| Popis: |
We present an analysis of casing deformation mechanisms in one area of extensive casing shear in the Daqing Oilfield, China. Instead of qualitatively stating the cause of large areas of casing shear, as in most previous work, we developed a mathematical model to quantitatively compute the coupled effect between the tectonic stress field and the induced stresses from high-pressure water injection. Numerical simulation carried out in the area indicated that variation of injection pressure induced a substantial perturbation on the local stress field. Furthermore, the induced stress fields associated with each injection well overlap with each other. Once the maximum compressive stress parallels or nearly parallels the differential pressure, the stability of strata with respect to shear is compromised, and when the thrust stress imposed exceeds the shearing resistance, the strata will slip in a direction coaxial with the vector from high to low-pressure areas. Simulation results of various schemes show that as long as the injection pressure and pressure differential between blocks are controlled to be less than 12.7 MPa and 0.86 MPa respectively, formation shear slip along a horizontal surface will no longer occur. The study in this article is somewhat different from previous studies in that a full coupling of the in situ and the induced stress fields was performed quantitatively and based on field experiments, and also in that the results were used to explicitly determine pressure limits on individual wells and differential pressures for the entire region of casing shear. The method and the results can serve as reference for other similar oilfield circumstances. |
