Modelling Orthorhombic Anisotropic Effects for Reservoir Fracture Characterization of a Naturally Fractured Tight Carbonate Reservoir, Onshore Texas, USA

Abstract

Abstract-In this study we present a step-by-step theoretical modelling approach, using established seismic wave propagation theories in anisotropic media, to generate unique anisotropic reflection patterns observed from three-dimensional pure-mode pressure (3D-PP), full-azimuth and full-offset seismic reflection data acquired over a naturally fractured tight carbonate field, onshore Texas, USA. Our aim is to gain an insight into the internal structures of the carbonate reservoir responsible for the observed anisotropic reflection patterns. From the generated model we were able to establish that the observed field seismic reflection patterns indicate azimuthal anisotropy in the form of crack induced shearwave splitting and variation in P-wave velocity with offset and azimuth. Amplitude variation with azimuth (AVAZ) analysis also confirmed multi-crack sets induced anisotropy which is characteristic of orthorhombic symmetry, evident as multiple bright and dim-amplitude azimuth directions as well as complete reversal of bright-amplitude to dim-amplitude azimuth direction as the angle of incidence increases from near (B15°) to mid (C30°) offsets. Finally, we fitted the generated P-wave velocity into an ellipse to determine the intensity and orientation (N26E) of the open crack set as well as the direction of the minimum in situ stress axis (N116E) within the reservoir. The derived information served as an aid for the design of horizontal well paths that would intercept open fractures and ensure production optimization of the carbonate reservoir, which was on production decline despite reservoir studies that indicate un-depleted reserves.

Orthotropic anisotropy: on contributions of elasticity parameters to a difference in quasi‐P‐wave‐squared velocities resulted from propagation in two orthogonal symmetry planes

“…If we know the values of density and elasticity parameters, we can numerically show how the velocity is expected to change with various polar and azimuthal phase angles. Such an analysis was performed, for instance, inSchoenberg and Helbig (1997) or Osinowo et al (2017). However, it is a more difficult task to grasp how much the particular elasticity parameters contribute to the magnitude of phase velocity in general; especially, when we do not know their values.…”

Uma comparação numérica de aproximações de tempos de trânsito para ondas qP em meios ortorrômbicos

“…Como muitas rochas possuem matriz polar e são fraturadas devido às altas tensões a que são submetidas, desprezar o efeito azimutal na propagação das ondas devido à anisotropia ortorrômbica pode acarretar em imprecisões no imageamento e em processos de inversão. Em particular, identificar e caracterizar o sistema de fraturas em rochas reservatório e selante é importante para a exploração e a produção de petróleo, até mesmo em alguns reservatórios carbonáticos 13 (Osinowo et al, 2017).…”

“…Apesar da anisotropia ser um fenômeno elástico e influenciar as características da propagação dos três modos de onda, o efeito azimutal devido ao caráter ortorrômbico do meio na velocidade de fase pode ser mais significante na onda quase-compressional do que nas demais (Osinowo et al, 2017). Além disso, o custo computacional para tratar a equação completa da onda pode tornar o problema impraticável mesmo em meios mais simples, como o isotrópico ou polar.…”