Cementation factor and water saturation in carbonates vs. seismic amplitude

Krakowska Paulina* , Niepsuj Magdalena*

*Department of Geophysics, Faculty of Geology, Geophysics and Environmental Protection,

AGH-University of Science and Technology, Poland

ReferencesBorai A.M. 1987: A New Correlation for The Cementation Factor in Low-Porosity Carbonates. SPE Formation Evaluation, December, paper SPE 14401 Montaron B. 2008: Connectivity Theory- A New Approach To Modeling “Non-Archie” Rocks. SPWLA 49th Annual Logging Symposium, Edinburgh, Scotland, 25-28 May, 2008, paper GGGBG Group materialsBała, M. 2009: Shear wave velocity (Vs) determination on the basis of well log data. Geological Review, Vol. 57, No. 12

Cementation factor is one of the main coefficients in petrophysics. It is included in the basic petrophysical equation called Archie equation. Wrong determination of cementation factor causes incorrect evaluation of water saturation. The purpose of the analysis is to check alternative method reliability of cementation factor calculations in carbonates. Sampled carbonates indicate microporosity, fracturing and stylolites. Variety of P-wave and S-wave velocity and velocity ratio are the source of information about lithology of reservoir rocks and confirm also level of hydrocarbon saturation (show perspective areas). To achieve accurate and excepted results there is a need to combine high quality seismic data, petrophysical and geological knowledge.

WorkflowWorkflow1. Cementation factor m calculations usingthree new approaches:• Connectivity equation• Borai formula• Shell formula Φ

−Φ−−

=ln

)]1

ln()1[ln(*C

C SS

m

µ

Cementation factor vs. connectivity equation (Eq. 1)

Sc- critical water saturation,Φ- porosity,µ- conductivity exponent

042,0

035,02,2

+Φ−=m Borai formula (Eq. 2)

m- cementation factorΦ- porosity

Φ+= 019,0

87,1m Shell formula (Eq. 3)

Discrepancy between well log data (fig. 1) and reservoir tests was observed inanalysed interval (4800-4900 m). Reservoir tests indicated gas saturation incontradiction to well logs (decrease in resistivity measurement). What may be thecause of this discrepancy? Wrong determination of cementation factor m is one of theideas. There was made an attempt to calculate cemenattion factor m using three newapproaches. The result are: m from connectivity equation= 1.341 and 2.004, m fromBorai formula=2.042, m from Shell formula=1.987, input m=1.913.

2. Water saturation Sw calculations using new determined cementation factors m

3. Use of fluid substitution method, which assume diverse water saturation,to calculate P-wave and S-wave velocities, Vp/Vs ratio and also density

Fluid substitution - assumed water saturations - Brie’s equation - results

4. 3D Crossplots construction

5. Seismic syntheticanalysis

The best result was obtained after using connectivity equation for m=1.341 (SWTAF- blue curve on fig. 2). Water saturation SWTAFpresents the highest gas saturation in analysed zone what was provedby reservoir tests. Fig. 2 presents differences in P-wave and S-wavevelocities. P-wave velocities show strongest parting than S-wavevelocities. However Vp/Vs ratio is one of the best gas indicatorsbecause of the highest gas sensitivity what is observed as a curveseparation. Fig. 3 and 4 present 3D crossplots. Low P-wave and S-wave velocity values express low water saturation.

To create theoretic seismic synthetics (fig. 5) the Ricker signal was used. Theoretical modellingwas carried out in order to estimate wave field diversity depending on water saturation.Toosmall differences between these theoretic seismic synthetics were observed. They do not qualify to draw a conclusion about determination of gas saturation degree. Theoreticalmodelling can be use for qualitative seismic interpretation. Amplitude analysis are not enoughdistibutive. Mainly it is related with P-wave velocity decrease under influence of variable gassaturation in pore space.

Conclusions:Correct determination of cementation factor is essential for proper water saturation estimation. Changes in P-wave velocities are likely seen than in S-wave. Decrease of the Vp/Vs ratio may be treated as a gas indicator in reservoir rocks. Fractures and microporosity play an important role in geophysical interpretation.

Fig. 1. Well logs for well A: GR- gamma ray, CALI- calipper,, RXO- shallow resistivity , RTO- deep resistivity, RHOB- density, PHIN- porosity, DTCO- P-wave interval time, DTSH- S-wave interval time

Fig. 2. Result of fluid substitution method- determination of P-wave and S-wave velocities, Vp/Vs ratio and also density

Fig. 3. 3D crossplot after using connectivity equation m= 1.341: P-wave velocity vs. S-wave velocity vs. water saturation

Fig. 4. 3D crossplot after using connectivity equation m= 1.341:Vp/Vs ratio vs. water saturation vs. porosity

Fig. 5. Seismic synthetics for various P-wave velocities and density (SS_B- for m=2.042, SS_S- for m=1.987, SS_m1341_M- for m=1.341, SS_m2004_M- for m=2.004