Page 383 - Fundamentals of Gas Shale Reservoirs

EXPLORATION PHASE DISCUSSION 363
17.3.2 Screening Study Recommended Practices assessment consisting of detailed mineralogical, structural,
and geomechanical characterization (pulsed neutron spec-
Conduct a basin screening study prior to entering a new troscopy, nuclear magnetic resonance, and acoustic logs).
basin or area that involves gathering and analyzing data These data are more location specific, while information
including:
gathered through seismic is broad, spread out, and certainly
not as accurate as that obtained from an actual wellbore pen-
• Geology—sedimentology, stratigraphy, and deposi-
tional environment etrating the formation. The detailed discussion of this topic
• Geochemistry—TOC (initial reserve estimate), thermal is included later in this chapter. Seismic information is rele-
vant through the exploration, appraisal, development, and
maturity (type of hydrocarbon) rejuvenation phases of the life cycle (Kennedy et al., 2012a).
• Geomechanics—stress regime for well drilling and Also, during the exploration phase the operator should begin
fracturing design and placement to characterize the reservoir from the exploratory wells.
• Petrophysics—rock type, lithology/mineralogy,
porosity (from cores and logs)
• Existing well data 17.3.3 Reservoir Characterization—Current Practice
Many US operators will log and core vertical exploratory
To begin initial characterization of the reservoir, it is recom- and appraisal wells to gather data for overall reservoir char-
mended that 3D seismic be conducted over the potential play acterization; however, very little data are collected on the
area. From 3D seismic the typical information on faults, lateral of horizontal appraisal wells. Industry data indicate
formation thickness, depth, and lateral continuity can be that in the United States less than 9% of horizontal laterals
obtained. However, 3D seismic can also be used to of development wells are logged or any data gathered about
the reservoir along these 5000–6000 ft horizontals. Such
• Identify areas of highest TOC using acoustic data could be used to refine the characterization of the
impedance reservoir, and lead to more intelligent placement of future
• Increase understanding of natural fractures using development wells.
seismic attributes
• Provide azimuthal anisotropy data related to natural 17.3.4 Reservoir Characterization—Recommended
fracture orientation and horizontal stress anisotropy Practices
• Assist in identification of sweet spots using seismic Open‐hole logs (conventional, pulsed‐neutron, and spectros-
cross‐plots
copy) and cores from exploratory wells provide data for pet-
rophysical analysis for initial reservoir characterization for
An optimal reservoir characterization workflow for stress
analysis/seismic interpretation is described by Sena et al. shale reservoirs. Wellbore image logs and nuclear magnetic
(2011), and the primary steps are listed in the following: resonance (NMR) logs provide necessary information for
shale. An example of one of these special log and analysis
• Seismic rock properties (advanced rock properties techniques is shown in Figure 17.2. This shale gas facies
analysis) expert system provides operators with a quick and accurate
• Azimuthal analysis method of classifying shale gas reservoirs, identifying favor-
able zones for hydraulically fracturing, identifying frac bar-
° Azimuthal migration riers, and locating zones from which to drill horizontal
° Reservoir oriented gather conditioning laterals (Jacobi et al., 2009; LeCompte et al., 2009; Pemper
° Azimuthal velocity/AVO analysis et al., 2009). It should be noted that cores are a must, either
• Seismic analysis whole cores or sidewall (rotary) cores for analysis and direct
° Pre stack inversion measurement of certain parameters which are also used to
calibrate logs.
° Pore pressure prediction These logs assist in locating where to locate the hydraulic
• Stress estimation fractures. A fracture should be initiated in the most “fraca-
• Multiattribute/integration ble” (most brittle mineralogy) locations, so more energy
° Rock property prediction can be used to propagate fractures and create more complex
° Interpretive correlation of seismic attributes with fractures. In addition, these locations should also have the
geologic and engineering data higher TOCs in the well; thus, they are targeted at the sweet
spots. To determine this, lithofacies data are examined
Data from exploration and appraisal wells are used to pin- in context with TOC content, geomechanical properties,
point the location of sweet spots through further formation and porosity to identify specific intervals within the most