Shell No. 1
Gas Oil
Oil Filtrate Bound Water
Depth ft
810 820 830 840 850 860 870 880
> Gas, oil and water. The permeability (Track 2) is consistent throughout the zone from 820 to 880 ft except for two areas with lower permeability at 845 and 860 ft. The viscosity (Track 3) indicates light oil, but its source is the OBM filtrate. The continuous fluid-saturation logs from Shell No. 1 (Track 4) and Shell No. 8 (Track 5) show fluid changes occur with deeper DOI. The well was drilled with oil-base mud, and OBMF (dark green shading) displaces native fluids. In contrast, the Shell No. 8 data show more native oil (light green) and gas (red). There is very little free water in the wet interval below 880 ft, perhaps because the free water has been flushed by OBMF. The well has more oil than originally anticipated, and consequently, less gas.
0
Gamma Ray gAPI
100 0.1
Permeability mD
1,000 0.1
Oil Viscosity cP
10 40
Porosity, 1.5-in. Shell %
0 40
Shell No. 8 Gas Oil
Oil Filtrate
Porosity, 4-in. Shell Bound Water
% 0 0 1 1
T1, Log Mean ms
T1 Cutoff ms
Shell No. 1 T1, Distribution 63 9,000 9,000 0 1 1
T1, Log Mean ms
T1 Cutoff ms
Shell No. 8 T1 Distribution 63 9,000 9,000
calculated volume of gas and condensate available for export from the reservoir was greatly reduced. The initial objective of the well, to develop and produce a gas reservoir, was modified along with the development plans for the field.
Finding the Oil/Water Contact
Laminated sand-shale sequences, referred to as low-resistivity, low-contrast (LRLC) pay, are familiar to log analysts. They are often over - looked or improperly evaluated because the pay is not obvious using conventional logging tools. There are, however, high-resistivity, low-contrast (HRLC) reservoirs where everything looks like pay, and these provide an entirely different set of challenges. In HRLC reservoirs, resistivity changes caused by water-salinity variations and poor resistivity contrast between oil and water zones make determination of an oil/water contact (OWC) extremely difficult. Correct deter mination
of the OWC directly affects reserves calculations, completion designs and production decisions. Mistakes are costly, especially when high water cut reduces oil production while adding addi - tional water-disposal costs. Traditional evaluation techniques are based on resistivity contrasts between oil and saline formation water. Reservoirs containing fresh or brackish water may exhibit little or no resistivity contrast between the fluids. NMR fluid- saturation measurements are based on the volume of each fluid and are not depen dent on water salinity. Thus, oil and water saturations derived from NMR data offer an ideal solution for evaluating HRLC reservoirs and determining fluid contacts.
In a Middle East HRLC reservoir drilled with oil-base mud (OBM), determination of hydro - carbon saturation and the OWC was not possible using resistivity and porosity logs.18
measurements from wireline-conveyed pressure- sampling tools to determine fluid gradients. The condition of the borehole often deteriorated during drilling, and pressures were difficult to obtain because of seal failures, yielding inconclusive results. Costly and time-consuming DSTs were then performed to pinpoint the contact. Saudi Aramco discovered the subject field in the 1960s, but there had been no recent
16. Irreducible bound water in the reservoir remains in place during production, and hydrocarbons alone are produced.
17. White J and Samir M: “Continuous Characterization of Multiple Fluids in a North Sea Gas Condensate Reservoir by Integrating Downhole NMR with Downhole Sampling,” Transactions of the SPWLA 49th Annual Logging Symposium, Edinburgh, Scotland, May 25–28, 2008, paper X.
The method for finding the OWC was to use pressure
18. Akkurt R, Ahmad NA, Behair AM, Rabaa AS, Crary SF and Thum S: “NMR Radial Saturation Profiling for Delineating Oil-Water Contact in a High-Resistivity Low-Contrast Formation Drilled with Oil-Based Mud,” Transactions of the SPWLA 49th Annual Logging Symposium, Edinburgh, Scotland, May 25–28, 2008, paper Y.
Winter 2008/2009
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