EVALUATING THE SHALY SAND OIL RESERVOIRS OF EL TORDILLO FIELD, ARGENTINA, USING MAGNETIC RESONANCE LOGS

摘要

The accuracy in evaluating the clastic reservoirs in El Tordillo field using conventional logs is mainly affected by variations in formation water salinity, texture, and lithology (primarily the amount of volcanic tuff material) combined with extreme changes in rock permeability and oil viscosity. All of these variations affect most of the conventional logs responses in such a way that traditional log analysis models may fail to provide proper results and consequently may not achieve appropriate forecasts. Such failures have been proven by the high degree of mismatch between conventional log analysis and test/production results. As a consequence of concern about uncertainties in conventional log evaluations, operators may resort to excessive well testing for reservoir characterization. However, well testing is usually very costly (rig time and frac jobs are expensive). And, on the other hand, if the operator decides not to proceed with well testing, productive zones can be bypassed. In either case, effective log evaluation would be the optimum, cost effective method "if it worked". Magnetic Resonance Imaging "MRI" technology has demonstrated promising results in the shaly-sand reservoirs of El Tordillo field. MRI offers in-situ evaluation of the reservoir properties vital for producibility. The ability of MRI to estimate the type of fluid in terms of viscosity influences the selection and elimination of zones to be tested based on their mobilities. MRI also helps in properly preparing for the testing procedure by pre-identifying the type of hydrocarbon in a zone and by identifying its reservoir quality in terms of permeability and porosity. Thus, the MRI serves well when it comes to deciding on the need for well testing and in enhancing the effectiveness of any particular well test. This paper discusses the effectiveness of applying the MRI logs in characterizing the shaly-sands of El Tordillo field in both the Comodoro Rivadavia and Mina El Carmen formations. This paper also argues for the value of MRI technology in minimizing the cost (and associated risk) of well testing by identifying best candidate zones for testing and by providing "prior-to-testing" information on the fluid type (i.e., water, light oil, heavy oil or gas) present in a zone.