Abstract
Tight oil reservoirs are characterized by a low porosity, low permeability, and strong heterogeneity. The macropores, throats, and microcracks in reservoirs are the main seepage channels, which affect the seepage law in the reservoirs. In particular, oil-water two-phase flow in different types of pores requires further study. In this study, two groups of online NMR displacement experiments were designed to study the seepage characteristics of tight oil reservoirs. It was found that the main seepage channels for oil-water two-phase flow are the microcracks, large pores, and throats in the reservoir. The large pores are mainly micron and submicron scale in size. The oil in the small pores is only transferred to the large pores through imbibition to participate in the flow, and there is no two-phase flow. Based on the influence of different pore structures on the seepage law of a tight reservoir, the pores were divided into seepage zones, and a multistage seepage model for tight reservoirs was established. Based on this model, the effects of the imbibition, stress sensitivity, threshold pressure gradient, and Jamin effect on model’s yield were studied. The results show that imbibition is no longer effective after a while. Owing to the stress sensitivity, the threshold pressure gradient, and the Jamin effect, oil production will be reduced. As the parameter value increases, the oil production decreases. The production decreases rapidly in the early stage of mining while decreases slowly in the later stage, exhibiting a trend of high yield in the early stage and stable yield in the later stage.
Highlights
With the development of the petroleum industry, conventional oil and gas resources are gradually being depleted, and the recoverable amount of unconventional oil and gas resources is gradually increasing [1]
The theoretical basis of nuclear magnetic resonance (NMR) analysis is that the hydrogen nuclear 1H contained in oil and water will excite different NMR signals in different relaxation times when encountering a specific frequency of the magnetic field
A conceptual model of multistage seepage in tight reservoirs was established, and a corresponding mathematical model was established based on the characteristics of the concept model to study the seepage law of tight reservoirs under multiple factors
Summary
With the development of the petroleum industry, conventional oil and gas resources are gradually being depleted, and the recoverable amount of unconventional oil and gas resources is gradually increasing [1]. It is of great significance to study the seepage law under the influence of microcracks and the capillary force in tight oil reservoirs. Yang et al [19] used the nuclear magnetic resonance (NMR) T2 spectrum to monitor the oil migration dynamics in tight oil reservoirs Their results demonstrated that the small pores have a larger capillary pressure and stronger imbibition ability, and the small pores have a more effective oil recovery rate than the large pores. In this study, based on previous studies, two groups of online NMR experiments on water flooding and gas flooding were conducted using cores with different degrees of microcrack development, and the seepage law of a tight oil reservoir containing microcracks was studied. A multistage seepage model is established according to the seepage characteristics of tight reservoirs and the pore structure, which makes up for the deficiency of the previous classification of reservoirs only according to the pore structure, and provides a new research idea for the subsequent study of the seepage of tight reservoirs
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