Pressure and Compaction in the Rock Physics Space. Jack Dvorkin

Transcription

1 Pressure and Compaction in the Rock Physics Space Jack Dvorkin June 2002

2 0 200 Compaction of Shales Freshly deposited shales and clays may have enormous porosity of ~ 80%. The speed of sound is close to that in water ~ 1500 m/s. The S- wave velocity is small but not negligible. As a result, Poisson s ratio approaches 0.5. As the overburden increases, the shale compacts. decreases and velocity increases. Compaction in on-shore shale and in GOM. NPP 7 Depth (m) GOM Ip (km/s g/cc) Compaction GOM GR RHOB (g/cc) Vp (km/s) 2 NPP

3 Difference in Compaction of Shales and Sands Sands are much less compactable than shales (unless the grains break or diagenesis sets in). Compaction in dry kaolinite, Ottawa sand, and a 50/50 mixture thereof (Yin, 1992) % Sand 50% Clay DRY Vp Vp (km/s) MPa Sand Clay MPa

4 Compaction of Shales As long as shale is load-bearing, the compaction trend in the impedanceporosity space seems to be universal among wells logs and lab data. Compaction in on-shore shale and in GOM + Yin's clay and sand/clay data. 7 6 YIN 50/50 Ip Ip (km/s g/cc) 5 Compaction GOM 3 2 YIN Kaolinite NPP

5 Compaction and Undercompaction Due to Pore Pressure Abnormal pore pressure results in undercompaction and porosity and velocity reversals SHALE: 120 > GR > 90 Reversal Vp (km/s) 3 2 Velocity Reversal SHALE: 120 > GR > 90 1 km

6 Compaction and Undercompaction Due to Pore Pressure -- Same Rock Physics Trend Normally- and over-pressured parts of the well project onto the same rock physics trend, same as the lab data. 9 Ip (km/s g/cc) YIN 50/50 GOM 3 YIN Kaolinite

7 Universality of Compaction and Undercompaction in Rock Physics Space Moreover, well log data from different wells worldwide fall onto the same Ip-porosity trend. Different color means different well. 500 Depth (m) Ip (km/s g/cc) Ip (km/s) GR Ip (km/s g/cc)

8 Universality of Compaction and Undercompaction in Rock Physics Space The curves are from unconsolidated sediment model that relates elastic properties to porosity, lithology, and pore fluid compressibility. Compressional Modulus (GPa) Compressional Modulus (GPa) % C = 100% 30% NPP Shear Modulus (GPa) % C = 100% 30% NPP

9 Rational Effective-Medium Model Uncemented Particles Hertz-Mindlin Theory + Modified Lower Hashin-Shtrikman 100 SOLID MODIFIED LOWER HASHIN-SHTRIKMAN WITH HERTZ-MINDLIN Compressional M-Modulus Modulus (GPa) (GPa) Increasing Pressure HERTZ- MINDLIN Hertz-Mindlin theory provides expressions for the contact stiffness between two elastic particles. Based on these expressions, we can derive the elastic moduli for uncemented sediment at critical porosity depending on pressure and pore fluid.

10 Ip Ip (km/s g/cc) Compaction and Undercompaction Due to Pore Pressure in AI-EI Space Abnormal pore pressure also results in AI and Poisson's ratio reversals Ip Reversal ALL PR Reversal Poisson's PR Ratio ALL PR Reversal Hi-P Gas Differential Pressure (MPa) 1 km

11 Compaction and Undercompaction Due to Pore Pressure in AI-EI Space PR is very sensitive to mineralogy. We may want to use it to detect mineralogical changes associated with onset of overpressure. May help resolve the non-uniqueness of universality of Ip-φ trends Just Shales 7 Pressure Zone Above Pressure Zone Ip (km/s g/cc) g/cc) 6 5 Below Pressure Zone Poisson's Ratio

12 High Pressure in Gas Sands High pore pressure in rock with gas results in smaller Poisson s ratio. Velocity may vary a lot among rocks but PR behavior is universal..2 Plots based on lab data. PR PR.1 Sand 36% Pp (MPa) Sand 27% Pp (MPa) Sand 35% Pp (MPa)

13 Normal Compaction in Shales Log data show monotonic compaction versus depth. Depth (m) (m) GR RHOB (g/cc) Vp (km/s)

14 Normal Compaction in Shales Log data show monotonic compaction versus depth. Left -- color coding by GR highlights compaction trends for shale and sand. Right -- color coding by depth shows porosity collapse and impedance increase Ip Ip (km/s g/cc) Ip Ip (km/s g/cc) GR TVD

15 Undercompaction in Shales Log data show reverse compaction versus depth. Color coding by depth shows porosity and impedance reversal. Overpressured shales stay on the same rock physics trend as normally pressured shales. 9 Ip (km/s g/cc) Reversal -- Deeper Shale Plots to Low-Right TVD

16 Undercompaction in Shales -- Example 2 Mudweight Steps Approximately Match and Velocity Flattening MD (kft) 8 9 AT_136_1 UPSCALED UPSCALED UPSCALED kft GR Rt Vp (km/s) Ip (km/s g/cc) MW (lb/gal) Pp (kpsi) Peff (kpsi)

17 Undercompaction in Shales -- Example 2 Overpressured shales stay on the same rock physics trend as normally pressured shales. Color-Coded by Depth 7.5 Overpressured Shale.5 7 Ip (km/s g/cc) P-Impedance 6.5 SAND Depth (km)

18 Undercompaction and Vp/Vs Ratio In overpressured (softer) sediments, the Vp/Vs ratio is high and deviates from the established Vp/Vs relations. In overpressured gas sands the opposite is true -- the Vp/Vs ratio is small. Vp/Vs Overpressured Shale Mudrock Pore Pressure Increase Williams Sand Williams Shale Figure shows a Vp/Vs versus Vp plot for a well with an overpressured shale section (green) and overpressured gas sand (red). Black symbols are for normally pressured shales. Blue curves show established relations for water-saturated sediment. Overpressured Gas Sand Vp (km/s)

19 Compaction and Unloading in Sands Loading and unloading produce different strain paths in sand Compaction and elastic unloading in a sand. 0.3 Galveston 2 Galveston 1.2 Galveston Vp (km/s) 1 Envelope Vs (km/s) Envelope Pressure (MPa)

20 Compaction and Unloading in Sands Static and Dynamic Moduli SAND 1 SAND 2 Bulk Modulus (GPa) Bulk Modulus (GPa) Static Unloading Dynamic Static Static Loading Effective Pressure (MPa) Static Unloading Dynamic Static Static Loading Effective Pressure (MPa)

21 Conclusion The projection of compaction (loading) process into the impedance-porosity plane produces a universal trend typical for many soft shales and independent of depth. AI/EI technique may help detect pressureassociated lithology changes in shales. But what to do in frontier areas where impedance inversion is difficult? Unloading (uplift) is different from loading (compaction) and will produce a different trend because of irreversible porosity reduction.

22 Pore Fluid and Pressure Diagnostic from AI and EI The softer the rock with liquid the larger the Poisson s Ratio. The softer the rock with gas the smaller the Poisson s ratio. 0. BRINE Poisson's Ratio PR NORTH SEA SAND Pore Pressure Pore Pressure 0 GAS P-Impedance (km/s g/cc)