Physical basis of the PM VSP
The PM VSP method is based on recording on the earth surface of an artificial acoustic wave signal excited by three-component sensors located in the well bore of a logged well.
During processing of the registered wave field from each of the excitation points located on different directions with respect to the logged well, waves of various types (direct, exchange, reflected, etc.) are extracted,Fig. 1, calculation of the distribution of their kinematic and dynamic parameters along the wellbore, and a comparison of these characteristics, depending on the distance from the wellbore and the azimuth of the excitation points to each other and to the GIS data.
The problems solved by the PM VSP
- studying the shape of the acoustic signal, damping and absorption of its energy in the wellbore space in the seismic frequency range;
- determination of the interval model of the velocity of longitudinal and transverse waves in the near-wellbore space;
- study of geomechanical characteristics of subsurface rocks in the wellbore space in the seismic frequency range;
- clarification of the structure of the reservoir, assessment of reservoir properties of reservoir rocks;
- studying the microlaminationof the cut crossed bythe well;
- study of anisotropic properties of rocks in the borehole environment;
- assessment of the nature of rock stratification in the borehole environment;
- assessment of the direction of fracturing of rocks in the borehole environment
- determination of criteria for estimating the oil and gas potential of the penetrated section in the seismic frequency range;
- lithology prediction, velocity distribution, areas dangerous for appearance of brine, abnormally high pore pressure, etc. below the bottom of the well;
- coordination of log and ground seismic data.
Fig.1. Selection of the wave field of the PM VSP for various types of waves
Legend: DP-incident longitudinal wave; UP-ascending longitudinal waves; DS-incident exchange waves; US-ascending exchange waves; UPUPDP-multiple longitudinal waves; UPUPDS - multiple exchange waves.
Experience in the implementation of the method of PM VSP
The company Octopus performed the PM VSP work on 12 wells within the Astrakhan GCF, which has a complex geological structure. The complexity of the geological structure is determined by the complex configuration of the salt bodies (domes) (see Fig. 2), which complicates the use of natural (LFS, LMS, etc.) and artificial (CMP method 2 and 3 D) wavefields to solve geological problems.
The performed works of the PM VSP made it possible to construct an adequate model for the distribution of the elastic parameters of rocks (the interval velocity of longitudinal and transverse waves, Young's modulus and Poisson's ratio, etc.) in the seismic range of vertical resolution in the interwell space.Such a model makes it possible to effectively use the methods of the NSA, CMP method2 and 3 D for studying the inters well space within the Astrakhan GCF.
Fig.2. The geological structure of the territory of Astrakhan GCF along the line that crosses it from the north-west to the south-east