CBM network meeting April 2011 V1.7 - Win Cubed

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Coal Bed Methane (CBM)
Permeability Testing
WTN Network Meeting
April 28 - 29, 2011
ExxonMobil Exploration / Well Testing Team
CBM Flow Characteristics
Flow mechanism
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Gas desorbs when pressure drops below critical pressure
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After gas desorbs, it diffuses through the matrix
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Gas migrates into the cleats and fractures
Factors that affect system permeability
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Cleat system, stress, diffusion, relative permeability,
natural fractures other than cleats, heterogeneity
Coal bed methane production
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Production involves dewatering the formation to lower the
pressure to the critical gas-desorption pressure
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After first gas to surface, slow initial desorption and
relative permeability create a increase in gas rate
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Gas Rate
Water Rate
Cumulate gas production increases for a period of
months/years while coal is being dewatered
Producing Time (yr)
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Types of CBM Permeability Testing
Drill Stem Test (DST)
• Can be performed in both open-hole or cased-hole environment
• DST may be performed with high reservoir pressure, high
deliverability, and reservoirs with free gas
Advantages
o Coals may have less near-wellbore damage
o Ease of readily obtaining water and gas
samples
o Confirm gas production early in the program
Disadvantages
o Relatively high cost compared to other
permeability testing methods
Slug Test
• Inject volume of water into wellbore and measure pressure
response as the fluid level returns to equilibrium
Advantages
o Low cost, Simple to design and perform
Disadvantages
o Duration of the test may be long, especially if kh < 10 md-ft coal
seams
o Minimal radius of investigation
o It is limited to under-pressured reservoirs
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Types of CBM Permeability Testing
Diagnostic Fracture Injection Test (DFIT)
• Inject fluid above the fracture gradient to estimate the
reservoir breakdown and closure pressure
• To derive kh, after-closure analysis appears to be the
preferred technique
Advantages
o Short-duration test; economical for operator
o Results can be used to optimize stimulation treatments
Disadvantages
o Pseudo-radial flow signature must be observed to
estimate kh
Injection Fall-off Test (IFT)
• Can be performed in open- or cased-hole environment
• It is critical to inject below fracture gradient
Advantages
o Injection rate is controlled. Hence, it may cover a wider
range of permeability values than other methods
Disadvantages
o The injection pressure must be maintained below
fracture gradient, which is usually not known in an
exploration setting
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Equipment Requirements for IFT
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Injection pump that provides constant rate (0.05 GPM
to 10 GPM)
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Low- and high-rate flow meters connected to the data
acquisition system for real-time reading
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Minimize pump pulsation while maintaining constant
injection rate
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Water Filters & Assembly used to avoid plugging cleats
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Inflatable straddle packer assembly to isolate IFT zone
with injection capability from surface
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Surface read-out or Redundant gauges run in memory
mode
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Option for bottom-hole shut-in for zones with
permeability < 1 md
•
High shot density with dynamic under-balance
perforation for clean perforation tunnels and to ensure
good communication with the coal cleat system
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CBM IFT - General Observations
Operationally the system with straddle packers worked well
Surface readout was crucial to optimize program during operations
Measurement devices and pumps at limits in thin coal beds (< 0.5 m)
In general it seems that injection permeability > falloff permeability
• Could be partly due to stress
• Could also be attributed to fracture/cleat opening
Wellbore Storage
• Extremely small due to a stiff system
• Does not appear to mask any other flow
regime
Example Log-Log Derivative Plot
Skin
• Most cases show a stimulated reservoir
(negative skin)
• Dynamic under-balance perforation system
seems to have worked successfully
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Pressure Analysis Example
History Plot
SPE paper 133356
Log-log Plot
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CBM Permeability Test Design Consideration
Permeability Test Design
Design Basis
Testing objectives
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Type of permeability test
Cleat system permeability to water
Initial reservoir pressure
Skin
Relative permeability (only DST)
Formation water fluid samples (only DST)
Breakdown & closure pressure (only DFIT)
• DST vs. IFT vs. DFIT (or Slug or Tank tests)
• Open vs. Cased hole
Operations
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Reservoir conditions and ranges
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Initial reservoir pressure
Effective permeability
Breakdown pressure
Seam thickness and shale boundary
Formation fluid composition
Saturated vs under-saturated
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Wellbore conditions
• Stable Drilling conditions (wash-outs)
• Cementing conditions
• Susceptible to near wellbore damage
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Cost
• Value of information
• Quality of data
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Tubing vs. wireline vs. Coil-Tubing
Conventional vs. slim-hole design
Rig vs. Rigless operations
Larger diameter perfs vs. deeper perf
tunnel (with Dynamic-Underbalance
perforations) vs TCP vs. under-ream
Surface read-out vs. memory gauges
Stimulation: Under-ream and water flush,
Slick water frac, Gel type frac with proppant,
acid wash
Production (DST) vs. Injection (Fresh water,
inhibitive brine, weighted brine)
Surface discharge vs sub-surface injection
Cementing and Mud weight
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