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Using Buckles Plots to Aid Log Analysis By Kenneth Chaivre Wednesday June 25, 2008

PROBLEMS WITH ROUTINE LOG ANALYSIS High Sw wells may produce gas with very little or no water Low Sw wells produce water or fail to produce. Similar looking wells produce differently. Leads to Changing “m” and “n” We have all encountered zones where normal log analysis has failed. Especially vexing are zones that appear to be wet with 50% or more Sw that produce water-free or nearly water-free. We look to m and n and reduce them to make “wet” zones appear to have lower Sw. Changing the saturation and cementation exponents is the result of testing the zone. Is there some way to know beforehand?

Review of R. S. Buckles Paper - 1965 Part of Volumetric Calculation is Ø * (1-SW) Ø * (1-SW) = Ø – Ø*SW Buckles showed that Ø * SW irr is a constant (Ø * SW is the Buckles Number or Bulk Volume Water) For Hand Calculations Ø – Constant is a quick and easy way to calculate Ø * (1-SW) Showed that Ø * SW is related to grain size. In 1965 Buckles, who worked as an engineer for Imperial Oil in Canada, published a paper entitled “Correlation and Averaging Connate Water Saturations”. He was concerned with getting more accurate volumetrics. He found that phi * sw was a constant and that the constant was related to grain size.

BUCKLES PLOT High Perm Transition Fine Grain SWirr Transition Plot comes from SLB and I saw it in the 1970’s. This is the Buckles Plot and the Buckles Number is the product of PHIE and Sw. Either decimal (.02) or as Porosity units (200). Two versions in SLB using different Perm functions and reversing the axes. I use Sw on the X axis. Be consistent. I used it for permeability. Note that the perm decreases downward and to the right and increases upward and to the left. Grain size becomes finer away from the origin and zones that are at irriducible water saturation follow or are sub-parallel to the Buckles Lines (aka constant Bulk Volume Water) Coarse Grain Low Perm

Parameters for Plot The plot is very simple I put SwA on the “X” axis and porosity on the “Y” axis. The only filter is Gross Sand which is a Vshale cutoff.

BVW vs Grain Size and Lithology This needs to be take with a grain of salt. The lowest BVW value that I see in Carthage is .02 and the sand was described as fine- grained. Really coarse grained sand I believe should be down around .009 and lower. The Buckles numbers apply to carbonates as well as clastics.

Uses of the Buckles Plot Identify Swirr Zones for Analysis i.e. Calculate Capillary Pressure Identify One Type of Low Resistivity Pay Identify Stratigraphic Flow Units Environments of Deposition We will look at three uses of the Buckles Plot.

Calculations with Swirr If Swirr is Known and If Permeability (k) can be estimated then Capillary Pressure (Pc) can be estimated Pc= (19.5*Swirr^-1.7)*(k/100*Phie)^-0.45 Height Above Free Water (H) can be calculated H= (.35 for gas)*(Pc) H= (.7 for medium oil) * (Pc) You can calculate depth to free water if you know the permeability and that the zone is at irreducible water saturation. This equation is based on the bouancy of the gas or oil and how many feet of hydrocarbons are necessary to displace the water in the pores.

Capillary Pressure R35 = 5.395 * ((K^0.588)/(100*PHIE^0.864)) Pore Throat Radius (r) can be calculated r = (108.1) / Pc Winland r35 Values – delineate commercial hydrocarbon reservoirs R35 = 5.395 * ((K^0.588)/(100*PHIE^0.864)) R35 < 0.5μm (microns) – Tight R35 > 0.5μm (microns) – Will Flow In addition to finding the depth to free water, an estimate can be made of the size of the effective pore system. H D Winland of Amoco developed an empirical relationship among PHI, Ka and pore throat radius. He found that the effective pore system that dominates flow through a rock corresponds to a mercury saturation of 35%.

BUCKLES IN PRACTICE DEPTH TO FREE WATER Examples

Structure –Carthage CVS G Show monoclinal dip with well to find depth to free water

ZONES SELECTED FOR H free water These are the two zones selected for analysis.

MAUDE #13 ZONES FOR DEPTH TO FREE WATER Note the red zone is coarser grained than the yellow (cyan) zone but the yellow is more porous and permeable

RESULTS OF DEPTH TO FREE WATER Report modual in GES lists depth and the calculated curve “depth to free water” and calculate subsea. The two sands seem to have different free water contacts. The subsea values in both cases are off my structure map.

FREE WATER - CVS G 200 – 400 FT DOWNDIP Free Water calculates off the structure map.

Low Resistivity Pay Zones very fined grained rocks have high bound water

ASHTON 2 UNIT #10 CVS F All of the sands in the Cotton Valley were treated alike. The rule of thumb was that 10 ohms of resistivity were required to be productive. Note this sand has consistently less than 10 ohms. The porosity is good for CV 7-8%. But the Sw is 60% at best. There were gas shows.

ASHTON 2 UNIT #10 CVS F BUCKLES BVW 0.049 This zone has a high Sw but it is related to grain size – not producible water. The BVW is .049 and consistent. The points that move to the NE show water but the grain size is getting very small and probably will not produce water.

RESULTS - 677mcf & 8.6 bw This zone was perf’d and made 677 mcf and 8.6 bwpd

STARR CO - FROST FIELD

STARR CO-VXBG Shell had such good success rejuvenating McAllen Ranch field that they are rejuvenating Frost Field which was originally drilled by Fina. The pink zone in the depth track are the perfs. Note that contact between the green hydrocarbon and the blue water is almost straight. This is the bulk volume water and the straightness indicates that it is at irreducible water saturation. The Sw is about 30% and phie aver. is ~16 –17%.

STARR CO - VXBG This zone on the Buckles plot gives us more info. Note that Sw increases as porosity dips below 10%. This would be a good phi cutoff. The red points are coarser grained than the yellow or green and provide most of the reservoir. From 6610 to 6622 the grain size is smaller. The red points have been identified and appear as red on the log column.

STARR CO – VXBG - NOT PERFD This is the interval below the productive one. Same log. The BVW line is almost straight but there is wobble at 6910 - 6920. The aver phie is ~14% The Sw is ~35 –40% and gets higher downward. This zone was deliberately passed over. Is it wet, tight, or productive?

STARR CO – VXBG – NOT PERFD Notice that the Buckles # or BVW is .06 - .07. The upper zone was .04 -.05. There is no water here. The spread in data is due to changes in grain size. It may be tight but Winland R35 is the same as the zone above. This zone should be tested. I think Fina missed this one. Note that the wobble in the BVW line from 6910 – 6920 on the log is not caused by increased Sw but is a lithology change.

IDENTIFY FLOW UNITS

PETER 2 UNIT #10 CVS F We had a Totco gas detector on this well. It is totally unmanned so it can be a challenge correlating the gas shows with the zone on the log. However we have a gas increase going thru the CVS F.

PETER 2 UNIT #10 – CVS F This shows the art involved. The green, purple, and blue zones probably produce the gas and the magenta zone the water. Look at the distribution of the grain size from 9040 to 9060. This is a fining downward sequence.

COTTON VALLEY FIELD Webster Parish, Louisiana

FIRST BANK OF PD - S Sarepta Field This well is in S Sarepta field and has produced 17 Bcf from three Gray Sands of Smackover age.

FIRST BANK OF PD I chose this well so you could see what a good well looks like on this plot. Note the BVW is Very LOW ~.01 - .02 over most of the interval. Also most of the points are in the NW quad. This is the High perm area. 17 BCF

CLEMONS – UTZ GRAY This is the same sand about 4 miles to the east.

CLEMONS – UTZ GRAY Although the zone is thinner, the BVW is still about .01 - .02 This too is in the high perm area.

CLEMONS – 1ST GRAY Same well next highest sand.

CLEMONS – 1ST GRAY This zone is finer grained than the one below with lower porosity but perm is not too bad

CLEMONS - UTZ VS 1ST GRAY 1ST GRAY UTZ Here is the comparison. Lower porosity, finer grained, and higher Sw but still not wet as production showed. Also 3%-5% porosity has coarser grains and will probably contribute.

CLEMONS – 3RD GRAY Thin Beds Same well next highest sand. Coarse grain but more points are moving into finer grained and lower perm area.

CARTHAGE FIELD ENVIRONMENTS OF DEPOSITION

CARTHAGE - TYPE LOG The CVS G and CVS F are very close to one another but look at the Buckles plot.

From Xindi Wang Each point represents the average of the BVW for one zone. Note the separation. One Sw cutoff or Phi cutoff cannot properly characterize all the sands. The F and G are with 200 ft of one another. There is a fair amount of spread among the points for each sand. Lets contour up the BVW for the CVS G and see if there is some pattern.

CARTHAGE – CVS G Structure First. Here is a structure map of the CVS G. Monoclinal dip to the south and west. The trap is a huge structural stratigraphic trap.

CARTHAGE BVW G with Structure The BVW was overlain and shaded on top of the structure. There is no correlation as the Sabine Uplift postdates the deposition of the CVS G. Although this is a a small portion of Carthage field there is a marked decrease in grain size from the NW (blue) to the SE (orange). This fits the bigger picture. Blue – 0.02 Red – 0.04

Late Cotton Valley Time Cotton Valley Sand Wave reworked deltaic shoreline. Fine grained argillaceous sandstones Here is the big picture. The CVS G is part of the series of wave-reworked deltaic shorelines.

CARTHAGE BVW G with Structure Back again to the BVW distribution in the CVS G. Note the fine grained trend running NS in the center of the slide. Could this be a channel that was cut and later filled with finer grained sediments? Blue – 0.02 Red – 0.04

MAUDE #4 LOG Log to west of possible channel

MAUDE #4 The Maude 4 shows several grain sizes. The coarsest is at the top in red grading into the magenta and finally the finest at the bottom cyan.

CALLOW #9 LOG Log in channel

CALLOW #9 We have lost almost all of the coarser grained red and have more of the cyan.

MAUDE #4 – CALLOW #9 Here is the comparison.

CVS G Qgas vs BVW Qgas – from Production Logs BVW Fewer points BVW Channel? In area of poorer production.

RESULTS IN CARTHAGE FIELD Perf’ing more and better zones Better Frac and completion techniques

Qgas vs Time Increased Initial production during this round of drilling

Qwtr vs Time We have not increased the amount of water.

TIME VS BW/MCF The Barrels of water per MCF have actually deceased since the drilling program began in 2005

CONCLUSIONS Useful in Establishing Capillary Pressure Use several equations to find depth to free water Differentiate Between Zones that Look Alike on Logs but Produce Differently. Help Establish Sw Cutoffs Different Rocks need Different Cutoffs Suggests Environment of Deposition Makes You Ask Questions

REFERENCES Buckles, R. S., 1965, Correlating and Averaging Connate Water Saturation Data: The Journal of Canadian Petroleum Technology, v. 5, p.42 -252 Aguilera, R., 2002, Incorporating capillary pressure, pore throat aperture radii, height above free-water table and Winland r35 values on Pickett plots: AAPG Bulletin V86 No. 4 p. 605-624 Doveton, J. H., 1994, Graphical Techniques for the Analysis and display of Logging Information: Chapter 2 Vol CA 2: p 23-46 Geologic Log Analysis Using Computer Methods Doveton, J. H., 1999, Integrated Petrophysical Methods for the Analysis of Reservoir Microarchitechure – a Kansas Chester Sandstone Case Study: AAPG 1999 Midcontinent Section, Transactions, Geoscience for the 21st Century PfEFFEER Concepts: http://www.kgs.ku.edu/Gemini/Help/Pfeffer-theory Asquith, G. and Gibson C., 1983, Basic Well Log Analysis for Geologists: p.98

THANKS Xindi Wang Christy Demel Pat Noon Matt Pickrel Randy Nesvold

QUESTIONS