Experimental study of the anisotropic behaviour of the Naparima Hill argillite

Abstract

Argillite rocks are often anisotropic due to the presence of microfractures , clay minerals, laminations , and bedding planes . Understanding the anisotropic behaviour of organic-rich argillite rocks is critical for their successful exploitation as unconventional reservoirs. In this study, the anisotropic rock properties of the argillite in the Naparima Hill Formation, Trinidad, an unconventional reservoir, was studied in laboratory experiments. A series of laboratory measurements including permeability, P- and S-wave velocities, and compressive strength were conducted at effective pressures of up to 130 MPa. The rock samples tested were collected from the Naparima Hill outcrop described by three lithofacies : siliceous calcareous mudstones , calcareous mudstone, and siliceous mudstones. The anisotropy of the argillite was determined by the ratio of the rock property measured perpendicular to the outcrop bedding to the rock property measured parallel to the outcrop bedding. For all the studied argillites, the permeability anisotropy ratios is about 2, while the P- and S-wave velocities anisotropy ratios, and compressive strength anisotropy ratios are close to 1, indicating that the rocks are fairly anisotropic. With increasing effective pressure, the permeability, velocities, and strength anisotropy ratios all decreased marginally. A comparison analysis reveals that the anisotropies of permeability and strength, anisotropies of P-velocity and permeability and anisotropies of S-velocity and permeability are all unrelated. However, anisotropies of P-wave velocity and strength, anisotropies of S-wave velocity and strength, and anisotropies of P-wave velocity and S-wave velocity anisotropy all have a 1:1 relationship. Overall, the results of the study suggest that the Naparima Hill argillite is weak to fairly anisotropic, which can be attributed to the low clay content, poor laminations, lack of foliation and few microfractures in the rocks.