Study on Rock Mechanical Properties of Deep Volcanic Rock Reservoir

. Volcanic rocks exhibit complex mechanical properties due to their special diagenetic forms. Laboratory tests were conducted to study the influence of confining pressure changes on the mechanical properties of deep volcanic rocks, and outcrop volcanic rocks single triaxial tests were conducted simultaneously to study the mechanical properties of deep volcanic rocks. The maximum deviatoric stress and elastic modulus are used to study the variation of mechanical properties of volcanic rocks with confining pressure. The maximum deviatoric stress of underground core increases linearly with the increase of confining pressure. The elastic modulus reaches its maximum at 80MPa confining pressure. Although the variation trend of mechanical parameters of underground core with confining pressure is similar to that of outcrop core with confining pressure, the underground core shows a larger maximum deviatoric stress and elastic modulus. It can be seen from the 80MPa confining pressure comparison test that the mechanical properties of underground core under high confining pressure are superior to those of outcrop core. The mechanical properties of outcrop core under high confining pressure are significantly affected by natural fractures, while the mechanical properties of underground core are less affected by natural fractures.The brittleness index of volcanic rocks decreases with the increase of confining pressure, and the decreasing range is relatively low. The brittleness index of outcrop volcanic rocks decreases with the increase of confining pressure, but its sensitivity to confining pressure is much higher than that of underground core.


Introduction
The study value of volcanic rocks is not only reflected in the abundant gas reservoir resources 1 , but also reflected in the special rock mechanical properties of volcanic rocks, so that volcanic rocks are easy to form tectonic fractures under the action of tectonic stress, and these tectonic fractures will become effective seepage channels and reservoir space 2 .In addition, volcanic rocks tend to have a high Young's modulus 3 , so the ability to create fractures near the borehole is low in the process of hydraulic fracturing of volcanic rocks.Especially, the high initiation pressure in deep volcanic rocks leads to the difficulty of rock initiation near the borehole, which increases the difficulty of engineering.
According to the spatial distribution, occurrence conditions and appearance characteristics of volcanic products, Yang 4 divides the Carboniferous volcanic lithofacies of the Kebai fault zone in the northwestern Junggar Basin into eruption facies, exhalation facies, volcanic sedimentary facies and Asian volcanic facies.The controlling effects of volcanic facies, tectonic movement and diagenesis on the characteristics of volcanic reservoir are also revealed.Wang 5 conducted rock mechanics tests and acoustic emission tests on reservoir rocks in the study area to evaluate the compressibility and fracture-forming ability of reservoirs by integrating anisotropy, brittleness, stress sensitivity, natural fracture density and acoustic emission activities of rocks.On the basis of volcanic core observation, thin section identification, well logging data and seismic data analysis, Xie 6 analyzed the spatial types and physical characteristics of reservoir and the controlling factors of volcanic reservoir physical properties in the study area.The results show that the types of volcanic rocks in the study area are mainly volcanic breccia, andesite and tuff.Lithofacies mainly include volcanic eruption facies, exhalation facies and volcanic sedimentary facies, among which volcanic eruption facies is the most developed.Shen 7 adopted nuclear magnetic resonance (NMR) and pulse-decay permeability monitoring experiment to study the petrophysical properties of the volcanic reservoir, mineralogical effects and sample heterogeneity, which could provide a reference for fractured stage selection in engineering.Liu 8 obtained the characteristics of natural fractures in volcanic rocks based on outcrop observation and core analysis, and established a hydraulic fracturing model by using stochastic modeling method.The results show that the expansion rate of hydraulic fracturing fractures increases with the reduction of cementation strength ratio, and the initiation pressure also decreases significantly.
Liu 9 converts electrical imaging data into porosity maps to obtain porosity spectra and bin distributions for qualitative study of reservoir heterogeneity.By mercury injection test and low temperature nitrogen adsorption test, combined with fractal theory, Cang 10 studied the pore structure and fractal characteristics of micro pores of volcanic rock reservoir in Changling fault depression, Songliao Basin.In this paper, deep volcanic rocks 3500m underground are selected for triaxial compression test to study the influence of confining pressure changes on the mechanical properties of volcanic rocks, and the rock mechanical properties of deep volcanic rocks are compared with outcrop volcanic rocks to fully understand the rock mechanical properties of deep volcanic rocks.

Sample preparation
In order to ensure the accuracy of test data, cores selected in the study were prepared by wire cutting.The sample is φ25mm×50mm cylinder, as shown below:

Test instrument
MTS815 Flex Test GT program controlled servo rock mechanics test system made in the United States was used in this test, as shown in Figure 2. The unit is equipped with servo-controlled automatic three-axis compression and omni-directional deformation monitoring system.The system consists of loading part, test part, control part and program control part.
The maximum vertical loading force of the device is 4600kN; Vertical piston stroke range is 100mm; Can apply 140MPa confining pressure; Strain rate adaptation range of 10-2～10-71/s; Fatigue frequency: 0.001~0.5Hz;The overall stiffness of the test frame: 11.0×109N/m, to ensure the stability of the test system during loading; All the test process is controlled by computer, and the data collected during the test is automatically transmitted to the computer in real time, with high measurement and control accuracy.

Test method
As the buried depth range of underground core is 3000-4000m, which belongs to the category of deep rock, the depth range of core covered by four confining pressures of 20, 40, 60 and 80MPa is set.The test procedure is as follows: add pressure head to both ends of the sample and seal it with a sealing sleeve (thermoplastic tube).Install axial and radial strain sensors respectively and put them into the pressure chamber; Confining pressure is applied by controlling the liquid pressure in the pressure chamber; Axial pressure was applied to the sample until the sample was damaged under the condition of keeping the confining pressure unchanged, and the load and strain values of the sample were recorded during the test.The sample photos in the loading process are shown in Figure 3.
Where: P is the peak load; A is the cross-section area of the sample;   is the stress difference; 1   and 3   are axial and radial strain differences respectively.

Analysis of failure forms
As can be seen from Figure 4 (a~f), in underground core triaxial test, with the increase of confining pressure, the yield stage takes an increasing proportion in the deformation process.According to the triaxial stress-strain curve diagram of outcrop core in Figure 6 (a~c), with the increase of confining pressure, the proportion of yield stage is also increasing.However, the outcrop core is more sensitive to the change of confining pressure because the inner particles of outcrop core are more loose than those of underground core, which is more prone to plastic failure.
Based on the analysis of rock failure forms in the triaxial test, combined with Figure 5, under the condition of no confining pressure, the failure of underground core rock results in high-angle fractures with small fracture width and large interference degree from natural fractures.When the confining pressure increased to 20MPa, the fracture length increased significantly, and the fracture width was similar to that without confining pressure.The fracture morphology zigzagged under the influence of natural fractures.When the confining pressure increases to 40MPa, the most obvious change is that the fracture morphology approaches a straight line, indicating that the fracture morphology is less affected by natural fractures at 40MPa confining pressure.Another obvious change is that the Angle between the fracture and the horizontal plane decreases greatly.When the confining pressure increased to 60MPa, the dip Angle of the fracture continued to decrease, and a micro-fracture parallel to the main fracture was formed.In combination with Figure 7, failure forms of outcrop core are analyzed and compared with those of underground core.When confining pressure is 0MPa, failure forms of outcrop core and underground core are similar, both of which produce micro-fractures with high Angle, but when confining pressure is 40MPa, the fracture Angle of outcrop core is significantly higher than that of underground core.The dispersion of rock mechanical parameters under uniaxial compression of underground core is small.The mean value of uniaxial compressive strength is 154MPa and the mean value of elastic modulus is 23MPa.The sample shows shear failure.According to Figure 8 (a), under triaxial compression, the maximum deviatoric stress presents a linear increase trend with the increase of confining pressure.Combined with Figure 8 (a), the elastic modulus changes little at confining pressures of 20 and 40, while the elastic modulus increases significantly at confining pressures of 80 MPa.It can be seen from Figure 5 that all samples show oblique shear failure.

Mechanical index analysis
The uniaxial compressive strength of outcrop core is 65.63MPa, which is much lower than that of underground core.The average elastic modulus is 9.1MPa, which is about half of the downhole lithology elastic modulus.The failure model of sample under uniaxial condition is the same as that of underground core, both of which are oblique shear failure.It can be seen from Figure 8 (b) that under triaxial compression, with the increase of confining pressure, the maximum deviatoric stress presents a linear increase trend as well as that of downhole core.According to the analysis of the changing trend of elastic modulus in Figure 8 (b), it can be found that the biggest difference in the changing trend of elastic modulus with confining pressure between outcrop core and underground core lies in that the elastic modulus of outcrop core reaches the maximum value at 60MPa, while the elastic modulus of outcrop core decreases significantly at 80MPa.As can be seen from Figure 7, HS-3 sample did not suffer oblique shear failure like other samples.Artificial failure after HS-3 test showed that the failure interface of the sample was in a granular state, and the existence of natural cracks could be found in the artificially damaged sample, which was not found in downhole core.The compaction stage time of sample HS-3 is significantly longer than that of elastic deformation stage, indicating that outcrop rock is different from underground rock in that it is difficult to withstand the action of high confining pressure, and outcrop core is significantly affected by natural fractures, so it shows lower elastic modulus and different failure characteristics under the action of high confining pressure.
In conclusion, combined with Figure 4 (g) and Figure 6 (d), the variation trend of mechanical parameters of underground core in single triaxial test with confining pressure is similar to that of outcrop core in single triaxial test with confining pressure.However, the maximum deviator stress and elastic modulus of underground core are much higher than that of outcrop core, and underground core exhibits superior mechanical properties under high confining pressure.It can be seen from Table 6 and Table 7 that with the increase of confining pressure, the brittleness index of underground volcanic rocks shows a decreasing trend, but the decreasing range is not large.With the increase of confining pressure, outcrop volcanic rocks are much more sensitive to confining pressure than underground core, although they also show a downward trend.

Conclusion
In this paper, deep volcanic rocks were selected for triaxial compression test to study the influence of confining pressure changes on the mechanical properties of volcanic rocks, and the rock mechanical properties of deep volcanic rocks were compared with outcrop volcanic rocks to fully understand the rock mechanical properties of deep volcanic rocks.The following conclusions are reached: (1) The maximum deviatoric stress increases linearly with the increase of confining pressure in the single triaxial test of underground core; The elastic modulus changes little under the confining pressure of 20 and 40, but increases significantly under the confining pressure of 80MPa.
(2) The variation trend of mechanical parameters of underground core with confining pressure in single triaxial test is similar to that of outcrop core with confining pressure in single triaxial test, but the maximum deviator stress and elastic modulus of underground core are much higher than that of outcrop core.
(3) The mechanical properties of underground core are better than those of outcrop core under high confining pressure.
(4) The mechanical properties of outcrop cores under high confining pressures are significantly affected by natural fractures, while those of underground cores are less affected by natural fractures.
(5) The brittleness index of volcanic rocks decreased with the increase of confining pressure, and the decreasing range was relatively low.The brittleness index of outcrop volcanic rocks decreases with the increase of confining pressure, but its sensitivity to confining pressure is much higher than that of underground core.

Figure 1 .
Figure 1.Picture of original specimen required for test.

Figure 2 .
Figure 2. American MTS815 Flex Test GT rock mechanics test system.

Figure 3 .
Figure 3. Photo of rock sample during loading.

Figure 8 .
Figure 8. Variations of two core mechanical parameters with confining pressure.

Table 3 .
Summary of single triaxial rock mechanics parameters of underground core.

Table 5 .
Comparison of mechanical properties between underground core and outcrop core.

Table 6 .
Evaluation table of downhole core brittleness.