Stress Interpretation And Wellbore Stability Evaluation Of Xujiahe Formation Of Exploration Wells In Western Sichuan Depression

The natural gas resources of Xujiahe formation are rich in western Sichuan depression, which the achievements of oil & gas exploration and exploitation had indicated that the gas pool has the good prospect for exploring and exploiting. Because of complex geological conditions, such as super-high reservoir pressure, collapsed clay, development of natural fracture, the complicated conditions have occurred frequently in downhole, which influenced the drilling process seriously, fracturing, etc, and restricts the gas pool exploration and exploitation appraisal work.. Research results of domestic and foreign indicate that the relation between down-hole complex conditions and the local stress is close. Based on the Xujiahe formation complex drilling conditions, the rock mechanics trial, stress interpret ,reservoir pressure ,fracture pressure, collapsing pressure and wellbore stability appraisal are researched, guided by the petroleum geology, the structural geology, the geophysical well logging, the reservoir geology, rock mechanics, the rock-mass mechanics, the petroleum well drilling project, the oil gas reservoir transformation, the oil & gas pool engineering and the multi-disciplinary theories and methods.The results indicate that the wave velocity of rock water saturated is quicker than the dry rock sample's wave velocity. Under the same fluid medium saturated, the wave velocity is in reverse proportion with the rock granularity. According the actual P-wave and S-wave data, the thesis establishes the binomial formula of transformational relation between P-wave and S-wave using the least squares method. The thesis realizes transformation of the different frequency wave velocity using the viscoelasticity frequency dispersion equation, resolving the corresponding relation of lithological-electrical parameters, establishing the elastic parameter explanation model and dynamic and static elastic parameters transformation directly basing the simulation formation temperature, the formation pressure and the fluid saturated.Basing on different lithology, the thesis establishes mudstone's compressing strength and the new sand compressing strength formula including information of rock density, acoustic velocity and content of clay, resolving the problem of compressing strength interpretation difficulty. And the thesis forms the rock mechanics parameter experiment, well logging interpretation and the assessment method system. According to the test results, the thesis establishes the interpretation method of Biot's coefficient using non-linear fitting. Through the artificially spot, the thesis achieves the formation pressure explanation plate basing equivalent depth method, and tries to interpret formation pressure using the Eaton method and the effective stress method. Through the sound launch, the wave velocity anisotropy test, the sluggish residual magnetism test, the difference strain test, the special well logging, and the fracturing data, the thesis obtains the stress value and the direction of Xujiahe formation, and the biggest horizontal principal stress's direction is nearly E-W, which slightly changes at different structure position in the plane. In the foundation of comparing various logging stress computation pattern, the thesis selects the ADS method to commutate stress, and regulates the result using the fracturing results to obtain the more reliable stress section.According to the core and well logging, the thesis distinguishes the natural fracture effectively, recognizes the single well fracture basing the R/S method, thus divides the complete wellbore and the non-complete wellbore, and analyzes the wellbore collapsed discipline. The thesis establishes the collapsing pressure assessment method by selecting the Coulomb-Mohr of strength criterion and revising the wellbore non-linear stress coefficient with the drilling mud density. According to the clay mineral analysis, the mudstone inflation experiment, and the dispersive experiment, the thesis discusses and appraises the influence of drilling fluid immersion to the rock mechanical properties basing the simulation formation condition and the different rock sample criterion, as well as the rock mechanical properties influenced by the coupling of mechanics and chemistry. Basing on the core trial, the field test and drilling data analysis, the thesis verifies the rock mechanics strength section, the stress section, the formation pressure explained results, the fracturing pressure, as well as the collapsing pressure section effectively, which the results indicate that the interpretation section's precision is high, the results and drilling data is in a good agreement, which improves that the explanation result has the high reliability.The research establishes a set of engineering geology research technique system from the experimental trial, the rock mechanics parameter explanation, the stress explanation, three formation pressure explanation, and the wellbore stability analysis synthetically. And the results of research provide the powerful technical support on drilling and well completion project processing technology enhancement, guarantee the drilling engineering scientifically and highly effective, safely as well as fracturing .The analysis methods and technological of the thesis is of a certain guiding and referable significance for research the same gas reservoirs.