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Evaluating the Shaly Sand Oil Reserviors of El Tordillo Field, Argentina, Using Magnetic Resonance Logs*
By
Maged Fam1, Luis P. Stinco2, and Julio. A. Vieiro2
Search and Discovery Article #40157 (2005)
Posted June 27, 2005
*Adapted from extended abstract, prepared by the authors for presentation at AAPG International Conference & Exhibition, Cancun, Mexico, October 24-27, 2004.
1Halliburton Energy Services, Houston, Texas ([email protected])
2Tecpetrol S.A., Chubut, Argentina ([email protected])
Introduction
El Tordillo field was discovered in 1932 and was operated by YPF (the national oil company) from 1932 until 1991. In 1991, the Consortium El Tordillo, in which Tecpetrol S.A. is the operator, assumed operations.
The field is situated on the north flank of the San Jorge Basin in Chubut Province, Argentina. It is approximately 50 kilometers from the town of Comodoro Rivadavia and 1500 kilometers south of Buenos Aires (Figure 1). More than 1200 wells have been drilled in the field, and production is spread over approximately 57 square kilometers.
Three Cretaceous units produce the numerous fluvial sand bodies that structurally and stratigraphically trap hydrocarbons in the field. Relevant geological, geophysical, and engineering observations from studies performed within the field can be found in Muruaga et al. (2001) and Taboada et al. (2001). See also Fitzgerald et al. (1990).
The main objectives for running MRIL logs in El Tordillo field were:
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Identification of water vs. oil zones
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Determination of porosity
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Evaluation of permeability to help identify reservoir quality zones
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Determination of variations in oil viscosity, if any
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Petrophysical Properties of the ReservoirsBoth Comodoro Rivadavia and Mina El Carmen formations consist of feldspathic shaly sandstones with volcanic tuff and conglomerates. A summary of the reservoir characteristics is presented in Figure 2. Additional information can be found in Stinco et al. (2001).
MRIL TechnologyMagnetic Resonance Imaging Logging (MRILPrime ®) service measures several key factors that influence the ability of a well to produce commercial quantities of oil or gas. One of the fundamental measurements of the MRIL-Prime tool is total fluid-filled porosity. MRIL in general is a fluids-only measurement as the rock matrix is invisible to the tool; therefore, it is unaffected by matrix effects that can influence conventional logs. In fact, the total porosity measurement is the sum of clay-bound water, irreducible water saturation and free fluid volumes — all of which are measured separately by the tool. These measurements are utilized to predict hydrocarbon production reliably. They are critical in identifying low-contrast and low-resistivity pay, where high immovable water volumes or fresh formation water may cause overlooking productive zones when evaluated solely with conventional logs. In addition, the MRIL tool derives a permeability measurement and is able independently to determine fluid type, pick fluid contacts, and detect changes in oil viscosity.
MRIL Porosity and Permeability Coates et al. (1999) provide a thorough explanation of the theory and the procedures for determining MRI-based porosity and permeability. It should be noted that, in general, the MRIL permeability should be used initially to compare good-quality from poor-quality reservoirs in a relative fashion. When calibrated to MRI core analysis, MRIL permeability can be used in its absolute form. See Marschall et al. (1999) for discussion.
MRIL Analysis
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In order to achieve the objectives of the
study, different models of analysis and
Refer to Coates, et al., (1999) for explanations of these individual methods.
Evaluating The Shaly Sands Oil Reservoirs Of El Tordillo FieldThe main problem to solve when dealing with these reservoirs is that the responses of conventional logs do not always indicate fluid types. These shortcomings have been documented by post-logging well testing.
Pre-Job PlanningA job plan for acquiring MRIL data in El Tordillo field wells was made in this study, based on specific parameters from the well design and anticipated formation and hydrocarbon characteristics (Fam, 2003). The plan was designed to acquire MRIL data that would provide the best opportunity of meeting objectives.
MRIL-Prime
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