Abstract
Abstract The microscopic pore structure characteristics and the oil-water two-phase seepage law in the low permeability sandstone reservoir in Mobei oilfield in Junggar Basin were analyzed through laboratory experiments. The results of mercury pressure, constant velocity mercury pressure, thin slice of casting, and CT scan analyses showed that the reservoir had strong microheterogeneity with the presence of local large channels. The large channel had a small volume but considerably contributed to the permeability, which played a crucial role in the reservoir seepage. The relative permeability curve showed that with the increase of water saturation, the relative permeability of the oil phase decreased rapidly; the water phase relative permeability of glutenite, gravel-bearing sandstone, and coarse sandstone increased slightly; and the water cut increased rapidly. The relative permeability of the water phase of medium and fine sandstone increased, the water cut increased rapidly, and the residual oil saturation was high. In the process of core displacement, on-line CT scanning monitoring showed that before the breakthrough of the water drive front, the oil saturation decreased greatly along the way. After the breakthrough of the water drive front, the water cut increased rapidly and directly entered the ultrahigh water cut stage. Owing to the serious heterogeneity of the micropore structure, the fingering phenomenon was obvious in the process of displacement.
Highlights
The Mobei oilfield is located on the Mobei uplift in the hinterland of Junggar Basin
Through the comparative analysis of constant velocity mercury injection experiment results from the Sangonghe Formation reservoir in Mobei 2 oilfield and a low permeability sandstone reservoir (Figure 2), we found that the distribution of a large throat in the Mobei oilfield reservoir is considerably heterogeneous, especially for rock samples with large permeability, and the large throat is extremely developed
The main lithology of the J1s21 reservoir is represented by the relative permeability curve of fine sandstone (Figure 7): with the increase of water saturation, the relative permeability of the oil phase decreases rapidly, the relative permeability of the water phase increases slightly, and the water cut increases rapidly
Summary
The Mobei oilfield is located on the Mobei uplift in the hinterland of Junggar Basin. The reservoir lithology mainly comprises medium and fine sandstone, followed by coarse sandstone and glutenite. The average porosity of the J1s21 reservoir is 13.6%, and its permeability is mainly distributed between 1 and 40 × 10−3 μm, with an average of 8:05 × 10−3 μm; the reservoir belongs to the low porosity and ultralow permeability type. The reservoir type belongs to the gas cap reservoir with a low permeability under the action of edge and bottom water; this is vertically subdivided into two reservoirs: J1s21 and J1s22, which are developed separately. Development of the J1s21 reservoir started in 2001 and has been developed every year from 2007 to 2011. Development of the J1s22 reservoir started in 2000. Development effect is poor at 97.7%, and the recovery rate is 10.9%.
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