Schlumberger
Contents
G1.0 WATER WATER SATURATION SATURATION IN SHALY SANDS ....... ........ ......... ........ ........ ........ ........ ........ ........ ........ ... 1
G1.1 G1.1 INTRODU INTRODUCTI CTION...................................... ON............................................................. .............................................. .............................................. .................................1 ..........1 G1.2 THE DUAL DUAL WATER WATER MODEL....................................... MODEL.............................................................. .............................................. .....................................1 ..............1 G1.3 DUAL WATER MODEL FORMULAE:......................................................................................6 G1.4 PROCEDURE FOR USING THE DUAL WATER MODEL.........................................................7 G1.5 DWQL Pass One ............................................ ................................................................... .............................................. .............................................. .........................8 ..8 Inpu t...................................... t............................................................. .............................................. .............................................. .............................................. .........................8 ..8 Outpu t.......................................... t................................................................. .............................................. ............................................... .........................................8 .................8 G1.6 DWQL Pass Two................................. Two........................................................ .............................................. .............................................. ...................................1 ............1 0 Inpu t...................................... t............................................................. .............................................. .............................................. ..............................................10 .......................10 Output...............................................................................................................................10 G1.7 CYBERLOOK CYBERLOOK QUALITY QUALITY CHECKS CHECKS .............................................. ..................................................................... ..........................................16 ...................16 G2.0 WORK WORK SESSION SESSION....................... .............................................. .............................................. .............................................. .............................................. ...........................1 ....1 7
(05/96)
Introduction to Open Hole Logging
(05/96)
Schlumberger
G1.0 Water Saturation in Shaly Sands G1.1
INTRODUCTION
Since the introduction of CSU wellsite surface instrumentation to well logging, the dualwater model has been applied as a means of quick, effective interpretation of basic logs. This technique has been extended to the MAXIS 500 wellsite surface instrumentation and more recently to IBM-compatible PCs through QLA Quick Log Analysis program (version 2). This section discusses the dual-water model as it applies to Cyberlook wellsite openhole evaluation and QLA version 2. G1.2 THE DUAL-WATER MODEL
In 1972, the dual-water model was the sub ject of an SPE paper "The Theoretical and Experimental Basis for the Dual Water Model for the Interpretation of Shaly Sands" by Clavier, Coates and Dumanoir. Although this section discusses the important basic ideas about the model, reference should be made to this paper if a more detailed explanation is necessary. The dual-water model is an improvement over the Waxman-Smits model presented in 1967 and better fits their experimental data. The Waxman-Smits model proposed that a shaly formation behaved like a clean formation of the same porosity, tortuosity and fluid content except that the water appears to be more conductive than expected from its bulk salinity. The excess conductivity is due to additional cations held loosely captive in a diffuse layer surrounding the clay particles to compensate for the deficiency of electrical charges in the
clay crystal. This model did not take into account the exclusion of salt from part of the pore volume near the shaly surface. Ion distribution near a clay surface should be as shown in Figure G1. In other words, the layer of water bound to the shale surface contains more positive (Na + ) ions than negative (Cl–) ions. This fact is necessary to balance the negative internal charge distribution of the shale particles. The thickness of the diffuse layer of positive (Na + ) ions X is related to the salinity of formation water, being smaller for more saline waters. Hence, conduction of current flow through this bound water is mainly by positive ion transport. d
Figure G1: Schematic of Diffuse Layer Ionic Concentration
(05/96) G-1
Introduction to Openhole Logging
Actually, the positive (Na+ ) ions are kept some distance from the clay surface by the hydration water around each cation and the water absorbed on the clay surface (see Figure G2). As a consequence, the diffuse layer thickness cannot be less than X . However, X = X when the connate water is saline enough. In other words, when the formation water has deficient salinity, the resistivity of the bound water is relatively constant. d
d
Hence, when R is used as the resistivity of bound water for the shale contained in nearby reservoirs it could be incorrect. In practice, this is not found to be too much of a problem, and generally R derived from shales may be used in adjacent beds. WB
WB
h
For sodium clays, the distance X is about 6Å and the Na+ ions will be stacked in the Helmoltz plane whenever the resistivity of the brine in the pores is less than 0.425 at 75 oF [24oC]. h
b.
Free Water: All water that is not bound is
c.
Total porosity φ T : Total porosity is the
free water. Although free water, normally associated with the pore space, is not necessarily producible. It contains the fraction of water that is irreducible. fraction of unit volume of formation occupied by fluids, that is, bound water, free water and hydrocarbons.
This thin sheet of salt-free water (the clay water) is important because clays have tremendous surface area, as much as 91071 ha/m3 compared to 1.5 to 3.0 ha/m3 for a typical sand, and the volume of clay water is far from negligible in comparison with the total pore volume. We can now make certain definitions in relation to bound water, free water, the volumes they occupy and their saturations. a. Bound Water: This is the water adhered to shales as described. In addition to the bound layer, shales may contain water trapped within the structure and not expelled by the rock compaction. This water does not have the same ion distribution as the surface bound water and so it has a different conductivity. In the event that the resistivity of bound water defined here as R is derived from 100% shale zones, the value of R is affected by this trapped water. WB
WB
(05/96) G-2
Figure G2: Schematic View of Outer Helmoltz Plane
Schlumberger
d. Effective porosity φ : It is the fraction of unit volume of formation occupied by free water and hydrocarbons. It can be derived from the total porosity by removing the bound water per unit volume of formation. e
e.
Total-Water Saturation S WT : It is de-
fined as the fraction of total porosity occupied by bound and free water. f.
Bound-Water Saturation S WB: It is de-
fined as the fraction of total porosity occupied by bound water.
g.
The relationship between these terms is shown diagrammatically in Figure G3. Because we have separated the surface-layer water from shales we are left with a dry colloid fraction. As a formation becomes increasingly shaly the colloid plus bound water fraction increases until we have a 100% shale formation consisting of a certain fraction of bound water and the remainder of dry colloids. Under the definition of total porosity φ T , a pure shale therefore has porosity filled with bound water (S = 1, S = 0). The effective porosity, φ , as defined is, of course, zero. The evolution of a formation with increasing shaliness is shown in Figure G4. WB
WF
e
Free-Water Saturation S WF : It is de-
fined as the fraction of total porosity occupied by free water. h. Effective Water Saturation S : It is defined as the fraction of effective porosity occupied by free water. It can be derived from the total-water saturation. WE
(05/96) G-3
Introduction to Openhole Logging
Water Saturation Graphical Definitions
Figure G3
(05/96) G-4
Schlumberger
Evolution of φT with Shaliness
Figure G4
(05/96) G-5
Introduction to Openhole Logging
G1.3 DUAL WATER MODEL FORMULAS
The main objective of the dual-water approach whether it’s through Dual-Water Quick Log (DWQL), QLA software, or otherwise, is to reconstruct the wet formation resistivity R . 0
2)
S WB =
φ T
3)
φ T = φ WF + φ WB + φ H
(If hydrocarbon is present).
Consider a wet shaly formation where: C0
φW B
= wet true conductivity
From the Archie relationships: F =
CWB = bound-water conductivity
1/ φ and F = R /R 2 T
o
w
(Shale) CWF = free-water conductivity
(Note: For simplicity of derivation,
(connate water)
a = andm 1 = 2,although theycould
φ F = volume of free water
be other specific values.)
φ B
= volume of bound water
φ T
= total porosity.
Given these, then φ = φ + φ and hence T
B
F
R w = φT 2 R0
which gives C = φ C where: C is the conductivity of the bound- and free-water mixture. 2 T
0
S WB =
φW B
W
W
φ T
Because φ represents the volume of bound water, which thus represents the proportion of shale out of the total volume. Therefore, S is in effect the volume of shale in the formation under investigation.
Considering volumes, we have
B
φ T C W = φWB C WB + φF C WF
WB
C W =
By definition:
1)
S WT =
φ WF + φ W B φ T
= S
WB
φ F C W F
φT
φT
+
C WB + (1 – S WB)C WF
∴ C 0 = φT 2[S WBC WB + (1 – S WB) C WF ]
or, in resistivity terms R0 =
(05/96) G-6
φ BC WB
RWF RWB
φT 2[S WB RWF + (1 – S WB) RWB ]
Schlumberger
Displayed graphically, our results are as follows (Figure G5):
4.
S WB: related to V SH and for our purposes can be equated to V SH . Therefore S WB = V SH .
To this point, we have calculated R and V for our example and have determined a gas corrected φ . All that is now required is to calculate R . This can be done with the same φ and φ values determined in our previous section, along with a value for R at the same point(s) on the log. w
SH
T
WB
Figure G5
NS H
Water Saturation and Effective Porosity:
D SH
SH
S =
R0/R t
WT
φe
Utilizing all of this data, a value for wet resistivity, R0, can be determined from
= φΤ (1 – S B) W
vbwe = φe S w
R0 =
1
1 ×
1 - V
φT 2 G1.4 PROCEDURE FOR USING DUAL-WATER MODEL
RWF
a) To evaluate a shaly formation using the dual water model, four parameters must be determined:
WB
V SH RWB
R0
S WT 2 =
Rt
wa
2. R : generally calculated from the shale surrounding the zone using the R technique.
+
using
1. R : from the SP, R technique, water resistivity catalog or known value. WF
SH
where Rt = RILD corrected for environmental effects as requ
WA
R WB = φTSH 2 × RtSH
φT
3.
=
φ NSH + φ DSH
2
1
and
F=
To arrive at effective water saturation one more step is required: S WE =
S WT – S WB
1 – S
S we
where V = S SH
WB
WB
φT 2
φ T : total φ from average of φ N and φ D
after correction for gas effect, if necessary.
(05/96) G-7
Introduction to Openhole Logging
We have now taken a shaly sand, corrected the log data for the effects of shale on both resistivity and porosity, as well as gas effect on porosity, and determined the effective S and hence S . w
HYC
BVW eff = S we × φe
(see Figures G6 and G9) 1. SP—optionally baseline drift corrected 2. GR—borehole corrected if caliper available 3. apparent grain density
Output
ρGRA =
b) In using software, the dual-water model is usually presented in two passes. The first pass is used to perform simple corrections to certain measurements and act as an aid to picking parameters for the second pass. Pass two performs the main output calculations.
4. apparent fluid resistivity R = R × φ 2 FA
T
TA
5. φ and φ in desired matrix N
D
TA
N
Input
1. mud weight 2. desired output matrix 3. recorded CNT matrix 4. bit size 5. optional SP baseline drift correction 6. logs—CNL/Litho-Density tool and deep resistivity ( R , R , or R from tornado chart if necessary).
(05/96) G-8
1 – φ TA
6. apparent total porosity φ = F ( φ , φ )
G1.5 DWQL PASS ONE
IDPH
ρ B – φ TA
LLD
T
φ TA
=
D
φ NLS – φ DL S 2
7. R for correlation T
8. MP1, MP2, and MP3 - Mineral proportions from the three-mineral LithoDensity model.
Schlumberger
Figure G6
(05/96) G-9
Introduction to Openhole Logging
G1.6 DWQL PASS TWO Input
1) All inputs used for pass one. 2) Clean and shale parameters for GR and/or SP and/or optionally φ RHOB, MP3. GR – GR MSIGR = GR – GR N
CL
SH
CL
3) Free- and bound-water resistivities. - R = R in a clean, wet formation. - R = R in a good shale formation. 4) Maximum total porosity φ . φ = highest φ in good hole. a. eliminates computation in bad hole. b. determines S – MSI relationship. 5) Expected clean grain density – ρ If ρ < ρ a minor correction is made to total porosity based on either: a. grain density or b. hydrocarbon volume and gas density. FA
WB
FA
MAX
MAX
TA
WB
GEX
GA
(05/96) G-10
GEX
T
TA
TA
BWF
SPSH – SPCL
WF
e
T
SP – SPCL
MSI SP =
(see Figures G7 and G10) 1) Shale index – minimum of indicators chosen. 2) Grain density. 3) R0 – reconstructed 100%-wet formation resistivity. 4) Water saturation. 5) Differential caliper – caliper-bit size. 6) Effective porosity φ . φ is φ corrected for light hydrocarbon effect, φ ≥ φ . 7) Water volume V . 8) Flags - Producibility – shading between R and R 9) MP1, MP2, and MP3 as for pass one.
Output
0
T
Schlumberger
Figure G7
(05/96) G-11
Introduction to Openhole Logging
Figure G8
(05/96) G-12
Schlumberger
Figure G9: Cyberlook Pass 1 for the Basic Log Set used in Sections B, C and D
(05/96) G-13
Introduction to Openhole Logging
Figure G10: Cyberlook Pass 2 for the Basic Log Set used in Sections B, C and D
(05/96) G-14
Schlumberger
Figure G11: Computational parameters for the Cyberlook using the Basic Log Set found in Sections B, C and D
(05/96) G-15
Introduction to Openhole Logging
G.1.7
CYBERLOOK QUALITY CHECKS 1. R0 and RT should overlay in clean, wet zones (if not Rwf is incorrect).
2. R and R should overlay in shale zones (if not, R is incorrect). 0
V SH must appoach 0% in clean zones
and 100% in shales.
S w should approach 100% in wet
7. Grain density must conform to local knowledge in clean zones and approach 3000 kg/m3 in shales.
φ e must be comparable with log po-
8. Are shows on pass 1 also shows on pass 2?
zones.
4.
6.
T
WB
3.
5. Differential caliper must compare to log.
rosity considering shale, matrix and gas effects.
(05/96) G-16
Schlumberger
G2.0 Work Session 1. Calculate S on the shaly sand example (Figures F10 – F13). WE
Hint: Use the R equation developed in this section. 0
(05/96) G-17
Introduction to Openhole Logging
(05/96) G-18
