Rock Typing - Understanding Productivity in Tight Gas Sands

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 114164, "Rock Typing - Keys to Understanding Productivity in Tight Gas Sands," by J.A. Rushing, SPE, Anadarko Petroleum, K.E. Newsham, SPE, Apache, and T.A. Blasingame, SPE, Texas A&M University, prepared for the 2008 SPE Unconventional Reservoirs Conference, Keystone, Colorado, 10-12 February. The paper has not been peer reviewed. A workflow process is presented to describe and characterize tight gas sands. The ultimate objective is to provide a consistent methodology to integrate both large-scale geologic elements and small-scale rock petrology systematically with the rock physical properties for low-permeability sandstone reservoirs. To that end, the workflow integrates multiple data-evaluation techniques and multiple data scales by use of a core-based rock-typing approach designed to capture rock properties that are characteristic of tight gas sands. Introduction Unconventional natural-gas resources—tight gas sands, naturally fractured gas shales, and coalbed-methane reservoirs—comprise a significant percentage of the North American natural-gas resource base. Unlike conventional reservoirs, unconventional gas reservoirs typically exhibit gas-storage and -flow characteristics that are tied to geology (i.e., deposition and diagenetic processes). Effective resource exploitation requires a comprehensive reservoir-description and -characterization program to quantify gas in place and to identify the reservoir properties that control production. Although many unconventional natural-gas resources are characterized by low permeability, this paper addresses only low-permeability sandstone reservoirs (i.e., tight gas sands). Understanding the pore structure and properties is critical in tight gas sands because diagenesis often modifies the original pore structure and reduces the average pore-throat diameter, typically causing an increase in both tortuosity and the number of isolated and/or disconnected pores. Some forms of diagenesis may increase porosity by creating secondary porosity, or microporosity. Regardless of the type of diagenesis, all tight gas sands retain some underlying traits of the depositional system, even though the original rock properties may have been altered (significantly). However, well productivity cannot be predicted accurately solely on the basis of the rock properties expected for those specific depositional environments and conditions. Therefore, a rock-typing work-flow process was developed specifically for tight gas sands. Many basin-centered gas-reservoir systems can be classified as tight gas sands. However, the characterization of basin-centered gas systems typically requires knowledge of the rock and multiphase-fluid properties and of the role of shale. Although basin-centered gas-reservoir systems are not specifically addressed in this paper, the proposed rock-typing approach is applicable to basin-centered gas systems.