Rational Rock Physics for Improved Velocity Prediction and Reservoir Properties Estimation for Granite Wash (Tight Sands) in Anadarko Basin, Texas

Durrani Muhammad Z. A., Willson Keith, Chen Jingyi, Tapp Bryan, and Jubran Akram

Abstract

Characterizing and evaluating elastic properties (seismic wave velocity and moduli) for the prolific Granite Wash formations (Pennsylvanian section) in the western Anadarko Basin Wheeler County (Texas), is quite a challenge. Complex nature of the reservoir (tight gas sands) and the diagenetic and pore shape influence on the P- and S-wave velocities (or elastic properties) are still indistinct and limit the applications of seismic methods in accurate prediction of reservoir properties. The elastic moduli of rocks depend directly on the rock texture, fluid-types, and pore-shape geometry. Many other factors such as consolidation, cementation and pressure indirectly affect the direct factors. Rational rock physics, precisely consistent with specific microstructure of sedimentary facies, is required to comprehend these factors that control the seismic velocities (or moduli) response.

Our main objective is to exploit a consistent methodology, which can systematically integrate small-scale rock microstructure using rock physic approach for low porosity-permeability sandstone reservoirs in order to describe the diagenesis of Granite Wash and an accurate estimate of seismic (P- and S-wave) velocity. Therefore, the significant part of this study is to build accurate rock-physics modeling of Granite Wash to quantify the rock texture (e.g., diagenetic cement, grain size sorting and clay effect). Here, physical-contact (Hertz-Mindlin) and cementation (Dvorkin’s) theories are applied to analyze the nature of the reservoir rocks (consolidated and cemented). The Hertz-Mindlin model is the most commonly used contact model for sandstone of perfectly elastic spherical bodies developed for grain contact under normal and tangential load/stress which mimics the elastic signatures of porosity reduction associated with depositional sorting and diagenesis. Cementation theory assumes that porosity reduces due to the uniform deposition of cement layers between the grains. This cement may be diagenetic quartz, calcite, or authigenic clay (such as illite). The contact cement dramatically increases the stiffness of the sand by reinforcing the grain contacts, which ultimately results an increase in elastic moduli.

Additionally, an effective medium modeling-based approach is adopted in order to understand the influence of pore geometry, a difficult parameter to quantify that affects the elastic moduli in a fundamental way. The uncertainty in elastic moduli is to a large extent due to the uncertainty in pore geometry. Pore geometry is a critical for the meaningful assessment of a geologic and quantitative integration. Three classical rock physics models (Empirical relations, Kuster-Toksöz, Berryman models) are comparatively analyzed for an accurate velocity prediction results which are aimed at determining the most favorable model to consider the influence of pore shape geometry. The relationships between the P- and S-wave velocities are discussed from different aspect ratios based on actual well data. These models predict the velocities of Granite Wash reservoir in different accuracies between the data measured and those predicted by different models. By employing the optimal rock physics model to comprehend the impact on reservoir quality due to variations in porosity and related elastic properties. Significantly, the results predicted by the Berryman’s model agree well with those measured. The model has the best predicted results because it can adjust for pore aspect ratio or the proportion of various 3D pores, thus effectively reconstruct the geometries of pore shape, which are the dominant factors influencing the velocities of the elastic-wave propagating in these type of complex reservoirs

In order to account for the influence of pore-shape on Vp-Vs relationship, an empirical (Vp-Vs) relationship is generated and calibrated with core data for Granite Wash. The generated relationship yields a reasonable trend of shear wave velocity, while other alternative relationships (Castagna’s mud rock line) for Granite Wash are not consistent and over-predict shear wave velocity. Furthermore, Berryman’s model (suitable for this particular reservoir) is applied to study the fluid substitution. Hence, the effects of various inclusion contents (i.e., brine and gas/oil) on seismic velocity and moduli-porosity relations show wide variations in logs-core data that might be due to variable crack porosity.

Based on the core study, numerical modeling analysis on well logs data exhibits the variations in the elastic properties of Granite Wash and can be explained by two main diagenetic phases in Granite Wash reservoirs: 1) In a proximal submarine fan setting, where Granite Wash is in thick-bedding (amalgamated) sand sheets, poorly sorted and coarse-grains lack cementation. At this phase, the Granite Wash remained less effected by the depositional period and has low elastic moduli. Elastic properties of this kind of Granite Wash can be calculated by using the Hertz-Mindlin contact model. 2) Microcrystalline quartz cementation between the grains causes a major decrease in porosity and an increase in elastic moduli, so the contact model (Hertz-Mindlin) is not in agreement with well logs and laboratory-measured data, and the cementation (Dvorkin’s) model best describes the elastic properties of the reservoir. An empirical Vp-Vs relationship, built on the basis of velocity prediction by effective medium modeling and calibrated with core data for the Granite Wash reservoir, provides a simple relationship honoring the reservoir’s pore shape geometry, while other alternative relationships (Castagna’s mud rock line) over- predict shear wave velocity. This work has important practical significance for AVO modeling and can to improve reservoir prediction results.

The research outcomes suggest that the adopted methodologies will certainly improve the industry’s ability to quantify the rock texture of Granite Wash based on well data, and the models established here, when calibrated with core study results, have the potential to be used as guides for relating elastic properties (elastic wave velocity and moduli) to key reservoir properties (Vsh, Phie, Phit and Sw), which provides us with a new understanding of AVO and seismic reservoir characterization.

Acknowledgement

We gratefully acknowledge the financial support of Newfield Exploration Company and Fulbright program under Bureau of Educational and Cultural Affairs, U.S. Department of State.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013