Horizontal drilling and hydraulic fracturing have been proven to significantly increase production from tight oil formations, however, one of the characteristics of these reservoirs is high production declining rate. In the long term, it is necessary to increase the development efficiency of block matrix, many studies have shown that spontaneous imbibition is a potential way. In addition, precisely predicting the behavior of the imbibition recovery under different viscosity ratios is thus critically important. The goals of this paper include theoretically and experimentally investigating the effect of the viscosity on the imbibition recovery.

In this paper a novel pore-scale spontaneous imbibition model is established by two steps. At first, the spontaneous imbibition equation in a single capillary tube considering that immobile oil is established based on Hagen Poiseuille formula. Then, pore-scale spontaneous imbibition model based on fractal theory has been derived. In addition, the corresponding spontaneous imbibition tests have been conducted on sandstone natural cores collected from Xinjiang oilfield.

The predictions of the imbibition recovery by the proposed model have been validated by comparing them with the experiments conducted. Data of cores H-56 and H-39 are used to comparing the calculated imbibition recovery with the measured imbibition recovery. Under four different oil/water viscosity ratio, 0.5, 1, 5 and 10, the relative errors between calculated data and measured data are less than 7%. And both the theoretical and experimental results show that the viscosity ratio of the mixed oil and brine has a significant effect on the spontaneous imbibition recovery. The predicted and experimental results demonstrate that a higher imbibition recovery corresponds to a lower oil/brine viscosity ratio. In addition, rate of spontaneous imbibition has a significant decline which is the same as the result of the calculation.

In general, since the established model has a high precision, it can be used to analyze the effects of different parameters like the contact angle, the interfacial tension and other structural parameters and the results can be useful for understanding the spontaneous imbibition law. What's more, the conclusion can guide the surfactant screening in surfactant-aided imbibition schemes.

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