Page 303 - Fundamentals of Gas Shale Reservoirs

13

PERFORMANCE ANALYSIS OF UNCONVENTIONAL
SHALE RESERVOIRS

Hossein Kazemi , Ilkay Eker , Mehmet A. Torcuk and Basak Kurtoglu 2
1
1
1
1 Colorado School of Mines, Golden, CO, USA
2 Marathon Oil Company, Houston, TX, USA

13.1 INTRODUCTION by Arps (1944). Interestingly enough, this mathematical
model is closely related to the pseudo-steady‐state flow in
Producing oil and gas from nano‐Darcy shale formations has high‐permeability conventional reservoirs. However, when
become possible because of intuitive insights of geoscien- applied to unconventional shale reservoirs, we will show that
tists and engineers, ingenuity of well completion engineers, it starts with transient flow behavior during early production
and persistent field tests in the last two decades of the twen- and converges to boundary‐dominated flow (BDF) later.
tieth century. Specifically, in late 1990s, Mitchell Energy Five topics are the focus of this chapter: shale reservoir
began producing commercial gas from Barnett shale using production, flow rate decline analysis, flow rate and pressure
slickwater for hydraulic fracturing instead of the polymer transient analysis, reservoir modeling and simulation, and
gel fracking fluids. Then, around 2006, EOG Resources, Inc. enhanced oil recovery (EOR).
(EOG) began producing more oil from its North Dakota
Bakken leases, using slickwater in the implementation of
multistage hydraulic fracturing (Zuckerman, 2013). This 13.2 SHALE RESERVOIR PRODUCTION
achievement spread to Eagle Ford shale and other US
shale resources. In 2014, the United States produces Shale is a fissile mudstone consisting of silt, 4–60 µm, and
nearly 3.5 million barrels/day of new oil from shale reservoirs clay‐size particles, less than 4 µm, which are largely mineral
across the country. fragments. Shale hydrocarbon reservoirs, in addition to min-
This chapter presents practical approaches for analyzing eral fragments, include a small amount of organic matter.
well performance in shale oil and gas reservoirs. In this Under large overburden stress and high temperatures, the
regard, the first notable observation is the vast contrast bet- organic material slowly converts to hydrocarbon components
ween core‐measured permeability versus field‐measured that create a large internal hydrostatic pressure locally, which
permeability from flow tests. Specifically, core‐measured could cause creation of microfracture pores because of the
permeabilities are two to three orders of magnitude lower fluid expansion force. The pore size in shale could be less
than field‐measured permeabilities. The prevailing explana- than 2 nm and as high as 2 µm. Nanopores create large capil-
tion for the larger field‐measured permeability points to lary pressures, lower the critical pressure, and temperature of
formation of extensive micro and macro fracture network as hydrocarbon components creating a shift in the phase
byproduct of the multistage well stimulation. The second envelope of the resident fluids, and cause capillary condensa-
notable observation is the rapid decline of well flow rates for tion and slippage of gas molecules at the pore walls (Knudsen
a short period of time but stabilizing to a gentler decline rate flow). Because of low matrix permeability, Darcy flow
for months and years. For long‐term forecasting, engineers (advection) becomes so small that molecular diffusion can
use an empirical analysis method, introduced to the industry play a significant role in the mass transfer of fluids from the

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