Stress sensitivity of tight reservoirs and its effect on oil saturation: A case study of Lower Cretaceous tight clastic reservoirs in the Hailar Basin, Northeast China

Abstract

To analyze stress sensitivity of tight reservoirs and the boundary conditions for oil filling, core samples of tight clastic reservoirs in the Lower Cretaceous Nantun Formation in the Hailar Basin were selected. The measurements include optical observation of thin sections, changes of porosity and permeability under overburden pressure, together with physical simulation experiments of oil filling. The results show that the stress sensitivity coefficient of permeability is a product of two parameters including the power exponent of porosity-permeability and pore compressibility, which reflect the geometric and mechanical characteristics of pores. The stress sensitivity and pore geometry of tight reservoirs of different lithologies vary greatly. Medium-coarse sandstones-conglomerates have relatively small stress sensitivity coefficient and large power exponent of porosity-permeability, indicating stronger anti-compaction, weaker stress sensitivity and dominated fissure-type pores. Fine sandstone with large stress sensitivity coefficient and small power exponent of porosity-permeability, indicating that stress sensitivity is high, and matrix circular/elliptical pores predominate. Oil filling in tight reservoirs displays two pressure gradient points with threshold characteristics (i.e., starting pressure gradient and critical pressure gradient) and two flow characteristics (i.e., low-speed nonlinear seepage and quasi-linear seepage). Only when the actual pressure gradient breaks through the critical pressure gradient into the quasi-linear seepage zone can oil saturation in the tight reservoir reach the lower limit of 30%, attaining higher oil saturation levels gradually during the subsequent filling process. The favorable tight reservoir is characterized by a low stress sensitivity coefficient and a high power exponent of porosity-permeability, and oil exhibits rapid filling characteristics with quasi-linear seepage. Another type of tight reservoir is characterized by a high stress sensitivity coefficient and a low power exponent of porosity-permeability, and oil exhibits slow filling rate with nonlinear seepage. It is therefore difficult in actual geological conditions for such reservoirs to achieve high oil saturation. Increase in oil saturation during the filling process is controlled by a combination of initial permeability, pore geometry, stress sensitivity and the displacement pressure gradient. These factors affect and compensate each other, comprehensively controlling the accumulation of tight oil, and could be used to classify the reservoir types.