Enhanced Iterative Formation Evaluation of El Tordillo Field, San Jorge Basin, Argentina: Using Electrofacies and Production Prediction Index Determination
- L.P. Stinco (Tecpetrol S.A.) | R.Y. Elphick (Schlumberger Holditch-Reservoir Technologies) | W.R. Moore (Schlumberger Holditch-Reservoir Technologies)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- October 2002
- Document Type
- Journal Paper
- 402 - 409
- 2002. Society of Petroleum Engineers
- 5.1.3 Sedimentology, 4.1.5 Processing Equipment, 5.1.1 Exploration, Development, Structural Geology, 5.6.3 Deterministic Methods, 1.6.9 Coring, Fishing, 5.1 Reservoir Characterisation, 1.8 Formation Damage, 1.6 Drilling Operations, 1.2.3 Rock properties, 5.5 Reservoir Simulation, 5.7.1 Estimates of resource in place, 5.6.1 Open hole/cased hole log analysis, 5.6.9 Production Forecasting, 4.1.2 Separation and Treating, 5.1.5 Geologic Modeling, 2.4.3 Sand/Solids Control, 4.3.4 Scale
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Formation evaluation, electrofacies, and production prediction index determination based on inductive and deductive methodologies were completed in El Tordillo field, north flank of the San Jorge basin, Argentina. The applied methodology consisted of a number of steps, including the following: editing and normalization of the log data; modeling of the deep resistivity; electrofacies determination; standard log analysis; computing a production prediction index; and summations. Using cutting and core data, the 15 electrofacies determined were associated with the geological facies encountered within the field and on the rock types. Consequently, porosity/permeability relationships were established between the core data and the productive electrofacies. These relationships permit computing permeability from the computed effective porosity. A production prediction index was derived from the computed results and correlated well with existing production data. Zones of possible bypassed production were identified, and some tested successfully. Finally, summations were used as input for original-oil- in-place estimation, geological modeling, and numerical simulation.
El Tordillo field is situated on the north flank of the San Jorge basin in Chubut Province, Argentina. It is approximately 50 km from the town of Comodoro Rivadavia and 1500 km south of Buenos Aires (Fig. 1). The field was discovered in 1932 and was operated by Yacimientos Petrolíferos Fiscales (YPF) from 1932 to 1991, when the Consortium El Tordillo (in which Tecpetrol S.A. is the operator) assumed operations. More than 1,000 wells have been drilled in the field, and production is spread over approximately 57 km2.
Hydrocarbons in El Tordillo field are structurally and stratigraphically trapped in numerous fluvial sand bodies that are grouped in three Cretaceous units. These formations are El Trebol, Comodoro Rivadavia, and Mina El Carmen. The latter two formations are analyzed in this work.
The fluvial reservoir units are sandstones with high lithic and pyroclastic (tuffaceous material) content. The quality of the reservoirs generally improves up-section as pyroclastic content decreases, but hydrocarbon accumulations throughout the producing interval are highly compartmentalized because of faulting and the discontinuous nature of fluvial sandstone reservoirs. Refs. 1 and 2 provide relevant geological, geophysical, and engineering observations from studies performed within the field.
In 24 wells located in the southwest part of the field, formation evaluation and electrofacies determination were performed by applying an appropriate set of inductive and deductive methodologies. The analysis was performed over the main producing intervals within the Comodoro Rivadavia and Mina El Carmen formations. The methodology was composed of a number of procedures, including data editing and normalization of the log data, modeling of the deep resistivity, electrofacies determination, standard log analysis, computing a production prediction index, and summations. The analysis procedures were calibrated with the help of cutting descriptions, cores, and production test data. The deep resistivity modeling greatly improved the tool response in thin bed layers and enhanced the quality of the readings. The Dual Water saturation estimation was optimized with the modeled resistivity and the electrofacies control. K-means cluster analysis in four dimensions was used to define 15 electrofacies. The 15 electrofacies were associated with geological facies and rock types using cutting and core data, as well as production data. A good match of the electrofacies data with the geological data was obtained with the remaining data. The consistency of the electrofacies and the computed results was used as a quality control to check the validity of the model parameters through the iterations to the final solution. Porosity/permeability relationships were determined from core data for each of the productive electrofacies. These relationships were used to compute permeability for all depths in all wells from the computed effective porosity and the electrofacies. The results of all the analyses were combined to derive a production prediction index. This index correlated well with existing production data and indicated a number of zones of possible bypassed production; some of these tested successfully. Summations on the results were used as input for original-oil-in-place estimation, geological modeling, and numerical simulation.
Petrophysical Analyses and Methodologies
The petrophysical data processing and analysis was performed following a number of interrelated procedures that were highly dependent on the quantity and quality of the data.
El Tordillo was discovered in the 1930s. The wireline data have been acquired from the initial discovery until the present time. The data, therefore, vary from the old electrical survey (ES) logs that have only spontaneous potential (SP), long normal (LN), and short normal (SN) resistivity - sometimes with the addition of micronormal (MNOR) and microinverse (MINV) curves - to modern suites of logs that have many curves. The wells in the study area were drilled recently enough that all of them have induction and sonic logs; in addition, some have neutrons and densities.
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