Experimental Study of Starting Pressure Gradient in Carbonate Gas Reservoirs

As the carbonate gas reservoirs continue to be developed, the water saturation and effective stress of the reservoir are constantly changing, which leads to the constant change of gas starting pressure gradient. In order to study the influence of fracture, permeability, effective stress and water saturation on the starting pressure gradient of matrix and fracture samples. In this paper, starting pressure gradient experiments are carried out on three types of carbonate rock. The results of the study indicate that (1) the water saturation of matrix type samples is greatly affected, while the water saturation of fracture type samples is a little affected; (2) As the effective stress grows, start-up pressure gradient gradually increases. Compared to matrix cores, when the effective stress increases, start-up pressure gradient of fractured core increases by 2–3 times; (3) The start-up pressure gradient of cores decrease as a power function when the permeability increases. The matrix type cores are more significantly affected by permeability, while the fracture type cores are less affected by permeability; (4) For matrix reservoirs, when the permeability get smaller, the influence of start-up pressure gradient on well control radius will get smaller; For fractured reservoirs, the well control radius is greatly affected by the start-up pressure gradient. The results of the study can provide technical support for similar reservoir gas seepage characteristics and fracturing construction scheme design.