Research on sensitivity damage of naturally fractured carbonate reservoirs in Ordos Basin

Understanding mechanisms and performance of formation sensitivity damage in naturally fractured carbonate reservoirs is crucial for avoiding productivity attenuation at any development stage. In this paper, based on the investigation of reservoir microscopic feature, a series of experiments were conducted to systematically evaluate formation sensitivity damage through a multistep coreflood platform. Firstly, the influence of flow velocity on core permeability was studied. Then, coreflood experiments under the condition of beneath the threshold velocity were performed to research the influence of the salinity and PH value for the injected water on core permeability. Finally, four types of cores from Ordos Basin-matrix cores, non-packed cores, semi-packed cores, and fully-packed cores-were used to conduct stress sensitivity damage experiments. Permeability is measured in the process of increasing confining pressure with a constant pore pressure. The experiment results show that the velocity sensitivity of the reservoir is medium to strong and the threshold velocity is 0.28 m/d. The water sensitivity is weak and the average water sensitivity index is 22.62%. There is no alkali sensitivity. The stress sensibilities of these four types of cores ranking from strong to weak are non-packed cores, semi-packed cores, packed cores, and matrix cores. With the decrease in packing degree of core fractures, the stress sensitivity of core permeability increases and the recovery degree of permeability decreases. The influence factors of sensitivity damage are systematically studied and it provides the suggestions for stimulation design and development of naturally fractured carbonate reservoirs in Ordos Basin.