Page 385 - Fundamentals of Gas Shale Reservoirs

APPRAISAL PHASE DISCUSSION 365
for this analysis comes from cuttings, rotary sidewall cores, displays steep initial decline rates and long periods of
conventional core plugs, and whole core. These different transient flow, realized that inaccurate forecasts would
types of analyses can often be combined and performed on result from using this transient production data. Also
the same sample. in 2008, Ilk et al. (2008) introduced the “power law
exponential decline” (form of power law loss ratio) con-
cluding that it offered a better match to production rate
17.3.5 Determining Initial Economic Value and than hyperbolic decline. Others, including Duong (2010),
Reservoir Potential
have also developed and proposed decline curve analysis
An initial assessment of reservoir potential and economic (DCA) methods, and some have offered new techniques
value can be determined from all the above data. Individual for using the material balance approach (Engler, 2000;
operators have different drivers and specific financial Payne, 1996).
and leasehold situations in the United States. Gas price, The industry has taken a traditional approach to devel-
regulatory, and infrastructure are all different for coun- oping shale gas/oil, looking at these unconventional shale
tries outside the United States. Martin and Eid (2011) plays in a statistical manner. Since it is simpler to apply the
cover these topics at length in their paper. The final step of classic decline curve analysis and type curve approach, it is
the exploration phase for the operator is to evaluate to being applied. Expected ultimate recoveries (EURs) are
determine whether or not to proceed with appraisal and also easily determined by calculating the cumulative pro-
development phases. duction from the type curves for 20–30 years. However, the
“average or type curves” that have been developed are not
truly representative of the physics of shale gas flow. Actual
17.4 APPRAISAL PHASE DISCUSSION performance has been found to be quite dissimilar from
these “average or type curves.” Since operators do not
17.4.1 Drill Appraisal Wells—Current Practice understand the exact reasons for the deviation, they have
been limited in their ability to optimize the development and
More wells are drilled during the appraisal phase than in properly prioritize operations based on sound engineering
the exploration phase; thus, data from these additional and geological information. A more reliable analysis and
wells should continue to be used to further characterize predictive approach is needed.
the reservoir.
17.4.4 Build Reservoir Models for
17.4.2 Drill Appraisal Wells—Recommended Practices Simulation—Recommended Practices

Both vertical and horizontal “appraisal” wells should be According to Vassilellis et al. (2010), conventional reservoir
drilled. Vertical wells are required to collect data, and some engineering tools have been found to be inadequate for use
horizontal appraisal wells should be drilled to test the with the change in reservoir characteristics after hydrauli-
hydraulic fracturing and mechanical well completion designs cally fracturing a “shale” well. This complex newly altered
to obtain estimates of initial production potential of the reservoir (after fracturing) must be described and properly
designs. Horizontal wells will also provide information to modeled in order to reliably predict long‐term production
assist in initially determining optimum lateral length, and to and recovery. Vassilellis and his coauthors introduced a mul-
begin early drilling optimization. tidisciplinary integrated approach called “shale engineering.”
Shale engineering involves building three models—reservoir,
well, and fracturing models—and tuning the models for
17.4.3 Build Reservoir Models for Simulation— reliable long‐term prediction and recovery. Data and anal-
Current Practice
ysis techniques involve the disciplines of geology, petro-
Applying classic reservoir engineering techniques to physics, geomechanics, geochemistry, seismology, and, of
unique shale reservoirs is problematic due to the length of course, reservoir engineering. Application of the shale engi-
time to reach pseudo‐steady‐state flow and/or establish neering techniques has been documented by Vassilellis et al.
a constant drainage area. This leads to the inability to (2011) and Moos et al. (2011). The workflow developed for
accurately estimate the recoverable reserves in a timely this process is shown in Figure 17.3. Cipolla et al. (2009a, b)
and consistent manner. Both decline curve and material also have introduced a new approach to more comprehensive
balance methods have been found to have serious draw- modeling of complex shale. Holditch (2006) concludes that
backs when applied to shale gas reservoirs that had not the most accurate reservoir analysis technique for tight gas
established a constant drainage area. Kupchenko et al. (and shale gas reservoirs) is to build a reservoir model that
(2008), upon recognizing that production performance includes layers, and it is also suggested that a dual porosity
from tight gas reservoirs (similar to shale reservoirs) model be used.