CBM network meeting April 2011 V1.7 - Win Cubed

Coal Bed Methane (CBM)
Permeability Testing
WTN Network Meeting
April 28 - 29, 2011
ExxonMobil Exploration / Well Testing Team
CBM Flow Characteristics
Flow mechanism
Gas desorbs when pressure drops below critical pressure
After gas desorbs, it diffuses through the matrix
Gas migrates into the cleats and fractures
Factors that affect system permeability
Cleat system, stress, diffusion, relative permeability,
natural fractures other than cleats, heterogeneity
Coal bed methane production
Production involves dewatering the formation to lower the
pressure to the critical gas-desorption pressure
After first gas to surface, slow initial desorption and
relative permeability create a increase in gas rate
Gas Rate
Water Rate
Cumulate gas production increases for a period of
months/years while coal is being dewatered
Producing Time (yr)
ExxonMobil use only
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
o Coals may have less near-wellbore damage
o Ease of readily obtaining water and gas
o Confirm gas production early in the program
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
o Low cost, Simple to design and perform
o Duration of the test may be long, especially if kh < 10 md-ft coal
o Minimal radius of investigation
o It is limited to under-pressured reservoirs
ExxonMobil use only
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
o Short-duration test; economical for operator
o Results can be used to optimize stimulation treatments
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
o Injection rate is controlled. Hence, it may cover a wider
range of permeability values than other methods
o The injection pressure must be maintained below
fracture gradient, which is usually not known in an
exploration setting
ExxonMobil use only
Equipment Requirements for IFT
Injection pump that provides constant rate (0.05 GPM
to 10 GPM)
Low- and high-rate flow meters connected to the data
acquisition system for real-time reading
Minimize pump pulsation while maintaining constant
injection rate
Water Filters & Assembly used to avoid plugging cleats
Inflatable straddle packer assembly to isolate IFT zone
with injection capability from surface
Surface read-out or Redundant gauges run in memory
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
ExxonMobil use only
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
Example Log-Log Derivative Plot
• Most cases show a stimulated reservoir
(negative skin)
• Dynamic under-balance perforation system
seems to have worked successfully
ExxonMobil use only
Pressure Analysis Example
History Plot
SPE paper 133356
Log-log Plot
ExxonMobil use only
CBM Permeability Test Design Consideration
Permeability Test Design
Design Basis
Testing objectives
Type of permeability test
Cleat system permeability to water
Initial reservoir pressure
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
Reservoir conditions and ranges
Initial reservoir pressure
Effective permeability
Breakdown pressure
Seam thickness and shale boundary
Formation fluid composition
Saturated vs under-saturated
Wellbore conditions
• Stable Drilling conditions (wash-outs)
• Cementing conditions
• Susceptible to near wellbore damage
• Value of information
• Quality of data
ExxonMobil use only
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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