Multiscale Investigation of Fluid Transport in Gas Shales. Rob Heller and Mark Zoback

Transcription

1 Multiscale Investigation of Fluid Transport in Gas Shales Rob Heller and Mark Zoback

2 Multiscale Fluid Flow Process Control Production July 5 July 6 Valko and Lee, 1 Production Rate July 4

3 Hypotheses: 3 Production Rate 1 Hypothesis 1: desorption and diffusion may be responsible for the flat production tails characteristic of many gas shale reservoirs.

4 Hypotheses: 4 Production Rate 1 Hypothesis 1: desorption and diffusion may be responsible for the flat production tails characteristic of many gas shale reservoirs. Hypothesis : achieving a large, conductive percolation area is essential for production.

5 1 Adsorption and Gas Shales 5 Conceptual model for flow in gas shales Desorption From Internal Surfaces Flow Through Intact Matrix Flow Through Fracture Network Adsorbed Gas Pore Pressure Adsorption is the pressure dependent attraction of gas molecules to the surface of a solid, resulting in a dense phase of gas at the surface The release of adsorbed gas is pressure dependent Rocks rich in minerals with a high surface area (organic matter and clay) tend to adsorb more

6 Adsorption Measurement Methodology 6 Adsorption Measurement Details All tests done at 4 C Boyle s Law gas expansion method: 1. First measure porosity of sample using nonadsorbing gas (Helium). Then expand adsorbing gas into sample and calculate adsorption amount based on measured pressure deficit 3. Repeat procedure at range of pressures Sample Preparation Samples from Barnett, Montney, Eagle Ford and Marcellus shale reservoirs Crushed samples (1 5 μm) Dried under vacuum at 5 C

7 Methane Adsorption Results: All Samples 7 7 Absolute Adsorption (SCF/ton) Barnett Montney Marcellus Eagle Ford 5.3% TOC, 37.4% Clay.% TOC, 3.7% Clay 1.% TOC, 51.4% Clay 1.8% TOC, 4.9% Clay Pressure (psia) Samples with higher TOC and clay adsorb considerably more.

8 Very little production comes from adsorbed gas 8 35 Barnett Marcellus Gas Produced (scf/ton) Total Gas Produced Free Gas Produced Adsorbed Gas Produced Gas Produced (scf/ton) Total Gas Produced Free Gas Produced Adsorbed Gas Produced Gas Produced (scf/ton) Pressure (psia) 5 Eagle Ford 17 Total Gas Produced Free Gas Produced Adsorbed Gas Produced Pressure (psia) Gas Produced (scf/ton) Pressure (psia) 5 Montney Total Gas Produced Free Gas Produced Adsorbed Gas Produced Pressure (psia)

9 Matrix Flow in Gas Shales 9 Conceptual model for flow in gas shales Permeability Desorption From Internal Surfaces Flow Through Intact Matrix Flow Through Fracture Network Pore Pressure. How does matrix permeability evolve during production? Stress effects

10 Matrix Flow in Gas Shales 1 Conceptual model for flow in gas shales Permeability Desorption From Internal Surfaces Flow Through Intact Matrix Flow Through Fracture Network Pore Pressure. How does matrix permeability evolve during production? Stress effects Flow regime effects

11 Matrix Permeability Influenced by: 11 Stress Effects Flow Regime Effects Zoback and Byerlee, 1975 Klinkenberg, 1941 Permeability (md) Permeability (md) Effective Stress, C p P p (bars) Reciprocal Pore Pressure, 1/P p (bars 1 ) At a given pore pressure, permeability decreases with confining stress eff k ( eff C p ) xp p Apparent increase in permeability at low pore pressure due to gas slippage b k ka k 1 p

12 Under what conditions is slip flow important? 1 As pore pressure decreases, distance between molecular collisions (mean free path) increases Diffusion (slip-flow) becomes increasingly more important

13 Permeability System Setup 13 Gas Cylinder QX-6 Pump Hydrostatic Pressure Vessel Pressure Generator

14 Sequence of Confining Pressure and Pore Pressure Steps Ln(P up -P down ) 14 4 Pressure (psia) P Upstream Natural Log P up -P down Slope = α P Downstream Time (hr) Time (hr) Pressure on one side of sample increased Downstream pressure monitored as pulse travels through Natural log of P linear in time Permeability calculated from slope t P( t) P e ka V L down Brace, 1968

15 Samples Tested 15 Sample Photo Perm Range (nd) Orientation Eagle Ford Horizontal Eagle Ford Horizontal Marcellus -18 Vertical All measured made with Helium data points/day ~6 points per experiment ~1 month/sample Measurement repeatability was monitored

16 Permeability vs. Cp Pp 16 6 Marcellus 5 Eagle Ford Eagle Ford 17 Pp=1 Pp= Pp=3 Pp=4 Permeability (nd) Permeability (nd) Permeability (ud) Effective Stress: Cp-Pp (psi) Effective Stress: Cp-Pp (psi) Effective Stress: Cp-Pp (psi) Permeability decreases with increasing simple effective stress. Can we describe this behavior with an effective stress law?

17 Permeability vs. Cp χpp 17 6 Marcellus 5 Eagle Ford Eagle Ford 17 Pp=1 Pp= Pp=3 Pp=4 Permeability (nd) χ =.15 Permeability (nd) χ =.4 Permeability (ud) χ = = Effective Stress: Cp-.15*Pp (psi) Effective Stress: Cp-.4*Pp (psi) Effective Stress: Cp-.6*Pp (psi) For each rock, successfully fit all measurements to a single trend So far, we have been able to explain all permeability variation with stress effects What about lower pore pressures?

18 Permeability vs. Cp χpp 18 Marcellus Vertical 1 Eagle Ford 174 Eagle Ford 17 Pp=1 Pp= Pp=3 Pp=4 Permeability (nd) Permeability (nd) Permeability (ud) Effective Stress: Cp-.15*Pp (psi) Effective Stress: Cp-.4*Pp (psi) Effective Stress: Cp-.6*Pp (psi)

19 Permeability vs. Cp χpp 19 Pp=5 Pp=5 Pp=75 Pp=1 Pp= Pp=3 Pp=4 Permeability (nd) Marcellus Vertical Permeability (nd) Eagle Ford 174 Permeability (ud) Eagle Ford Effective Stress: Cp-.15*Pp (psi) 4 6 Effective Stress: Cp-.4*Pp (psi) Effective Stress: Cp-.6*Pp (psi)

20 Permeability vs. Cp χpp Pp=5 Pp=5 Pp=75 Pp=1 Pp= Pp=3 Pp=4 Permeability (nd) Marcellus Vertical Permeability (nd) Eagle Ford 174 Permeability (ud) Eagle Ford Effective Stress: Cp-.15*Pp (psi) 4 6 Effective Stress: Cp-.4*Pp (psi) Effective Stress: Cp-.6*Pp (psi) eff = psi 16 Marcellus Vertical 7 Eagle Ford Eagle Ford 17 eff =3 psi eff =4 psi Permeability (nd) K =531.4 psi b K =854.7 psi b Permeability (nd) K =688. psi b K =916.7 psi b Permeability (ud) K =195.9 psi b K =.8 psi b 6 K b = psi K b = psi 1 K b =7. psi /Pore Pressure (psi -1 ) x 1-3 1/Pore Pressure (psi -1 ) x 1-3 1/Pore Pressure (psi -1 ) x 1-3

21 1 Assumptions: Total flow is sum of viscous (Darcy) flow and Knudsen/slip flow Slit shaped pore geometry Model viscous flow using Poiseuille equation P L w RT M P c P L w Q P L ka Q M RT wp c A w k 1 16 M RT w c k b 1 16 M RT k c w b P b k k k a 1 Effective Pore Size from Klinkenberg Slope

22 Effective Pore Size vs. Effective Stress 14 1 Eagle Ford Pore size 1- nm Effective Pore Width (nm) Eagle Ford 17 Marcellus Eagle Ford Effective Stress (psi) Pore width decreases with increasing effective stress Pore widths range from -4nm in Marcellus samples, ~13nm in Eagle Ford Klinkenberg pore sizes consistent with SEM images Pore size 1 s of nm Kohli and Zoback, 1 Image from Sondergeld, 1

23 To what extent does diffusion contribute to total flow? Eagle Ford 174 Marcellus Vertical 3 3 Diffusive Flux/Darcy Flux (-) eff = psi eff =3 psi eff =4 psi Diffusive Flux/Darcy Flux (-) eff = psi eff =3 psi eff =4 psi Diffusive Flux/Darcy Flux (-) Pore Pressure (psi) Eagle Ford eff = psi eff =3 psi eff =4 psi Pore Pressure (psi) Pore Pressure (psi) Diffusive flow contributes appreciably to total flow at pore pressures < 8 psi Diffusive flow is sometimes more important than Darcy flow at pore pressure < 5 psi As we increase effective stress for a given pore pressure, we narrow the pore aperture and the relative contribution of diffusion decreases

24 Matrix Flow Conclusions 4 Gas slippage seems to enhance permeability at low pore pressure Effective pore widths are estimated to be 1-15nm, consistent with SEM images At low pore pressures, Knudsen diffusion (or slip flow ) becomes increasingly more important, in some cases surpassing Darcy flow Heller, R.J., Vermylen, J.P., & Zoback, M.D. (13). Experimental Investigation of Matrix Permeability of Gas Shales. AAPG Bulletin, In Press. Heller, R.J, and Zoback, M.D. (14). Adsorption of Methane and Carbon Dioxide on Gas Shale and Pure Mineral Samples. In Review. Heller, R. J., & Zoback, M. D. (11, June). Adsorption, Swelling And Viscous Creep of Synthetic Clay Samples. In 45th US Rock Mechanics/Geomechanics Symposium. Heller, R., & Zoback, M. (13, August). Laboratory Measurements of Matrix Permeability and Slippage Enhanced Permeability in Gas Shales. In Unconventional Resources Technology Conference.