Estimation of anisotropy parameter γ in Kangan and Dalan Formations by DSI in a well at South Pars field

Anisotropy has an important role in exploration and reservoir characterization. Inpractice, the determination of seismic anisotropy is not easy, but it has importantconsequences in enhancement of seismic data recording and processing. Anisotropyinteracts with reflection seismology, acquisition, processing and interpretation. Ignoringanisotropy can lead to poor seismic imaging, misleading the seismic reflector responses,inaccurate location of well-ties, and incorrect interpretation of seismic arrival times andamplitudes for the determination of lithology and fluid content.Shear wave velocity anisotropy is commonly referred to as shear wave splitting,because a shear wave traveling in an heteregeneous medium splits into two shear waves.At a given receiver, shear waves are characterized by their orthogonal polarizationdirections (fast and slow) and a delay between their arrival times.The most common anisotropic models have been related to the framework oftransverse isotropy or a hexagonal isotropy system. When the symmetry axis is alignedwith the vertical axis, the model is called vertical transverse isotropy or VTI. For a VTImedium, there are five stiffness coefficients and three independent phase velocities.Thomsen (1986) replaced these stiffness coefficients with two vertical velocities (Vp0 andVS0) and three dimensionless anisotropy parameters (namely, ε, γ and δ). Anisotropyparameters can be determined in several ways, including velocity measurements on coresamples in a laboratory or from field data in a VSP experiment. A common form ofanisotropy observed in many geological area (thinly horizontal layers or fractures). Thisinvolves the reference axis of symmetry being normal to the bedding surfaces. Thomsen(1986) introduced three anisotropic parameters (ε, γ and δ) to describe weak anisotropy,which is believed to be the simple model of anisotropy. Thomsen parameters can becomputed with the stiffness tensor considered in anisotropic media. Alkhalifah andTsvankin (1995) showed that, for P-wave Moveout, there exists a range of kinematicallyequivalent models which are governed by the stacking velocity and introduced by theparameter η.The Dipole Shear Sonic Imager (DSI) is an example of devices that are used to obtainand analyze sonic measurements of formations surrounding a borehole. The DSI Imagercan measure the components of shear slownesses in many directions in a planeperpendicular to a borehole axis. The DSI tool is a full waveform acoustic tool thatdelivers measurements of sonic waves in a wide variety of formations. In theconventional DSI logging tool, one can present compressional slowness, Δtc, shearslownesses, Δts, and Stoneley slowness, Δtst, each as a function of depth. The DSI tool canestimate the orientation and magnitude of stress from velocity dispersion. By invertingthe dispersion curves from DSI logs, one can estimate the horizontal stresses. One type ofthese special dipole modes enables the recording of both inline and crossline(perpendicular) waveforms. These modes, both called cross receivers (BCR) which areused for anisotropy evaluation.In this paper, one of the anisotropy parameters of Thomsen (γ) was determined by theuse of S-wave velocities and their relationship with the DSI tool used in the Kangan andDalan gas zones of the South Pars field. Subsequently, the γ parameter was comparedwith the Gamma Ray log in depth. The results show anisotropy behavior in shaly zones ofKangan and Dalan Formations. It is found that the average of the γ parameter for theKangan and Dalan Formations are 0.015 and 0.02, respectively. Also, this parameter wascompared with slowness based on anisotropy. A good correlation was observed betweenanisotropy parameter γ and the slowness based on anisotropy (slowness vector).