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Similar to Barber_GSA_2014_Vacuum Saturation v3
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Barber_GSA_2014_Vacuum Saturation v3
- 1. Applying Vacuum Saturation to Study The Pore Structure of Tight Shales Troy J. Barber and Q.H. Hu 2014 GSA Annual Meeting Vancouver, British Columbia October 21, 2014
- 2. Outline I. Tight gas production decline and fracture- matrix interaction II. Pore topology and macro scale fluid migration III. Vacuum saturation: how we use it IV. Mapping edge-accessible pores V. Preliminary Results VI. Summary and Looking forward
- 3. Recovery factor of 5‒10% for tight oil (Hoffman, 2012; SPE 154329) Problem: Steep production decline in tight shale gas Chong et al., 2010, SPE-133874 Refrac rebound ??? Slow matrix diffusion
- 4. Pore structure: Geometry and Topology
- 5. LA ICP-MS Multiple approaches to pore characterization Nitrogen sorption Imbibition Tests Traced vacuum saturation • Fluid (API brine; n-decane) and tracer imbibition tests • Edge-accessible porosity after traced vacuum saturation • Liquid and gas diffusion tests • Mercury intrusion porosimetry and hysteresis • N2 adsorption isotherm and hysteresis • Ar ion milling, FE-SEM, and TEM • 2-D/3-D tracer mapping using Laser Ablation-ICP-MS • Small-Angle Neutron Scattering (LANL; ORNL; NIST) Today’s talk will focus on these two methods
- 6. T What is traced vacuum saturation? FLUID (water, brine, n-decane) CO2 TO VACUUM CO2 T T T T T T T T T T T CO2 highly soluble in water Several cycles over 6-12hr removes air After ~1 hr, vacuum in connected space = (0.01/743)Torr = 99.999% Evacuation Duration: 12-24 hr API Brine - water wetting ReO4- (nonsorbing) Cs+, Co2+, Ce+, Eu3+ n-decane – oil wetting Re (nonsorbing) I- Different fluids to see effect of wettability. Sorbing and nonsorbing tracers. After saturation, apply CO2 pressure to liquid surface 12-24 hr. T T T T T
- 7. • Large volumes • Prone to leaks, 99.91% vacuum • No flushing or positive pressure capability Our previous apparatus
- 8. Our current apparatus Smaller chamber = less waste 99.99% vacuum Mechanically sealed, allowing CO2 flushing/positive pressure Sample holder for easy organization
- 9. Solid Liquid Granite Laser Ablation-ICP-MS for micro scale profiling 100 µm hole diameter
- 10. Rb (intrinsic) ReO4 - (non-sorbing) 224 µm 2 µm 12 µm 54 µm 100 µm spot size 2 mm Saturating Surface -45 -35 -25 -15 -5 0 20 40 60 80 100 120 140 160 180 200 Verticalheight(µm) Horizontal distance (µm) 1 pulse 5 pulses 10 pulses 25 pulses 50 pulses 3D Tracer Distribution
- 11. 2D Interior cross section 9.5 mm Saturation Saturation Epoxied Sides 100um spot size 500um spacing ~ 2 order of magnitude drop within 500um from sample edge.
- 13. Summary and Looking Forward • Steep 1st year decline and low overall hydrocarbon production observed in hydraulically-fractured shales. • Investigating pore structure in natural rock requires several complimentary approaches. • Traced vacuum saturation paired with LA-ICP-MS is effective at characterizing the edge-accessible pores. • Results indicate low pore connectivity in shales, which reduces gas diffusion from matrix to stimulated fractured network – driving steep production decline What’s next? • Elevated pressure saturation • Comparing samples of different mineralogy, maturation, TOC, bedding orientation, wettability, etc. • SANS/USANS o Inaccessible pores o In-situ P-T conditions o Pore structure and flow dynamics
- 14. Acknowledgements Q.H. “Max” Hu GSA On to the Future Program Thank You
Editor's Notes
- My name is Troy Barber. I am an undergraduate at the University of Texas at Arlington. In my lab we characterize pore structure and fluid flow in porous media. Today I am going to talk to you about using vacuum saturation to study pore structure in tight shales.
- 1) Here are some typical tight shale gas production decline curves. These are from the Barnett Shale in Texas. The blue curve is averaged from horizontal wells completed between 00-04. The red curve from 05-09. Despite the jump in productivity from 2000 to 2005 (which probably results from improving technology and techniques in unconventional), the declines are similar and quite steep, roughly 60% after the first year. 2) And of course one solution is to refracture a well, which results in a rebound in production. But, again, always followed with the steep decline curve typical to unconventional. For the most part, production decline seems to be play independent, with the Barnett, Marcellus, Woodford, Utica, etc. all exhibiting first year declines between ~55-75%. So we know that outside of just refracking a well, technology hasn’t been able to overcome this phenomenon. 3) We also know that despite heterogeneity between plays, the phenomenon is common to pretty much all tight shale gas production. 4) So the million dollar question here is what controls this slow production and can we develop a physics based model to predict it. 5) CLICK Now we know that flow in the fracture network, which is typical Darcian flow, is not the rate limiting characteristic here, 6) CLICK so it appears that the bottleneck is communication between the fracture network and the matrix pore network. So there are a number of factors that are probably contributing to this anomalous matrix diffusion: molecular dynamics, P-T conditions, pore grain composition, multiphase fluid flow and pore structure.
- 1) In our lab, one of the aspects we focus on is pore structure, in particular pore topology, or connectivity, a largely overlooked aspect of pore structure. 2) CLICK In a porous medium, total porosity can be separated into connected and isolated porosity, which in turn can be split into edge accessible pores and what is known as an infinite cluster, wherein a cluster of connected pores is large enough to extend entirely from one end of the system to the other.
- (CLICK) Evacuate and flush several times with CO2 over the course of half a day to remove air. Then we evacuate the sample for at least 24 hours at 0.01 Torr = 99.999% vacuum before. . . (CLICK) . . . introducing the saturating fluid. For pore distribution mapping we use both oil wetting and water wetting fluids containing sorbing and non-sorbing tracers. (CLICK) After the fluid has been introduced, the system is subjected to positive pressure for 12-24 hrs. Admittedly, we can currently only reach 60 psi, because the pressure gauge we are using is only rated that high, but we should be able to reach upwards of 15 bar with the current chamber and hope to reach higher pressures with an improved chamber design. By flash freezing, and then freeze drying the saturated samples, the fluid is removed, and the tracers remain in the edge accessible pores.
- This is our previous setup Very large volume chamber, requiring large volumes of saturating fluid and precarious sample organization System used plastic tubing and was prone to leaks, decreasing the vacuum efficiency. Lacked the capability to flush with CO2 or apply positive pressure, which was something we wanted to do. Now I knew nothing about vacuum systems or vacuum saturation when I was given this task, but I had taken some engineering courses and so I did a little research and got to work. It took a couple of iterations, including one ultimately dangerous prototype, but this is what I ended up with. (CLICK)
- (CLICK) - The new design has a smaller sample chamber, so we don’t end up wasting as much fluid. (CLICK) - By replacing most of the plastic tubing with stainless steel, replacing many of the fittings and redesigning the sample chamber, I was able to increase the vacuum efficiency to 99.999%. (CLICK) - I constructed an easy to use sample holder capable of accommodating up to 64 cubic cm samples. (CLICK) - By mechanically sealing the chamber lid, I was able to add the ability to flush the system with and apply positive pressure using CO2. (CLICK)
- We then analyze these samples using laser ablation paired with ICP-MS (inductively coupled plasma – mass spectrometry) in order to map the tracer (and hence the pore) distribution in 2 and 3D. Here is our LA ICP-MS setup. We have a UP-213 Nd:YAG laser ablation system, which ablates the sample in craters as small as 4um diameter. We use He as a transport gas to carry the aerosol to our Perkin Elmer ELAN DRC II ICP-MS. We use a constant input of liquid internal standard as well as periodic sampling of NIST glass and USGS rock solid standards for instrument calibration. (CLICK) The laser software allows for generating preset ablation grids, and by calibrating the depth of penetration as a function of laser pulse intensity (Point out the Depth/pulse X-section), we can generate a stackable series of 2 dimensional elemental distribution maps. (CLICK)
- Saturating surface grid 100um spot size 2x2mm grid 4 scans of increasing penetration depth corresponding to 2, 12, 54, and 224 um ReO4- (nonsorbing tracer) vs Rb (intrinsic element). (CLICK) Tracer concentration falls off by 2 orders of magnitude within 225 um from the saturating edge. Again, by plotting concentration vs distance from the edge, we see that the results agree with our percolation model. (DO THEY?) (CLICK)
- Then we cut the sample parallel to the saturation direction and analyze the interior face, to look at penetration from sample edge in 2 dimensions. Point out epoxied sides, saturation direction, spot size, resolution Wall effect between epoxy and sample.
- 1) This concept of the infinite cluster is based on percolation theory, a mathematical tool which can be used to model the behavior of randomly connected systems. Percolation theory says that for a randomly connected finite lattice of nodes or, in our case, pores, if the probability that any one node is connected to a neighbor exceeds what is known as the percolation threshold, then an infinite cluster is expected to exist between opposite boundaries of the lattice. Previous studies in our lab have confirmed that percolation theory is an effective way to model pore connectivity and its affect on anomalous diffusion in tight geologic media. 2) For example, percolation theory predicts that for a system with connectivity far enough above the percolation threshold, classical Darcian flow and Fickian transport can occur. We have performed spontaneous imbibition experiments, wherein a porous sample is placed in contact with a liquid surface and allowed to imbibe the fluid through capillary pressure. Percolation theory predicts that if a log-log plot of mass imbibed vs time is generated, a medium with a high connectivity probability will yield a slope of 1/2, whereas for a medium very near the percolation threshold a slope of ~1/4 will result. 3) CLICK So here we have two such plots, one for a sample of Berea Sandstone and the other for a sample of Barnett Shale. The sandstone ( a well connected rock) exhibits an imbibition slope of 0.495 and the shale (which we hypothesize to be poorly connected) yields a slope of 0.262, as predicted for a rock of low connectivity by percolation theory. 4) For replicate tests of four other Barnett shale samples the resulting slope is always ~1/4. 5) But this test alone doesn’t tell us anything about the accessible pore distribution, which in samples of low pore connectivity decreases with distance from the sample edge – which is where vacuum saturation comes in handy. (CLICK)