Research On Brittleness Evaluation And Main Controlling Factors Of Tight Oil Reservoirs

The research of rock mechanics is a major problem for the unconventional tight oil reservoir,because the conventional reservoir evaluation can no longer meet the current research needs of tight oil reservoirs.How to combine conventional reservoir parameters with engineering parameters to evaluate is not only the focus of researchers,but also the objective requirements of tight oil exploration and development.Brittleness index is the most important parameter in brittle evaluation,studying the evaluation method and analyzing the main controlling factors of brittleness index has important guiding significance for further engineering sweet spot prediction.This thesis conducts a systematic evaluation on brittleness,and study its main controlling factors of Chang7 reservoir of Triassic Yanchang Formation in Dongrengou oil area,based on the basic data of drilling,logging and production of 295 wells and combined with the core observation and analysis of test data,using petroleum geology,reservoir sedimentology,rock mechanics,brittleness characterization and other multidisciplinary methods.The distribution ranges of dynamical Young’s modulus and dynamical Poisson ratio of Chang7 reservoir of Yanchang Formation in the study area obtained by array sonic logging are34～45GPa and 0.25～0.35,respectively.The static rock mechanical parameters can be obtained by uniaxial and triaxial compression experiments on rock samples,which are static Young’s modulus ranging from 15 to 45 GPa and static Poisson’s ratio ranging from 0.1 to 0.3,respectively.The static rock mechanical parameters are generally smaller than the dynamic parameters,which are be applied to the engineering evaluation of the whole area by converting the dynamic to static rock mechanics parameters.The brittleness index of the study area was evaluated by mineral components,rock mechanics parameters,and stress-strain curves,that the average values of the brittleness indexes are 0.74,0.46,and 0.92,respectively.The value ranges of three methods are different,but there was a certain similarity of variability.The maximum and minimum horizontal crustal stress of the Chang 7 member in the study area is 64.01 MPa and 46.98 MPa,respectively,indicated by the combining of rock mechanics experiment and array sonic logging to evaluate horizontal crustal stress and anisotropy,which show the direction and strength of anisotropy is relatively stable,and the maximum principal stress of the interval is about NE60°-75°.The brittleness index of the whole area is obtained by the rock mechanics parameter method,which carried out continuously brittleness evaluation of single well.The high sand content,with higher acoustic travel time and lower gamma ray,also show higher brittleness index,such as better compressive resistance and Young’s modulus.The brittleness index is further used as the main parameter to research the brittleness distribution,and the study showed that the distribution of the brittleness index in the study area extends along the NE～SW direction in a strip shape.Rock brittleness is mainly controlled by lithology,physical property,horizontal crustal stress anisotropy and sedimentary facies zone.Rock brittleness index is higher with the higher content of brittle minerals and the better the physical properties,and it also higher along the direction of the minimum horizontal crustal stress or closer to the center of the channel with the larger sand body thickness.