Page 321 - Fundamentals of Gas Shale Reservoirs

14

RESOURCE ESTIMATION FOR SHALE GAS RESERVOIRS

Zhenzhen Dong , Stephen A. Holditch and W. John Lee 3
2
1
1 PTS, Schlumberger, College Station, TX, USA
2 Petroleum Engineering Department, Texas A&M University, College Station, TX, USA
3 UH Energy Research Park, University of Houston, Houston, TX, USA

14.1 INTRODUCTION the target zone and has been successfully fracture treated,
the free gas flows quickly, causing an initial high production
Many gas shale plays are currently under development in the rate. Production then plateaus as the natural gas absorbed in
world oil and gas industry. The use of horizontal drilling in the rock is removed. Thus, for a typical shale gas well, pro-
conjunction with hydraulic fracturing has greatly expanded duction declines between 70 and 90% in the first year, and an
the ability of producers to profitably produce natural gas overall average well life may be of the order of 20–30 years.
from low‐permeability geologic formations, particularly
shale formations.
14.1.2 Petroleum Resources Management
System (PRMS)
14.1.1 Unique Properties of Shale
The terms “resources” and “reserves” have been used in the
Shale gas refers to natural gas (mainly methane) in fine‐ past and continue to be used to represent different classifications
grained, organic‐rich rocks (gas shales). When talking about of mineral and/or hydrocarbon deposits. In March 2007,
shale gas, the word shale does not refer to a specific type of the Society of Petroleum Engineers (SPE), the American
rock. Instead, it describes rocks with more fine‐grained par- Association of Petroleum Geologists (AAPG), the World
ticles (smaller than sand) than coarse‐grained particles, such Petroleum Council (WPC), and the Society of Petroleum
as shale (fissile) and mudstone (nonfissile), siltstone, fine‐ Evaluation Engineers (SPEE) jointly adopted and published
grained sandstone interlaminated with shale or mudstone, the PRMS to provide an international standard for classification
and carbonate rocks. of oil and gas reserves and resources (Fig. 14.1a). Technically
Gas is stored in shales in three ways: (i) adsorbed gas is and ERR, commonly used by the Energy Information
the gas attached to organic matter or to clays; (ii) free gas is Administration (EIA), are not formally defined in PRMS.
the gas held within the tiny spaces in the rock (pores,
porosity, or microporosity) or in spaces created by the rock 14.1.3 Energy Information Administration’s
cracking (fractures or microfractures); and (iii) solution gas Classification System
is the gas held within other liquids, such as bitumen and oil.
Gas shales are source rocks that have not released all of their According to the Energy Information Administration (EIA),
generated hydrocarbons. In fact, source rocks that are “tight” TRR is the subset of the total resource base that is recover-
or “inefficient” at expelling hydrocarbons may be the best able with existing technology. The term “resources” repre-
prospects for potential shale gas. sents the total quantity of hydrocarbons that are estimated, at
Since natural gas occurs both as free (as around the rock a particular time, to be contained in (i) known accumulations
structure) and within the rocks, once the wellbore reaches and (ii) accumulations that have yet to be discovered

Fundamentals of Gas Shale Reservoirs, First Edition. Edited by Reza Rezaee.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.