Strength, creep and frictional properties of gas shale reservoir rocks

The deformational properties of gas shale reservoir rocks from the Barnett and Haynesville shale formations were investigated using a triaxial apparatus. The samples tested varied in their mineralogical composition, the degree of diagenesis, the total organic content and the degree of maturity of the organic material. In general, rocks with more cement and less clay show higher elastic moduli, higher intact strength and higher frictional strength. In addition, the amount of time-dependent creep under constant triaxial load correlates strongly with the Young’s modulus and clay content in the samples, but does not show any correlation with the Poisson’s ratio. Viscoplastic creep in these rocks will impose challenges in effectively stimulating production by slick-water hydraulic fracturing by raising the frac gradient and reducing the amount of induced brittle deformation. various depths of about 2600 m and we categorize them into 2 groups according to their visual appearance as “light” and “dark” colored groups which are thought to reflect their carbonate and clay content based on log elemental analyses. Haynesville samples come from depths of about 3500, and the mineralogical contents are known from XRD analyses of nearby samples (Table 1). Although visual appearances of Haynesville samples do not vary significantly as in Barnett samples, we call samples with more carbonate content as the “light” group and the other as “dark” group to match the description of the Barnett samples. Samples were kept under room humidity condition prior to the test and experiments were performed under dry and drained condition. Due to the dry condition of the samples, we were able to eliminate poro-elastic effects so our data represents the mechanical behavior of the dry framework. All samples were shaped to 25.4 mm (1 inch) diameter. For all Haynesville samples, sample lengths were 50.4 mm (2 inch) and the axes of the cylindrical samples were perpendicular to the bedding plane. Lengths of Barnett samples varied between 38-56 mm (1.5-2.2 inch) except for one short sample (28 mm, 1.1 inch) whose rock strength data was neglected from the analyses. Barnett samples included those cored parallel and perpendicular to the bedding plane. 2.2. Experimental Procedures Cylindrical samples were tested for its mechanical properties in a triaxial apparatus (Autolab2000, New England Research Inc.) under hydrostatic and triaxial loading conditions. Axial and radial deformations were measured by two pairs of individual sensors, and the confining pressure and differential load on the sample were monitored by a pressure gauge and an internal load cell, respectively, during the experiments. A typical experiment is performed in 3 stages, hydrostatic, triaxial, and failure & friction, as shown in Figure 1. In the hydrostatic stage, samples were subject to 3 steps of isotropic pressures, after which the pressure was held constant for 3 hours to observe any possible hydrostatic creep deformation. After each 3 hour holds, the pressure was decreased and increased over several minutes before proceeding to the next pressure step, in order to measure the elastic bulk modulus of the sample. In the triaxial stage, the axial differential load was increased in two steps while holding confining pressure constant at 20, 30, or 60 MPa. Differential load was increased in 2 steps, after which the load was held constant again to observe creep deformation and un-/reloaded to measure the Young’s modulus and Poisson’s ratio. The size of the differential loads was chosen to reach about 50% of the rock strength after 2 load steps. Finally in the failure and friction stage, the sample was taken to failure while the differential load was servocontrolled to produce constant axial strain rates of about 10 s. After rock failure was observed, the sample was allowed to slide along the failure plane until a steady frictional strength was observed. After the experiment, the angle of the failure plane was measured relative to the sample axis to obtain the shear and normal stresses resolved on the failure plane. The experimental condition for each sample is summarized in Table 1. Table 1. Mineral contents of the sample groups and summary of main experimental results. Composition of Barnett samples are rough estimates based on geophysical logging data. Sample Group Qtz/Felds [%] Carbonate [%] Clay [%] Others [%] Pc Triax [MPa] Creep Stress 1/2 step [MPa] Frictional Coefficient UCS [MPa] 30 0.923 30 46.5 / 92.0 0.805 30 46.5 / 92.1 0.706 20 47.3 / 95.2 0.899 30 0.896 Barnett Dark 60 10 25 5 40 49.2 / 94.9 0.791 160 30 0.928 20 44.7 / 89.7 1.10 Barnett Light 35 55 5 5