Fundamentals of Gas Shale Reservoirs

Reza Rezaee

• Publisher: Wiley
• Year: 2015
• Print ISBN: 9781118645796
• eBook ISBN: 9781119039266
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Fossil Fuels
• Index: Technology & Engineering

Provides comprehensive information about the key exploration, development and optimization concepts required for gas shale reservoirs

• Includes statistics about gas shale resources and countries that have shale gas potential
• Addresses the challenges that oil and gas industries may confront for gas shale reservoir exploration and development
• Introduces petrophysical analysis, rock physics, geomechanics and passive seismic methods for gas shale plays
• Details shale gas environmental issues and challenges, economic consideration for gas shale reservoirs
• Includes case studies of major producing gas shale formations

1
GAS SHALE: GLOBAL SIGNIFICANCE, DISTRIBUTION, AND CHALLENGES

REZA REZAEE¹ AND MARK ROTHWELL²

¹ Department of Petroleum Engineering, Curtin University, Perth, WA, Australia

² HSEassist Pty Ltd, Perth, WA, Australia

1.1 INTRODUCTION

The central geological properties of a shale gas play are generally assessed in terms of depositional environment, thickness, organic geochemistry, thermal maturity, mineralogy, and porosity. The key features of successful shale gas plays include high total organic carbon (TOC) content (>2%), thermally mature (Ro 1.1–1.5%), shallow for the given maturity, and a low clay content/high brittle mineral content. However, porosity, in situ stress regime, stress history, and mineralogy are also significant factors.

Technically recoverable (although not necessarily economically recoverable) gas shale is abundant across the globe. It is also located in a very wide range of geographical regions, and in many of the nations with the highest energy consumption. For certain nations, shale gas therefore has the potential to reduce energy prices and dependence on other nations, hence impact on both the political and economic outlook. However, the prospects for and significance of shale gas are greater where there is a lack of existing conventional gas production, where there is proximity to demand (i.e., population), and where some form of existing gas distribution infrastructure exists.

The definition of a “resource” can follow a number of classifications. However, in the context of this chapter, the class of “technically recoverable resources” (TRRs) has been adopted, which includes both economic and uneconomic resources.

The assessment of the global data included the identification of the shale depositional environment and basin type. A brief summary of the shale gas plays is presented for each country, which is followed by a statistical assessment of certain data subsets to illustrate where shale gas is located, the expected range of properties in terms of TOC, depth, age, and basin type.

There are a number of key challenges that the industry faces, including environmental issues and commercial challenges. The key issues relate to the management of the hydraulic fracturing process, the prediction and improvement of EUR/well, and the consideration of variable production costs in different regions.

1.2 SHALE GAS OVERVIEW

In very simple terms, shale gas refers to gas produced from fine-grained gas-prone sedimentary rocks (i.e., organic-rich shale) (Lakatos and Szabo, 2009). Shale gas is considered an “unconventional” gas resource, since conventionally gas is produced from granular, porous, and permeable formations (i.e., sandstone), within which gas can readily flow. Although shale gas is considered an unconventional hydrocarbon resource, the gas produced essentially serves the same market (Staff, 2010). The term “unconventional,” therefore, only refers to the rock from which the natural gas produced in this particular case.

In a conventional gas play, gas shale¹ is often present, but it serves as the source rock rather than the reservoir. Shale, as a function of its traditionally low permeability, also often serves as a sealing lithology within the trapping mechanism of a conventional gas play, which prevents oil and gas accumulations from escaping vertically (Gluyas and Swarbrick, 2009).

Generic global hydrocarbon estimates have always somewhat reflected resources in-place within tight formations and shale. However, it is the relatively recent technological developments and higher gas prices that have now resulted in a vast resource being considered potentially economic, which had previously been considered uneconomic to develop (Ridley, 2011).

Sources indicate that shale is present in a very wide range of regions across the globe, with an estimated 688 shale deposits occurring in approximately 142 basins (Ridley, 2011).

1.2.1 Shale Gas Geology

Shale gas is a natural gas produced from organic-rich fine-grained low-permeability sedimentary rocks, such as shale, where the rock typically functions as both the “source rock” and the “reservoir rock,” to use terms associated with conventional plays (US DOE, 2009). The relationship between conventional and unconventional gas is illustrated in Figure 1.1.

Figure 1.1 Schematic geological section illustrating the fundamental geological principles associated with conventional and unconventional hydrocarbons. Shale gas is designated as “gas-rich shale” (from EIA, 2010).

Gas shale is similar to traditional shale in terms of the range of environments of deposition. For example, Caineng et al. (2010) note that organic-rich shale can be divided as marine shale, marine–terrigenous coal bed carbonaceous shale, and lacustrine shale. The depositional setting directly controls key factors in shales, such as organic geochemistry, organic richness, and rock composition. According to Potter et al. (1980), the organic matter preserved in shales depends on the dissolved oxygen level in the water.

Shale gas organic geochemistry is a function of the depositional environment and is similar to conventional source rock geochemistry. Marine shale is typically associated with Type II kerogen (i.e., organic matter associated with a mixture of membraneous plant debris, phytoplankton, and bacterial microorganisms in marine sediments). Lacustrine shale is generally associated with Type I kerogen, due to the organic matter being associated with an algal source rich in lipids (typically only in lacustrine and lagoona...

Table of contents

• COVER
• TITLE PAGE
• TABLE OF CONTENTS
• CONTRIBUTORS
• PREFACE
• 1 GAS SHALE
• 2 ORGANIC MATTER-RICH SHALE DEPOSITIONAL ENVIRONMENTS
• 3 GEOCHEMICAL ASSESSMENT OF UNCONVENTIONAL SHALE GAS RESOURCE SYSTEMS
• 4 SEQUENCE STRATIGRAPHY OF UNCONVENTIONAL RESOURCE SHALES
• 5 PORE GEOMETRY IN GAS SHALE RESERVOIRS
• 6 PETROPHYSICAL EVALUATION OF GAS SHALE RESERVOIRS
• 7 PORE PRESSURE PREDICTION FOR SHALE FORMATIONS USING WELL LOG DATA
• 8 GEOMECHANICS OF GAS SHALES
• 9 ROCK PHYSICS ANALYSIS OF SHALE RESERVOIRS
• 10 PASSIVE SEISMIC METHODS FOR UNCONVENTIONAL RESOURCE DEVELOPMENT
• 11 GAS TRANSPORT PROCESSES IN SHALE
• 12 A REVIEW OF THE CRITICAL ISSUES SURROUNDING THE SIMULATION OF TRANSPORT AND STORAGE IN SHALE RESERVOIRS
• 13 PERFORMANCE ANALYSIS OF UNCONVENTIONAL SHALE RESERVOIRS
• 14 RESOURCE ESTIMATION FOR SHALE GAS RESERVOIRS
• 15 MOLECULAR SIMULATION OF GAS ADSORPTION IN MINERALS AND COAL
• 16 WETTABILITY OF GAS SHALE RESERVOIRS
• 17 GAS SHALE CHALLENGES OVERTHE ASSET LIFE CYCLE
• 18 GAS SHALE ENVIRONMENTAL ISSUES AND CHALLENGES
• INDEX
• END USER LICENSE AGREEMENT