IDENTIFICATION AND EVALUATION OF FRACTURED TIGHT-SAND OIL RESERVOIR IN DEEP ZONE OF BOHAI GULf

摘要

In this paper, three fracture models are considered. First, we study the plane fracture model that represents the formation consisting of fractures with equal spacing and opening. Second, the matchstick model in which two sets of perpendicular plane fractures exist is examined. Third, we also study the cubic one where three sets of plane fractures perpendicular to each other. These models can be represented by a macroanisotropic medium using equivalent conductivity theorem. Using a 3-D FEM codes, dual laterolog responses in the models may be computed. The simulated results of plane fracture model show that tool response in the equivalent medium equals to that of the original fracture-free medium only when the spacing between fractures is small , and that deep laterolog response (Rlld) is greater than shallow laterolog response (Rlls) for large dip angles and Rlld is smaller than Rlls for small dip angles. The computed responses of the matchstick model show that when the porosity of main set fractures is much greater than that of another set, the response of the model has the same characteristics as the plane model; and when the main fracture porosity decreases, Rlld decreases for large dip angles and increases for small dip angles. Rlls decreases for large dip angles and changes very little for small dip angles. Same characteristics are found in the response of cubic situation. An inversion code has been developed to compute fracture porosity and dip angle from dual laterolog response, and used to evaluate the fracture porosity and dip angle in tight-sand oil reservoir in Bohai Gulf. A Well “A” was drilled in Dagang Oilfield in Bohai Gulf in 1999. When drilling through a tight-sand formation with low porosity and low permeability, more than 67 cubic meters of mud was lost. Oil testing of the reservoir with 12.2mm nozzle produced 543 cubic meters of oil and 90,500 cubic meters of gas daily. Such high production in this kind of reservoir was beyond expectation of all experts working on this project. Based on the characteristics of dual laterolog curves and related geometrical data in this region, this layer and two others were named as fractured reservoirs, and their fracture porosity and dip angles were computed by using the inversion code. Oil testing to another reservoir obtained 220 cubic meters of oil and 79,984 cubic meters of gas daily.