Abstract
Fracturing fluid imbibition and retention are treated as a main mechanism for oil production from shale reservoirs. However, the oil–water exchange phenomenon during post-fracturing soaking periods has not been thoroughly studied. In this study, a water–oil flow model is built to investigate the water imbibition and oil drainage phenomenon in hydraulically fractured shale. With the developed numerical simulator, the main characteristics of post-fracturing soaking, that is, pressure diffusion, water imbibition, and especially, the oil–water exchange behavior are simulated. Three key time points, that is, oil–water exchange equilibrium, steady exchange efficiency, and oil breakthrough in fracture are found. The oil–water exchange efficiency and exchange volume are also calculated. Moreover, the proposed model is validated by field wellhead pressure dynamics, indicating a relevance of time between the oil–water exchange efficiency and the wellhead pressure falloff derivatives. Finally, the influences of shale permeability, wettability, fracture complexity, and oil viscosity on the oil–water exchange behavior are investigated. Results indicate that the matrix of oil-wet shale almost does not suck water and discharge oil, and only the oil in natural fractures exchanges with the water in hydraulic fractures. The water-wet shale with low permeability, high oil viscosity, and few natural fractures needs extra soaking time to achieve good oil–water exchange performance. The suitable soaking period for the water-wet base case in this study is from 17.25 to 169 days, among which 64 days is the optimal soaking time.
Highlights
Compared with conventional oil reservoirs, shale has relatively high clay content and covers a wide range from 16.8 to 70.1% (Yang et al, 2013)
1) The simulation results prove the occurrence conditions for water–oil exchange, that is, water-wet rock can make the oil in the matrix replaced by the water in fractures, while for oil-wet rock, the oil in the matrix cannot be replaced out by well soaking and the oil in natural fractures will be further squeezed into matrix pores with the extension of soaking time, resulting in production difficulty
2) The simulation results reflect the water–oil exchange behavior in water-wet shale, that is, the water imbibition and oil drainage occur in the matrix at the beginning of the soaking periods, but the oil breakthrough will be delayed to 1.29 days
Summary
Compared with conventional oil reservoirs, shale has relatively high clay content and covers a wide range from 16.8 to 70.1% (Yang et al, 2013). Odumabo et al (2014) shows the relation between imbibition volume and soaking time It shows that the change of permeability in the invasion area of the fracture surface and the distribution of water saturation near the fracture surface are two important factors affecting the post-fracturing shale gas production. Zhang et al (2017) established a triple-porosity numerical model for hydraulically fractured shale gas wells, which considers the effects of imbibition, stress sensitivity, and gravity differentiation Their simulation results show that the spontaneous imbibition rate of fracturing fluids in a tight matrix is very low during the well-soaking process. To investigate the oil–water exchange behavior in a well/reservoir scale, we build a fracture-matrix water–oil flow model to simulate the fracturing fluid imbibition and the oil drainage process during the post-fracturing soaking periods. The results of saturation as well as fluid pressure are input as initial conditions for the following 1 year soaking simulation
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