indicated these two main sand layers are com- partmentalized (below). Lack of connectivity resulting from compart-
mentalization is a significant risk in deepwater development because its existence requires additional wells to contact untapped reserves. Extremely high well costs can make a project uneconomical. Because of the thick salt canopy overlying the Tahiti field, delineating reservoir architecture and potential compartments from seismic data is challenging. Moreover, many sealing barriers are too thin to be visible in seis- mic data. To understand the reservoir connectiv-
ity, reservoir engineers have focused on the properties of sampled fluids. Data from 14 DFA sample stations in the M21 sands were analyzed in the study.38
M21A data M21A model
Downhole
and laboratory measurements show undersatu- rated black oil with GORs ranging from 550 to 650 ft3/bbl [99 to 117 m3/m3]. Geochemical fin- gerprinting from gas chromatography confirmed pressure-data results: The M21A samples are similar to but distinct from those recovered from the M21B sand. The DFA data indicated an asphaltene compositional gradient, as revealed by an increase in fluid color with depth, in both
2X,700 2X,650 2X,600 2X,550 2X,500 2X,450 2X,400
2X,750 Tahiti Field, M21A and M21B Sands 2X,000 2X,500 2Y,000 2Y,500 2Z,000 19,XXX 19,XXX 19,XXX Formation pressure, psi North
GC 596 1 ST1
GC 640 1 ST2 BP1
Tahiti Field, M21 Cross Section GC 640 1
GC 641 1 ST1
GC 640 2 ST1
19,XXX
M21A sand M21B sand
M21A sand, GC 596 1, ST1
South
GC 640 2 ST2 BP2
sand bodies. This gradient was corroborated by laboratory fluids measurements. For development-well planning, engineers
M21A M21B
integrated information from this study to predict the DFA measurements at proposed well loca- tions. Synthetic Fluid Profiling logs, based on asphaltene analysis, were generated for a subse- quent well and matched the DFA data. This vali- dated the model and verified connectivity within the sand layers found in the new well. Had there been no match, DFA stations could have been reacquired for validation or the geologic model adjusted to account for differences. In the Tahiti field, crude oil has a low GOR and
Oilfield Review Autumn 09
> Tahiti field, two separate sands. The petrophysical cross section (bottom) of the Tahiti field, developed from several wells and sidetracks (STs), exhibits considerable heterogeneity. The M21A and M21B sands are the primary targets and, although similarly pressured, are in two different pressure regimes (top left). The two primary sands are thus disconnected. The gas chromatography (GC) starplot diagram (top right) indicates geochemical fingerprints that distinguish M21A crude oils (blue) from those from the M21B sand (red). Oil from the M21A sand in a subsequent well, drilled in the north area of the field, had its own GC fingerprint (green), indicating possible separation from the rest of the reservoir.
is fairly incompressible. Consequently, gravity determines the asphaltene distribution. In an EOS the gravity component consists of Archimedes buoyancy for the asphaltene nano aggregate in a Boltzmann distribution. Fluids experts developed an EOS model based on a fixed asphaltene particle size, correlating optical density to depth. As an indication of connectivity, a simple equation was developed from field data that accounted for the asphaltene distribution in almost the entire field.
FluidsLab Fig. 17 ORWIN09/10-FluidsLab Fig. 17 M21A M21B
Geochemistry Analysis Based on High-Resolution Gas Chromatography
0
M21B data M21B model
M21A North data M21A North model
0.5
1.0
1.5
2.0 Optical density at 1,000 nm
> Optical density trends and asphaltene modeling. The Boltzmann distribution model predicted color (OD) using a fixed particle size but with different asphaltene concentrations. Data from samples and the predictive model again demonstrate that the M21A (blue) and M21B (red) are two separate sands. Data from a subsequent well drilled in the northern area of the field (green) yields a different trend because oil from the M21A sand in the northern section has a lower asphaltene concentration than that in the south and central regions.
2.5
3.0
50
Oilfield Review
Subsea depth, ft
Depth, ft
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