The impact of organic matter on geomechanical properties and elastic anisotropy in the Vaca Muerta shale

The Vaca Muerta formation and many other well known shale plays are often characterized by high quantities of kerogen and bitumen. The organic-rich intervals within these formations often hold large quantities of moveable hydrocarbons and are frequently the intended drilling target. An accurate estimation of the quantity of organic matter (TOC) in the formation is important for a successful well. When core and cuttings-derived TOC values are not available, well logs are used to estimate the TOC. One common method of predicting the TOC with well logs is the Passey method. This method provides more of a qualitative estimation of the quantity of organic matter in the Vaca Muerta formation, as changes in the rock fabric often produce fluctuations in the resistivity curve that are not related to changes in the TOC. Direct correlations between the sonic, bulk density, neutron porosity, or uranium content logs may provide a better means of predicting the TOC. A good estimation of the TOC in the Vaca Muerta shale for well PRo.x-1 was achieved when a linear correlation to the bulk density from well LJE.x-1010 was utilized, even though the wells are about 26 miles (42 km) apart.;As kerogen and bitumen are not as stiff as most of the other mineral components of the rock, a large volume of TOC should affect the stiffness of the shale rock. A strong correlation between Young's modulus and the TOC exists for the Vaca Muerta formation. This correlation is apparent in both laboratory and well log data. However, there does not appear to be a strong correlation between the TOC and Poisson's ratio. Furthermore, Thomsen's anisotropy parameters do not appear to be strongly correlated to the TOC. However, there appears to be a small negative correlation between the ratio of horizontal and vertical Young's modulus and the TOC and a small positive correlation between the ratio of horizontal to vertical Poisson's ratio and the TOC. Pore pressure also has a strong positive correlation with the TOC up to a value of about 6 wt%.;The pore shape that creates the least stiff rock is a penny-shaped crack, a type of shape often associated with kerogen in situ. Also, when lower quantities of kerogen and higher quantities of formation fluids fill the pore spaces, the rock stiffness decreases, which is expected as kerogen is a stiffer material than oil, gas, or formation water.