Study Of The Fluid Flow Mechanism Simulation And The Distribution Of Residual Oil In Oil Field Developing

This paper made an integrated study of fluid flow in oil-bearing beds in the North Second Block of the Shaertu oil field, Daqing. It dealt with aspects such as geology, natural fractures, stress perturbation, coupling of fluid flow and solid skeleton in beds, residual oil, and adjustment of well network. Major conclusions drawn from this study are listed as follows.18 NNE-trending normal faults are recognized in the slightly dipping North Second Block, and all have relatively good seal ability. Oil beds are buried at a depth of 70 to 1,200 meters. They consist of intercalated sandstone and mudstone, of fluvial and lacustrine facies. They are substantially heterogeneous in space in that a number of 10 sedimentary for independent sandstone beds are discriminated.Stress perturbation was observed by changing work scheme and water well row. The in-situ maximum principal stress may vary in a range of 3 to 5MPa, and shear stress reaches a value or 0.35MPa, after the work scheme for 9 water wells at row 6-3 is changed. The perturbation would drive underground fluid flow, and enhance the nucleation and growth of micro-cracks in sandstone, enlarging the porosity and permeability of it. On the other hand, it should compress the pore in sandstone, reducing the porosity and permeability.A mathematical model is established for coupled stress and fluid flow at the middle and late stages of water injection development. It is implemented by adopting a numerical algorithm. In this model, the nucleation and spatial distribution of micro cracks are addressed.Simulation of stress field through the development shows that principal stress in oil-bearing bed ShaII_7-8 varies slightly before the development, and greatly at the middle and late stages. The maximum principal stress is in a range of -21.0 to -20.0 MPa before the development, and of-25.0 to -24.0 MPa at the middle and late stages.According to the simulation of fluid (namely, oil and water) potential, it is high to the west of wells B2-3-35, B2-D4-429, B2-D5-426 and B1-D1-26, and low to the east of wells B2-3-35, B2-D4-429, B2-D5-426 and B2-D1-24. The variation of fluid potential is small between them. Residual oil tends to remain in areas of low fluid potential in a close configuration.Fracture predication reveals that, at the middle and late stages tensional fractures are widely distributed in oil-bearing bed Sha II_7-8, in contrast with the local distribution of shear fractures. These tensional fractures are trended along the NE to SW direction, and extended in a stable way with a great variation of fracture density. Dense fractures have fracture index of greater than 7.5 are predicted in the eastern and western sides, respectively.The relation between stress perturbation and fluid flow is modeled to examine the impact of perturbed stress on fluid flow. The impact is both positive and negative, as described above. The fluid potential generally increases with the stress of rock skeleton acting upon the fluid in pore. Elastic deformation or plastic flow or even brittle fracturing might have resulted from the increase of stress, thus changing the properties and transmissivity of the rock. Both porosity and transmissivity in rock tend to decrease in highly stressed areas, and to decrease in slightly stressed areas.The potentialability and origin of residual oil, as well as its relation with reservoir of variable sedimentary kind, is analyzed on the base of detailed geological study. The distribution of residual oil is related in some way to factors including a low value of stress perturbation, a small fracture index, fracture orientation, and so forth. In oil-bearing bed, west of the North Second Block, there is a large possibility of having residual oil to the west of B1-D1-2, B2-6-420, B2-5-14 and B2-4-122, to the east of B2-4-122, and to the north of B2-4-126, because of the low value of stress perturbation. Additionally, residual oil is comparatively enriched in areas having small fracture index.Based upon the above study of residual oil, fractures and faults, injection-production well pattern in the North Second Block is rearranged, and a second infilled well pattern are optimized. Reservoir simulation of variable schemes confirms the best water-injection program, and adjustment pattern of injection-production well in the vicinity of faults and large fractures is promoted. It is shown by pre-production that in the new network the productivity may increase 0.82%, giving an additional amount of 53.38×10⁴ tons active oil.