Coal Bed Methane CBM Permeability Testing WTN Network

Coal Bed Methane (CBM) Permeability Testing WTN Network Meeting April 28 - 29, 2011 Exxon. Mobil 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 • Cumulate gas production increases for a period of months/years while coal is being dewatered Gas Rate Water Rate Producing Time (yr) Exxon. Mobil use only 2

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 Exxon. Mobil use only 3

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 Exxon. Mobil use only 4

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 Exxon. Mobil use only 5

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 Exxon. Mobil use only 6

Pressure Analysis Example History Plot SPE paper 133356 Log-log Plot Exxon. Mobil use only 7

CBM Permeability Test Design Consideration Permeability Test Design Basis Testing objectives • • • 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 • • 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 Exxon. Mobil use only 8 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