AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Analysis of Reservoir Heterogeneity and Applications of Representative Core Sampling for Gas Shale Characterization
(1) Innovation Center, TerraTek, A Schlumberger company, Salt Lake City, UT.
(2) Texas Engineering Experiment Station, Texas A&M University, College Station, TX.
Natural gas from organic rich shale formations has become an increasingly important energy resource worldwide over the past decade. Extensive hydraulic fracture networks with massive contact surface areas are frequently required to achieve satisfactory economic production in these highly heterogeneous reservoirs, with permeability in the nano-Darcy range. Current operational experience in gas shale plays indicates that the loss of productive fracture area and conductivity, both immediate and over time, are major factors leading to reduced flow rates, poor gas recovery, and marginal production. The RPSEA funded project on “Sustaining fracture area and conductivity of gas shale reservoirs for enhancing long-term production and recovery”, addresses these problems of loss fracture area and conductivity, and proposes to find adequate solutions for various reservoir types. The present study is an important component of the overall RPSEA project.
Unconventional gas reservoirs are formed by interbedded, fine-grained, lithofacies of varying texture and compositions, and exhibiting significant post-depositional alteration. In unconventional plays, the same depositional unit represents the reservoir, the seals, the petroleum source, and the containment for hydraulic fracturing. This means that minor changes in texture and composition result in measurable changes in reservoir and mechanical properties, leading to these various roles. Understanding and mapping the vertical and lateral variability of shale systems is facilitated by identifying principal rock units with characteristic bulk log responses, using Heterogeneous Rock Analysis (HRA). The analysis identifies subtle changes in texture and composition by their effect on the log responses, allowing us to identify changes in rock types within seemingly uniform geologic units. Mapping heterogeneity and identifying the principal rock types representative of the entire system provide us with a consistent sampling methodology. The goal is to maximize representation of the variability while minimizing redundancy. The goal is also to obtain better scaling from plug-scale properties to log-scale behavior, and to develop better models. Results indicate that analysis of heterogeneity and its use in sample selection is fundamental for adequate characterization of heterogeneous systems. The method provides effective representation of the inherent variability of gas shales, and results in better models.