| In the development of oil reservoir with water flooding,the injection capacity of any individual well is closely related to the wellhead injection pressure.The wellhead pressure for most of injection wells in BH oilfield are controlled about 15MPa.It follows the principle that the bottom injection pressure is no more than 85%of the formation fracturing pressure.However,with consideration of some safety factors,the wellhead pressure is strictly restricted as usual in the water injection process(e.g.the wellhead pressure is limited at 10 MPa,12 MPa,etc.).That greatly exacerbates the contradiction between the water quality of injection treatment and the requirement of injection allocation.In order to alleviate this contradiction,common used measures such as deep treatment of water quality,high-frequency injection and so on further increases the difficulty of "cost reduction and efficiency increase" actually.There’s no doubt that the limiting wellhead pressure during water injection is reasonable from the view of safe production.The reasonable wellhead pressure is restricted by the breakdown pressure,what’s more,the breakdown pressure directly determine the maximum wellhead injection pressure.Thus,accurate prediction and test of the breakdown pressure of the formation is the key point to solve this problem.Firstly,the triaxial test machine is used to simulate the rock stress state and rock mechanics parameters are obtained by indoor rock mechanics tests.The profile of rock mechanics parameters along the whole well segment are calculated according to acoustic logging,density logging,gamma logging data etc.Combining with the laboratory test and mine test data,in-situ stress prediction analysis is carried out using prediction model of the ground stress.The rock mechanics parameter profile and the ground stress profile are established as well.Then,the studies on the prediction of breakdown pressure for BH oilfield are performed using breakdown pressure prediction method,shear failure and tensile criteria.According to that,a set of method to test mine burst pressure is proposed.Combining with the experimental results of the breakdown pressure test,Fracture mechanism of the loose sandstone for BH oilfield is analyzed.The fracture mechanism verifies the accuracy of the prediction model of the breakdown pressure for different well injection wells.The prediction model of the breakdown pressure is further corrected and the calibration of the model is performed to make it more applicable for actual field.It provides reference for the determination of reasonable injection pressure and Technical support to sustainable development of water injection.Finally,through the analysis of the injection pressure level and breakdown pressure level for some field cases of BH oil zone,it has been found that there is space to increase injection pressure for target injection wells.Therefore,it’s highly feasible to increase the injection pressure to improve the water injection in BH oil field. |
