Study On Borehole Stability In Sidewall Rock Of Petroleum Drilling

The wellbore stability problem is widespread existence in the process of the artesian well and have been an iMPortant problem which perplexs the petroleum industrial boundary. It has been being subjected to domestic and international field experts concern of the petroleum artesian well, and has been the iMPortant contents which the domestic and international artesian well technique field's unremittingly efforts work for . The test indicates that the wellbore stability problem not only including the mechanics problem, also chemistry problem, the chemistry problem still reflects to the mechanics problem in the end .The wellbore stability study is the results of many subjects development,such as chemistry, elastic mechanics, log interpretation and so on. The paper's point is to carry on mechanics analysis, the application of logging method is also involved.This paper will conclude the logging method of the stress, rock mechanics parameters.As the loggingg well method itself is a coMPlicated subject, the text didn't do deep research on it.Drilling fluid density played a pivotal role in controling the wellbore stability ,it is the only factor that can control in many factors. This text's mainly aim is calculate the drilling fluid density to prevent sidewall instability. This text will establish modle based on linear elastic theory and deduce the mechanical formula about sidewall surrounding rock, also the shear failure scriteria was commonly used, they are Mohr-Columb scriteria , Drucker-Prager scriteria and Mohr-Coulomb scriteria. The generation goes into rock parameters of the depths rock underground,it can solve to the drilling fluid extreme limit density which can prevent the wellwall from collapsesing. The research indicates that under the same parameter condition, there is great difference of three results which use the three shear failure scriteria.The result which use Mohr-Coulomb scriteria get the safe density is too conservative, according to Druck-Prager scriteria, the safe drilling liquid density probably with great difference from the reality, so it draw erroneous conclusions. The Mohr-Coulomb scriteria didn't consider the mid-principal stress of rock's breakage function, but the Druck-Prager scriteria is too exaggerates the mid-principal stress of rock's breakage function. Therefore, the Mohr-Coulomb criterion and the Druck - Prager criteria are not well reflected intermediate principal stress on the role of rock damage. The Modified-Lade criteria made up for the shortcomings of these two criteria, can accurately reflect the intermediate principal stress on the role of the rock damage effects.The drilling liquid density play the role in the wellbore stability, it is the only factor that can control in many factors. The main task is calculating the liquid density to keep wellbore from collapsed or fracture. So this text is mainly studying of the mechanics to dint formula , respectively with the Mohr-Columb rule,Drucker-Prager rule,Lade rule to calculate a the liquid density to keep wellbore from collapsed or fracture.Beascuse the great majority of rock have shown a certain degree of elastic-plastic, that is, rock will through a period of plastic deformation before destruction, rather than direct brittle failure. So make use ofthe large-scale finite element software ANSYS to carry on elastoplastic analysis on the sidewall rock, coMPareing the two results. The paper presents parameters to perform numerical analysis, observeing Borehole Collapse and Well-hole Enlargement from simulation results.We can know, it could be that the density of drilling fluid have great iMPact on well diameter enlargement ,when the density of drilling fluid is less than a critical value, well diameter enlargement increased dramatically.when the density of drilling fluid is greater than the critical value, well diameter enlargement was less affected. In the practical drilling, as long as the density of drilling fluid controled greater than the critical value, it can prevent the wall collapsed, enhance mechanical separator speed, ensure the drilling was operated safety and reduce costs. Although the elastic modle of the sidewall stability can be a good description of the brittle formation's damage property, it can not reflect the mechanical behavior of elasto-plastic strata coMPletely. In this sense, the finite element method is more appropriate to the calculation principle and controling of material properties, finite element analysis can express more intuitive to deep rock's elastic-plastic properties.The formation stress alsoly plays the role in the wellbore.Law of the hole collapse pressure of azimuth, hole angle and perigon is studied.When the both horizontal stresses are equivalent, oblique angle is zero and collapse pressure is not change with azimuth angle and perigon;when the horizontal stresses are unequal and the vertical pressure is the most large, the different azimuth of wellbore stability is difference alongπ/6到π/3 of hole angle. It is safety alongπ/6 toπ/2 of hole angle;when the vertical stress is the intermediate value, it is safety alongπ/3 toπ/2 that the wellbore can't collapse;when the vertical stress is the least value,it is safety alongπ/3 toπ/2 that the wellbore can't collapse and the the position of collapse lies to the 0 andπperigon. It is different from the previously cases and similar td the third case. Above the law mentioned in this paper will be meaning forfurther study of centering theory.In practice, the spot is according to the oil-field concrete circumstance, combine together theories and fulfillment then can prevent°from and handle a well eye to lose steady trouble in time.The well wall lose steady is a coMPlications and need to open an exhibition research further of problem, applying limited dollar number to imitate can provide an accurate reference for the artesian well engineering basis.