siderite) and ductile (clays) should be consider in favorable for hydraulic fracturing [56], [57]. The higher order to accurately determine brittleness Index based on magnitude of brittleness index makes the rock brittle and mineralogy. However, there may be different value of BI suitable for fracturing [13]. The mineralogy can be on same rock using different approaches because the finding from laboratory analysis such as X-ray diffraction lithology and mineralogy is changing from rock to rock. testing (XRD). Therefore, along with mineralogy the other properties like Each mineral has its own brittleness factor as grain size, cementing material, acoustic properties and shown in Figure-4 [21] BI > 40% considered brittle and BI mechanical properties of rock should be considered. Figure-1. Brittleness factor of several minerals [21]. 2.2 Brittleness based on dynamic mechanical for brittleness as shown in Figure-2. The elastic properties parameters can be estimated both in laboratory and Brittleness can be quantified from Young’s through wireline logs. The calibration is required between modulus and Poison’s ratio because they represent the log-based and core-based parameters, and calibrated mutual effect of stress and strain and more accurate than elastic parameters should use to estimate brittleness index individual use of stresses and strains [23]. For brittle rock (Equation 1-3) [57]. the Young’s modulus should be high while Poison’s ratio should be low and in such condition the shear modulus …..…………. Equation (1) will be high under brittle failure [26]. The high young’s 2 2 2 3푉푝 −4푉푠 modulus and low poison s ratio under brittle failure has 2 2 ’ 푌푀. 푙표푔 = 휌푉 푉푝 −푉푠 been confirmed by many researchers [20], [22] [27]. The 2 2 …………….Equation (2) relationship between young’s modulus and poison’s ratio 푉푝 −2푉푠 2 2 푃푅. 푙표푔 = 2푉푝 −2푉푠 100(YM .Cali.YM _ Cali ) 100(PR _ Cali PR _ Cali ) min max (YM _ Cali YM _ Cali ) (PR _ Cali PR _ Cali ) BI 1_ max min min max 2 …………….Equation (3) 3272 VOL. 14, NO. 18, SEPTEMBER 2019 ISSN 1819-6608 ARPN Journal of Engineering and Applied