This paper proposes an approach to determing the optimal cluster spacing for volume fracturing in shale oil reservoirs based on three scales, i.e. microscopic capillary displacement, large-scale core imbibition, and macroscopic reservoir nuclear magnetic resonance (NMR) logging. Through flow experiments using capillary with different diameters and lengths, and large-scale core counter-current and dynamic imbibition tests, and combing with the NMR logging data of single wells, a graded optimization criterion for cluster spacing is established. The proposed approach was tested in the shale oil reservoir in the seventh member of the Triassic Yanchang Formation (Change 7 Member), the Ordos Basin. The following findings are obtained. First, in the Chang 7 reservoir, oil in pores smaller than 8 μm requires a threshold pressure, and for 2–8 μm pores, the movable drainage distance ranges from 0.7 m to 4.6 m under a pressure difference of 27 mPa. Second, the large-scale core imbibition tests show a counter-current imbibition distance of only 10 cm, but a dynamic imbibition distance up to 30 cm. Third, in-situ NMR logging results verified that the post-fracturing matrix drainage radius around fractures is 0–4 m, which is consistent with those of capillary flow experiments and large-scale core imbibition tests. The main pore-size range (2–8 μm) of the Chang 7 reservoir corresponds to a permeability interval of (0.1–0.4)×10 −3 μm 2 . Accordingly, a graded optimization criterion for cluster spacing is proposed as follows: for reservoirs with permeability less than 0.20×10 −3 μm 2 , the cluster spacing should be reduced to smaller than 4.2 m; for reservoirs with permeability of (0.2–0.4)×10 −3 μm 2 , the cluster spacing should be designed as 4.2–9.2 m. Field application on a pilot platform, where the cluster spacing was reduced to 4.0–6.0 m, yielded an increased initial oil production by approximately 36.6% over a 100-m horizontal reservoir section as compared with untested similar platforms.
Fan et al. (Sun,) studied this question.