The mineral composition,porosity,microstructure and mechanical characteristics and relations of tight gas reservoir in Daqing

1. Introduction

Rock strength and deformation characteristics and their respective petrographic interpretation are important in many fields of earth science, petroleum exploration, development and drilling(Álvarez-Calleja et al. [1]; Sousa et al. [2]; Sabatakakis et al. [3]). The factors influencing the strength and deformation characteristics can be divided into material factors and environmental factors (Li et al. [4]; Baud et al. [5]; Ündü [6]).The material factors include mineral composition, porosity, microstructure, texture et al. (Engelder and Plumb [7]; Hudec [8]; Prikryl [9]; Jeng et al. [10]). The environmental factors include water content, confining pressure, stress path, loading rate, temperature, et al. (Haimson [11]; Mogi [12]). Many scholars choose the uniaxial compressive strength (UCS) as the most important parameters to evaluate the rock mechanical characters. The relationship has been widely investigated between UCS and influenced factors. Lower porosity (Ulusay et al. [13]; Jeng et al. [10]; Sousa et al. [2]; Cantisani et al. [14]), higher quartz content (Bell and Lindsay [15]), larger grain size (Olsson [16]; Fredrich et al. [17]; Robertson [18]; Wong et al. [19]; Hatzor and Palchik [20]; Meng and Pan [21]), greater grain contact (Dobereiner and De Freitas [22]) and greater packing density (Bell [23] and [15]) result in a higher UCS for granites, marbles and sand stones in general. With the development of the unconventional reservoir, brittleness becomes more and more important for evaluating hydraulic fracture networks (Dahi-Taleghani [24]; Yao [25]; Xia [26]), which is expressed and calculated not only by the uniaxial compressive strength (UCS) but also by elasticity modulus (E) mineralogical composition, full stress-strain, triaxial residual stress (TRS), strain at peak point ( ε p ), elasticity strain ( ε e ) and fracture diagram parameters (Bishop [27]; Hucka [28]; Geng [29]; Kahramana [30]; Jarvie [31]; Rickman [32]; Fred [33]; Zhang Kuangsheng [34]; Liu [35]). So, it is necessary to evaluate the above mechanical properties and reveal influencing factors and regularities. Motivated by these, 18 specimens of tight gas reservoir are cored in Da Qing oilfield. The experiments are thus carried out for triaxial stress-strain, XRD and thin section observations. The experimental results of stress-strain curve, mechanics parameters, mineralogical composition and porosity are obtained in the same pressure and temperature environment. The above factors are then analyzed synthetically and it also reveals the effects of material factors on mechanical properties for tight gas reservoirs.

2. Porosity and mineral composition

The specimens evaluated in this study are obtained from exploratory sampling boreholes in Da Qing oilfield of china. The experiments are carried out for porosity and x-ray diffraction (XRD). The porosity, mineralogical composition and content are acquired, as shown in Table 1.

The 18 thin sections have been examined by a petrographic microscope. Cementation type, contact relation, interstitial material, particle size of the debris, sorting features and so on are observed and quantified, as shown in Table 2.

For specimens A1 to A4, the cementation type is pore-contact, the main contact type is line-spot, the main interstitial filling is clay, and the grain size is from 0.03 mm to 0.25 mm.

For specimens B1 to B5, the cementation type is membrane-pore, the main contact types are spot and spot-line, the main interstitial fillings are clay and little calcite, and the grain size is from 0.03 mm to 0.24 mm. For specimens C1 to C4, the cementation type is pore-basement, the contact type is spot, the main interstitial fillings is calcite, and the grain size is from 0.12 mm to 0.32 mm. For specimens D1 to D2, the cementation type is membrane, the contact type is spot-line, the interstitial fillings is calcite, the grain size is from 0.03 mm to 0.20 mm. For specimens E1 to E2, the cementation type is pore, the contact types are line and spot-line, the interstitial fillings are clay, calcite and other minerals, and the grain size is from 0.16 mm to 0.36 mm.

Mechanical properties are tested by triaxial compression experiment. Cylindrical specimens are prepared by cutting and polishing with the diameter is 25 mm and the ratio of height to diameter is 2 ( ± 0.03 ). All shrinkable-tube-jacketed specimens are sealed with rubber belt, avoided the infiltration of hydraulic oil by accident when applying confining pressure. The rate of loading is 300 N/s. The confining presser is 13.25MPa. The mechanical parameters can be tested and calculated such as stress-strain curve, elasticity modulus (E), Poisson’s ratio (μ), triaxial peak strength (TCS), triaxial residual stress (TRS), drop amplitude of stress-strain curve (DROP), strain at peak point ( ε p ) and recoverable strain ( ε e ) for representing strength, deformation and brittleness. Those parameters are shown in Fig. 1.

E is the slope of the straight line in stress-axis strain curve. μ is the ratio of radial strain to axial strain. DROP is the difference of TCS and TRS. The E, μ, DROP can be expressed as follows: (1) E = tan ( α ) = σ ε 1 where σ is axial stress, MPa; ε 1 is axial strain, %. (2) μ = ε 2 ε 1 where ε 2 is radial strain, %. (3) DROP = TCS − TRS .

The stress-strain curve, E, TCS, TRS, μ, ε p , ε e and fracture diagram were obtained for different specimens, as shown in Table 3.

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Fig. 1.

Stress-strain curve and mechanical properties calculation.

According to Table 3, the stress-strain curve can be classified two types: the first type shows four stages including the compaction, elasticity, yield and failure, such as A3 and A4. The second type shows three stages of elasticity, yield and failure except A3 and A4. The patterns of the rock macro failure mainly demonstrate shear failure with one fracture in common, but it has main fracture and muti-branch fractures for C1, C2 and C3.

To investigate the relationship between mechanical parameters and cementation types, we plot in Fig. 2 the measured mechanical property results of elasticity modulus (E), triaxial residual stress (TRS) and drop amplitude of stress-strain curve (DROP), together with cementation types. The cementation types are shown in the horizontal axis, the TCS and DROP are represented by histogram, the E is represented by various symbols connected by types of line with values displayed on the secondary vertical axis.

Figure 2 reflects the effects of cementation types on the E, TCS and DROP, the regularities will be described in the following discussions.

6. Conclusion

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Fig. 2.

Mechanical parameters and cementation type.

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Fig. 3.

E and other material factors.

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Fig. 4.

Poisson’s ratio and material factors.

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Fig. 5.

TCS and material factors.

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Fig. 6.

TRS and material factors.

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Fig. 7.

DROP and material factors.

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Fig. 8.

ε p and other material factors.

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Fig. 9.

ε e and other material factors.