Coal Bed Methane (CBM) Permeability Testing WTN Network Meeting April 28 - 29, 2011 ExxonMobil Exploration / Well Testing Team.

Coal Bed Methane (CBM) Permeability Testing

WTN Network Meeting

April 28 - 29, 2011

ExxonMobil Exploration / Well Testing Team

ExxonMobil use only 2

CBM Flow Characteristics

Producing Time (yr)

Gas RateWater Rate

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

• Cumulate gas production increases for a period of months/years while coal is being dewatered

ExxonMobil use only 3

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 damageo Ease of readily obtaining water and gas

sampleso Confirm gas production early in the program

Disadvantageso 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 equilibriumAdvantages

o Low cost, Simple to design and performDisadvantages

o Duration of the test may be long, especially if kh < 10 md-ft coal seams

o Minimal radius of investigationo It is limited to under-pressured reservoirs

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Types of CBM Permeability Testing

Advantageso Short-duration test; economical for operatoro Results can be used to optimize stimulation

treatmentsDisadvantageso Pseudo-radial flow signature must be observed to

estimate kh

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

Injection Fall-off Test (IFT)

• Can be performed in open- or cased-hole environment

• It is critical to inject below fracture gradientAdvantages

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

ExxonMobil use only 5

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 mode

• 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 6

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

CBM IFT - General Observations

Example Log-Log Derivative PlotWellbore Storage

• Extremely small due to a stiff system• Does not appear to mask any other flow

regime

Skin• Most cases show a stimulated reservoir

(negative skin)• Dynamic under-balance perforation system

seems to have worked successfully

ExxonMobil use only 7

Log-log Plot

History Plot

Pressure Analysis Example

SPE paper 133356

ExxonMobil use only 8

CBM Permeability Test Design Consideration

Testing objectives• Cleat system permeability to water• Initial reservoir pressure• Skin• Relative permeability (only DST)• Formation water fluid samples (only DST)• Breakdown & closure pressure (only DFIT)

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

Cost• Value of information• Quality of data

Type of permeability test• DST vs. IFT vs. DFIT (or Slug or Tank

tests)• Open vs. Cased hole

Operations• 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

Design Basis Permeability Test Design