The first production well encountered black
oil that correlated with the asphaltene concen- tration predicted from the discovery- and appraisal-well data (right). This analysis con- firms that the asphaltenes are in an equilibrium distribution in both the M21A and M21B sands. Consequently, each sand is predicted to have large-scale connectivity. This prediction was later confirmed during production. Distinct asphaltene trends are visible in the
data from the M21A and M21B sands (previous page, top right). A subsequent well drilled in the north section of the field revealed a lower con- centration of asphaltenes in the M21A sand than is found in wells drilled elsewhere. There was no pressure differential within the sand because the reservoir was at virgin pressure. With almost all other hydrocarbon properties being equal, the asphaltene distribution was the primary means of determining a lack of connectivity between the northern well and the rest of the reservoir. Interpretation following reprocessing of the seismic data confirmed the possibility of fault separation between the regions (below right).
Integration Is the Key The downhole laboratory provides a wealth of real-time information. But if DFA data are to be maximally utilized, it is important to treat them as pieces of a larger puzzle. Reservoir engineers integrate measured fluid properties with existing geologic models. Fluid predictions based on EOS models are either corroborated by the downhole measurements or the models can be adapted to fit the data. For example, in 2002 a North Sea operator
identified a large compositional gradient in a dis- covery well containing oil and gas.39
DFA technol-
ogy was fairly new, and the original sampling program was modified in real time to profile the complex and depth-variant fluid properties. From analysis of the data, reservoir engineers picked the depth of the gas/oil contact (GOC) higher in the reservoir and moved the oil/water contact
38. Betancourt SS, Dubost F, Mullins OC, Cribbs ME, Creek JL and Matthews SG: “Predicting Downhole Fluid Analysis Logs to Investigate Reservoir Connectivity,” paper IPTC 11488, presented at the International Petroleum Technology Conference, Dubai, December 4–6, 2007.
39. Gisolf A, Dubost F, Zuo J, Williams S, Kristoffersen J, Achourov V, Bisarah A and Mullins OC: “Real Time Integration of Reservoir Modeling and Formation Testing,” paper SPE 121275, presented at the SPE EUROPEC/EAGE Annual Conference and Exhibition, Amsterdam, June 8–11, 2009.
Wellbore
Well Section DFA Channel Data Measured DFA Well M21A
M21A sand
Measured DFA Predicted DFA
Predicted DFA
M21B sand
M21B
> Predicting DFA response. The DFA spectrometer measures the optical density from discrete channels focused on specific frequencies. The OD is computed from these data and used to quantify oil color. Asphaltenes are the primary source of this color. Using a modified Boltzmann distribution equation from nanoaggregate particle-size estimations of the asphaltenes, engineers developed a predictive color model. This model used DFA data from the original Tahiti discovery well to predict the response of spectrometer channels (shown as color bands in Track 3) for oil in a subsequent development well. The DFA data from the M21A and M21B sands (Track 2) matched the model, suggesting reservoir connectivity. Recent production data confirmed this connectivity, validating the original model.
Asphaltene concentration, %
1.5 3.0
4.0 5.0 6.0 7.0
Oilfield Review Autumn 09
N
Possible fault
FluidsLab Fig. NEW 16
ORWIN09/10-FluidsLab Fig. NEW 16
> Field-wide asphaltene concentrations. This 3D model of the M21A reservoir shows asphaltene concentration versus depth that is consistent with an equilibrium distribution of asphaltenes and indicates reservoir connectivity in the central and southern clusters of wells. The two well penetrations in the north show a similar but different distribution, which could indicate that this area is separated by a fault. A recent seismic reinterpretation also indicates a possible fault in this orientation.
Winter 2009/2010
51
Gamma Ray Resistivity
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