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2010 01 25 Pore Geometry Update
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2010 01 25 Pore Geometry Update


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Discussion on Pore Geometry Observations in Carbonate Reservoirs

Discussion on Pore Geometry Observations in Carbonate Reservoirs

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  • This is the combination of 2 papers presented to the SPWLA in Abu Dhabi.
  • Formation evaluation in Middle Eastern carbonates can be challenging primarily due to the varying texture in the rock assemblages. Presented in this slide are 4 examples of the rock types with improved rock quality improving clockwise from the upper left. It should be noted that all rocks contain some component of micrite. The pore geometry is the key to producibility and the control over fluid flow.
  • Here is my little cartoon explaining what I believe is happening in the rock system. Note that the total porosity is what one measures in both the lab (CMS-300) and with the porosity tools. Classically the petrophysical community has recognized changes in carbonates. Nugent and others tried to use the sonic in conjunction with ND to compute a ‘secondary’ porosity in much the same way I compute Occluded porosity. However what we see affecting the sonic is a change in the poro-elastic properties as texture changes.
  • Here is an example of rock type #1. You will note very low entry pressures due in turn to large, well connected pore throats. It is NOT the highest porosity rock, but it represents the highest permeability. The rock pictured here is a peloidial grainstone. Most of the grainstones represent the best rock types and are found in the high energy environments. In Kuwait they represent the end of a parasequence in the coarsening upward cycles. You will note the presence of minor micrite which contributes to what is commonly expressed as micro-porosity.
  • Rock Type #3 is very bimodal, both in pore body size and the connectivity of pore throats. Secondary diagenisis controls the connectivity of the pore bodies.
  • The rock texture is multimodal in both pore body size and pore throat size. Our oolitic grainstones are one extreme and the micritic mudstones the other. Unfortunately our rocks are a mostly in between the two end members. In the example and in the slides that follow I have bucketed Sw and display it in the color axis with regard to total porosity vs. air perm (800 psi). I have imposed the red line to identify an area (to the right of the line) that is susceptible to a significant percentage of non-efficient porosity. I define this term as the volume of porosity that is connected by small cross section pore throat systems. As this volume is difficult to saturate with low buoyancy pressures it will tend to remain water wet and resist the initial hydrocarbon charge. Rocks to the left of the line has a much lower percentage of non-efficient porosity.
  • This is the first slide in the animation. It is at 2 psi. Please ignore the point at 5 PU that is cyan. I did not eliminate this bad data point when I made the slides.
  • At 5 psi you can get an indication of the influence of the well connected pore systems. The ellipse roughly approximates the K/phi response predicted by Dale Winland.
  • At 10 psi you note that the K/phi relationship drops down indicating that you are charging a significantly large volume with Hg.
  • At 20 psi the trend continues. Note that the decrease in saturation is leveling out indicating a significant volume of non-efficient porosity that is not charging.
  • At 40 psi we are finally starting to charge the more challenging pore geometries.
  • At 60 psi I approximate the greatest influence of the non-efficient porosity. The crescent moon represents a vector for the flash charging. We have crossed a threshold of pressure (Washburn pore throat size) where upon the meniscus forces are defeated and a large proportion of the non-efficient porosity is subsequently charged.
  • Increasing pressures…. Note Sw’s below 20% starting to develop.
  • And so on…
  • I use the classical definition of micro-porosity (pore throats <1u) to differentiate between Occluded and Efficient pore systems. This is fairly analogous to what our friends in silici-clastic reservoirs have done, however in most carbonates there is little or no clay. The limestone muds and micrite contribute to the ‘micro-porosity’ and there is no easy mechanism to compute its volume. Using the MICP one can volumetrically compute the occluded volume by integrating under the incremental curve from the high pressure end stopping at 1u. Subtracting this from total porosity yields the Efficient Pore volume. The center P&P plot illustrates the impact of the occluded volume. The red points are the conventional total porosity vs. permeabilty as obtained from the CMS-300. The blue points are the same values except that Efficient porosity is presented. Note that our cloud of data tends to coalesce as what could be interpreted as two or three Fontainebleau curves. Considering the grainy composition of these rocks, it fits. I have not done any work with crystalline carbonates and I reserve opinions there as per Jerry Lucia’s investigations. The right hand plot of SWi @ 1u vs. porosity is significant. It illustrates that as the Efficient volume increases that there is a reasonable decrease in SW. This suggests that within low to mid column heights one can rock type via saturation in reservoirs with little or no voidage.
  • The first clues that there were serious questions on saturation determination came from comparing TDT results with OH. My predecessor here blamed it on the TDT’s and operations. When we finally drilled an aquifer dump flood well, part of the answer is clear (left figure). The figure on the right shows saturation as a function of height above the OWC for a well with a bottom water rise. OH Sw is the blue curve and TDT Sw is in yellow. The red tabs in the depth track are rock type #1 as determined from core inspection, thin section, and HPMI. A representative RT-1 is the red Sw trace while RT-4 is in violet. I haven’t gone to the extent of trying to compute a conductive porosity derived Sw at this time. Standard Archie Sw just does not honor the capillarity of the system.
  • This slide tells a significant story. In it we have the open hole movable oil plot and integrated the TDT surveillance upon it. The dark blue within the volumetric track is the CH BVW signifying the aquifer influx in this case as bottom water. The rightmost track illustrates the total mobile volume with the light green signifying the pore volume of mobile hydrocarbons. The yellow stippled volume indicates the change in pore volume affected during acidization. It is significant to note that acidization only affects water wet rocks. Most of the ME carbonates are predominately oil wet due to rock and fluid characteristics. Therefore it is the more micritic portions of the rock that is being preferentially being affected. The large decreases in skin and corresponding increases in permeabilty are caused by affectively increasing the pore throat radius of the smaller pore geometry. Pore volume can be increased by as much as 30-40 porosity units, especially if significant micrite is present.
  • Here is my little cartoon explaining what I believe is happening in the rock system. Note that the total porosity is what one measures in both the lab (CMS-300) and with the porosity tools. Classically the petrophysical community has recognized changes in carbonates. Nugent and others tried to use the sonic in conjunction with ND to compute a ‘secondary’ porosity in much the same way I compute Occluded porosity. However what we see affecting the sonic is a change in the poro-elastic properties as texture changes.
  • Low resistivity pay is a common theme in carbonate reservoirs, especially low in the column. A simple ‘dual’ porosity conductivity equation can be constructed to capture the effects. David Allen with Schlumberger is attempting a similar mechanism with 3 poro groups based upon NMR and some work by Mario Petricola. I don’t necessarily agree with their application of NMR but the methodology has merit. Schlumberger just doesn’t understand pore geometry but perhaps discussions I have had with them will help.
  • Two critical observations should be noted. Lab cementation exponent measurements suggest higher ‘m’s for grainy rocks and low ‘m’ for the more micritic. I use an m~2.1 for efficient porosity and 1.7 or so for the occluded. In oil wet rocks I do not believe that one can obtain a drainage ‘n’ in so much that I do not believe one can place the hydrocarbons in the correct pore spaces in the lab. Therefore I use 2 for the want of anything better. For reservoir parameters noted I compute an Rt & Ro curve respectively assuming SW=20% in the efficient pore volume and 100% in the occluded. You will note that even though there are hydrocarbons present in the rock it is not resolvable with resistivity measurements when the efficient porosity is less than about 7pu and it takes about 15pu of efficient porosity (out of 25) to exceed 1 ohm. Note that the total porosity for this model is 25pu.
  • I add SWtotal to the plot to illustrate that within the normal Kuwait range of efficient porosity (7pu to 20pu) for 25pu rock, one can expect Sw’s to range from 90% down to about 30%. This agrees nicely with the MICP and the log computations.
  • Low in the column (upper left plot) most of the rock types contain a significant but varying volume of efficient porosity. The efficient pore volume WILL saturate at low buoyancy and will minimal drawdown, it should produce water free depending upon the relative permeability. As height increases (up to about 50-60psi) one can easily note the influence of pore geometry upon Sw. Above about 200 feet the cap pressure curves coalesce and rock type discrimination becomes difficult, however residual saturations are discriminatory. The lesson here is to drill with conductive muds and record a decent micro resistivity.
  • Here is my little cartoon explaining what I believe is happening in the rock system. I removed the animation describing drainage but you get the point. Carbonate rock systems are a collection of varying pore body sizes (primarily controlled by secondary diagenisis) and pore throat sizes. The pore throat distributions and the connectivity of the large pore throat system is the control on permeability. As you will shortly see this is also the control over the saturation distribution.
  • Transcript

    • 1. Pore Geometry Effects in Carbonate Reservoirs
    • 2. Common Cretaceous Carbonate Rock Textures Unlike sandstones, carbonate pore systems do not generally exhibit a relationship between pore throat size and pore body size. The connectivity between pores in carbonates is generally fairly chaotic.
    • 3. Pore Geometry Model  t =  o +  e Hi K Low K Pore Body Plug Scale Pore Throat Pore Body Pore Body Pore Body Pore Body
    • 4.
      • Rock Type 1
      • Largest pore throats
      • Well connected
      • Lowest Swi
      • Highest K
      • Intermediate porosity
      • Little or no microporosity
      Rock Typing Pore Geometry
    • 5. Back-ups?
      • Rock Type 3
      • Strongly Bimodal -Large and small pore throats
      • Some macro pores connected via micropores
      • Higher Swi at a given H
      • Intermediate K
      • Intermediate to low porosity
      • Abundant microporosity within grains and matrix (where present)
      Rock Typing Pore Geometry
    • 6. Greater probability of large pore bodies Pore Body Size Pore Body Size Throat Size Throat Size
    • 7.
    • 8. Winland Rock Type f(k/  )
    • 9.
    • 10.
    • 11.
    • 12. Non Winland Behavior
    • 13.
    • 14.
    • 15.
    • 16. Significant volume of poorly connected porosity
    • 17. Hydrocarbon Habitat Pc = 0 Swi Transition Zone 60 to 90 psi Oil in both efficient and occluded pore volumes Oil in efficient pore volume only  t =  o +  e
    • 18. Impact of Efficient Porosity Pore Geometry Effects
      • Logs & Core sense total porosity .
      • Efficient porosity contains hydrocarbons at low buoyancy.
      • Pores connected by efficient porosity predominately oil wet.
      • Occluded pores water wet and charged only at high buoyancy.
      Efficient Occluded Efficient Phi Total Phi  t =  o +  e Total Porosity Efficient Porosity
    • 19. Pore Geometry & Formation Evaluation What is your confidence on Sw? a=1 m=2.03 n=1.98 Rw=0.018@FT
      • Uncertainties:
      • How to vary Archie exponents as a function of rock type (incorporate Sw from TDT?)
      • Capillary Pressure variables (  cos  )
      • Pore Geometry effects upon wettability and water saturation
      • With confidence:
      • Connate water properties (Rw,  water)
      • Oil properties (  oil)
      • Pc=0
       Sw Aquifer Well m = 2.38 m = 1.75
    • 20. Effects of Acidization Acid Effects on CH CNL
    • 21. Pore Geometry Model  t =  o +  e Hi K Low K Pore Body Plug Scale Pore Throat Pore Body Pore Body Pore Body Pore Body
    • 22. Pore Geometry Model
      • Conductivity Equation – conductive matrix model
      Efficient  Efficient 
    • 23. Conductive Matrix Model Assumptions: Sw o = 100% Sw e = 20%  t = 25%
    • 24. Conductive Matrix Model Assumptions: Sw o = 100% Sw e = 20%  t = 25%
    • 25. Pore Geometry Model Efficient  filled low in column Multiple rock types with similar degrees of Efficient  Occluded  filling slowly with height
      • Multiple Rock Types can exhibit the same type of Efficient  , making them indistinguishable at low buoyancy pressures.
      • The high conductivity of the Occluded  dominates the resistivity measurement, especially at low column heights.
      Occluded  increasing
    • 26. Questions & Considerations Discussion
    • 27. Full Cycle Impact of Pore Geometry
      • Well Planning
        • The position of the target interval as a function of buoyancy is a critical factor.
          • High in the column there is a greater probability that the oil is charging the occluded pore systems.
          • Near Pc=0 only the large pore systems will contain oil.
        • Near the contact low drawdown is necessary to prevent coning.
          • Horizontal wells that will have small drainage radii.
          • Consider using MRC wells to effectively increase drainage area.
          • Minimize porpoising. Sumps cut down the effective flowing cross section, especially in low influx wells.
          • Strongly consider OBM drilling fluids combined with UBD and/or CTD. Also pre-consider the deployment of ICDs.
    • 28. Full Cycle Impact of Pore Geometry
      • Drilling the well
        • Over balance drilling with non-wetting fluid introduces a relative perm dominated skin which can be significant in low perm rock.
        • Over balance drilling conveys cuttings into the large pore system further reducing to eliminating perm by reducing or blocking pore throats.
        • OBM allows you to easily detect if water is the mobile phase.
      • Completions
        • Acid is conveyed as ions in water suspension and will only react if they come in contact with the rock surface. Occluded pore volumes are connected by small pore throats and are most likely effected. Significant volumes of “trapped” water can be mobilized.
        • Acidization dramatically alters the Kv/Kh in the near wellbore region.
        • Acid rinds in perforated completions can never be isolated by conventional squeezes. The first continuous filament of water entering is conveyable through the entire continuous perforation length.
        • If required in perforated production wells, consider short perforation intervals and numerous blanks to facilitate conformance at the well.
    • 29. Full Cycle Impact of Pore Geometry
      • Formation Evaluation
        • Computing total porosity is straight forward but what of efficient porosity?
        • Do you trust your Sw computations?
          • Archie exponents should be a function of RRT
          • Induction tools are conductivity seeking devices
          • Does the Archie Sw favorably match capillary response?
          • How do you measure drainage RI in oil wet rocks?
          • Is there a better way to obtain Sw using logs? Limitations?
        • Do you understand your flow SCAL?
          • Which volume of porosity are you flowing through in the lab?
          • What would your Rel-k curves look like if the occluded pore volume was eliminated (low buoyancy model)?
          • Is the hydrocarbon in the right pore geometry when you execute the test?
          • Do you do Deane-Stark on results to verify flood out ROS?
          • Have you made experiments that cover the entire column height range?
    • 30. Full Cycle Impact of Pore Geometry
      • Recovery Process
        • Do you understand the relative distributions of hydrocarbons in the efficient and occluded pore geometries?
        • At low sweep speeds, displacement should be more piston-like within the efficient pore system.
        • Caution that water is often more mobile than the oil.
        • ROS will most probably be contained within the occluded pore volume. The higher the column, the greater that volume.
          • What mechanisms facilitate mobilizing hydrocarbons locked in occluded pore volumes?
    • 31. backups
    • 32. Heterogeneity at the Plug Level Hi K Low K Pore Body Plug Scale Pore Throat Pore Body Pore Body Pore Body Pore Body
    • 33. Sample Preparation for RRT 1 ½ x 3 inch host plug (Routine & SCAL) 1 inch RHC P&P @ 800 psi FF Kbrine, miniperm MICP 1 inch RHC P&P @ 800 psi Miniperm TS MICP spare 1 inch RHC P&P @ 800 psi Miniperm TS spare MICP spare 1 ½ inch RHC P&P @ 800 psi P&P @ elevated PSI FF Kbrine, miniperm