Hydrocarbon migration in fracture-cave systems of carbonate reservoirs under tectonic stresses: A mechanism study

Abstract

The fracture-cave systems of carbonate reservoirs have almost stored 30% of recoverable oil and gas around the world. However, it is still doubtful about the mechanism of hydrocarbon migration in the fracture-cave systems. In this work, deducing from the Eshelby’ solution, we derived the equation to calculate the bulk strain of elliptic cylinder caves applied by stresses. Calculated results indicate that the bulk strain of caves negatively increases with the radius ratio of the elliptic cave axials under fixed stresses. In the case of the effective horizontal stress increasing from 30 MPa to 80 MPa, the bulk strain difference of a cave could be up to 0.5%. It may result in 0.4% of the total cave volume of fluid transporting through the fracture-cave systems within such a stress cycle. Since the tectonic stresses transform in a cyclic way, the volumetric ratio of new-to-old fluid in a cave would increase with the number of stress cycles. As a result, we proposed that the periodic fluid flow induced by cyclic tectonic stresses could be an important mechanism for hydrocarbon migration in the fracture-cave systems of carbonate reservoirs.

Keywords

• Eshelby’ solution
• Bulk strain
• Cyclic tectonic stress
• Periodic fluid flow
• Fracture-cave system
• Carbonate reservoir