Fluid flow and permeability analysis of tight gas carbonate reservoir rocks using fractures and dynamic data

Abstract

The porosity, permeability, rock material and fracture networks of the newly discovered tight carbonate reservoir rock, the Early Cretaceous Sarmord Formation in Zagros basin are examined. The methods used to investigate the formation are rock cores, micro-resistivity image logs, drill stem tests, drilling fluid data, petrophysical wireline data and drilled cutting samples.The analysed data show that the Sarmord Formation is characterized by extensive open and partially open macro-fractures formed by the local stress from fault and fold propagation. These fractures are sub-vertical with an average dip of 73°, fracture aperture of 0.44 mm and persistence (length) of 11.0 cm. The dominant strike of the fractures is NE-SW, parallel to the observed faults and perpendicular to the main Miran West structure.The vast majority of reservoir porosity was found to be in the rock matrix, with the fractures having very limited impact (average fracture porosity is 0.032%) on porosity. In contrast the fracture permeability averages 240 mD which is four orders of magnitude greater than the matrix permeability. As a result, the inter-connected fractures create effective fluid flow pathways and enhanced hydrocarbon migration perpendicular to the fold structure. Faulting associated with the field structure led to fault damaged zones which act as zones of inter-connected fractures with higher permeability and preferential fluid flow pathways. It is demonstrated that the fracture permeability is driven by the fracture aperture and spacing, whilst the fracture persistence has limited control on permeability.The findings of this study highlight that the importance of fractures and faults on reservoir's quality in tight gas bearing rocks. The network of inter-connected fractures, provide the fluid flow which control the permeability. In contrast the matrix controls the pore spaces and the hydrocarbon storage. This work emphasizes the importance of understanding the impact of structural geology in tight carbonate reservoirs.