El Lenguaje

SPECIAL SECTION: dT i g hr s g a s m a n d s r a t i o n
Tight gas san Reve t e ti s e mig
s

MIKE MILLER, Cimarex Energy Company
KEITH SHANLEY, The Discovery Group

U

nconventional gas resources continue to be a growing part of the
total gas production in the United
States and have captured the interest
of energy firms around the world.
Unconventional resources comprise
threemajor categories; tight-gas sandstones (low-permeability), shale gas,
and coal-bed methane. Tight-gas
reservoirs currently comprise approximately 35% of the uncon-ventional
gas production in the U.S. Lower 48
(Wood Mackenzie’s Unconventional
Gas Service).
It has long been understood that
significant gas resources are located
in low-permeability rocks. In the late
1970s, the U.S. FederalEnergy Regulatory Commission (FERC) offered
tax incentives for gas produced from
reservoirs with permeability less than
0.1 mD, which accelerated the widespread development of these resources.
Despite more than 30 years of industry activity and the realization that
unconventional gas is a necessary component of U.S. domestic production, Figure 1. (top) Typical pore arrangements in a conventionalreservoir (a) and (b) slot- or
significant challenges remain. This (c) crack-pore arrangement typical of tight gas reservoirs. Petrophysical properties in lowpaper focuses on several fundamental permeability rocks (k < 0.1 mD) are highly sensitive to changes in pressure, saturation,
petrophysical issues that continue to temperature, and time. (bottom) Photomicrographs of the Frontier Formation,Phi = 9.4%
challenge those working in tight gas and Ka = 0.091 mD, in the Greater Green River Basin. (Courtesy of Randall Miller, Core
Laboratories)
provinces.
The intent of this paper is to provide an overview of some significant petrophysical challenges tems can be readily demonstrated by the simple and comthat must be faced if tight gas sandstone resources are to be mon observation ofextensive gas shows encountered during
fully exploited, and if less densely drilled provinces, particu- drilling. Unfortunately, finding gas is only the first step (and
larly those outside North America, are to be explored for new perhaps the easiest) in exploiting these resources. Fundamenresources in tight gas sandstone reservoirs. The petrophysi- tally, the petrophysical challenges surroundidentifying procal challenges that confront tight gas sands can be broadly ducible, economic pay (“moveable” gas), and distinguishing
grouped into four categories: (1) porosity and permeability, it from noncommercial, or subeconomic volumes of gas and/
(2) electrical properties, (3) water resistivity and water satura- or water. In most conventional reservoir systems, insights to
tions determined fromcore, and (4) capillary properties and producibility can be gained from dynamic measurements of
irreducible water saturation. There are, of course, complex in- pressure and sampling reservoir fluids (e.g., from RFT, MDT,
terdependencies between each of these general categories. For or DST tests). In low permeability in tight gas sand systems,
the purposes of this paper, however, each categoryis treated there is often little meaningful real-time dynamic data, and
as a distinct topic in order to provide the reader with a fun- little direct fluid sampling in advance of completion decidamental overview of the key challenges encountered when sions. As a result, wireline data and scattered core data form
evaluating wireline log data or core measurements from tight the basis for the vastmajority of formation evaluation decigas sand reservoirs.
sions. In many tight gas provinces, the uncertainty in formation evaluation is such that wells are routinely drilled, cased,
Petrophysical challenges
perforated, and completed with a seeming inability to disThe presence of ubiquitous natural gas in tight gas sand sys- criminate economic from noneconomic reservoirs and wells.
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