Study On Numerical Calculation Method Of Staggered-Grid Finite Difference For Response Of Anomalous Body Near Well

Logging is one of the important methods for obtaining oil and gas reservoir data.Electrical logging is a widely used logging method.The existing electrical logging tools can no longer meet the needs of geosteering while drilling.With the deepening of oil and gas exploration and production,there is an urgent need to detect and identify anomalous bodies and stratum structures further away,avoid harmful formations in advance,and maximize production efficiency and economic benefits.This paper mainly studies the numerical calculation technology of the response of anomalous bodies based on the staggered grid finite difference numerical calculation method,analyzes the response logging characteristics of underground faults and anomalous bodies,and develops corresponding software for the inversion of anomalous body parameters around the well and surrounding resistivity imaging provides a tool for numerical calculation of forward response,and provides software support for the engineering implementation of a new logging tool for remote detection.The main research results are divided into the following four parts:The first part describes the basic concept and classification of formation electrical anisotropy.Combining the basic theory of dual-coil system and triple-coil system induction logging in isotropic formation,the structure and principle of induction logging tool for electrically anisotropic formation are studied.The second part studies the numerical calculation method of induction logging for electrically anisotropic formations.Starting from the Maxwell equations,the discrete calculation formula of the three-dimensional time-domain staggered grid finite difference method for induction logging tools is derived,and then the three-dimensional time domain problem is converted into a 2.5-dimensional frequency domain problem,which simplifies the calculation process and uses preprocessing.The double conjugate gradient method is used to solve the large-scale sparse equation system formed by the difference discrete,at the same time,the numerical solutions of the electric field strength and the magnetic field strength are obtained,and finally the apparent conductivity of the formation is calculated.Completion of the 2.5D staggered grid finite difference numerical calculation program.The third part studies the response characteristics of fault logging.Calculate and analyze the logging response when the factors such as fault dip angle,fault distance,fault thickness and instrument trajectory in the fault change,and establish corresponding numerical calculation models for different factors.The results show that when the instrument enters the lower reservoir from the upper reservoir,the change of fault dip angle is proportional to the change of apparent conductivity.In the case of other borehole trajectories,the change of fault dip angle has little effect on the value of array induction logging response;the long array is more sensitive to the detection of the change in fault distance and reservoir thickness;the location of the fault plane can be determined according to the change of the logging curve.The fourth part studies the response of abnormal body induction logging.The numerical calculation model of anomalous body was established,and the array induction logging response when the factors such as conductivity contrast,anomalous body geometry and anomalous body distribution mode were changed was calculated and analyzed.The research shows that the intersection of the short array induction logging curve is the location of the interface between the anomalous body and the surrounding rock;the change of the anomalous body thickness(z-axis direction)and width(x-axis direction)will affect the logging response.The interval between the intersection points of the rate curve is the thickness of the anomaly;when two anomalies are placed side by side horizontally,the different horizontal distances mainly affect the value of the long-field logging response value.The distance between the intersection points can determine the thickness of the abnormal body and the longitudinal distance between the abnormalities.