TX 75083-3836, U.S.A., fax 01-972-952-9435. ProposalComplexities in the properties of tight gas sandstone formations, due to low effective porosities, presence of clays, the effect of mixed mineralogy, etc. require advanced methods of evaluation. Likewise, adverse effects brought on by drilling, (e.g., altered formation, unknown mud filtrate effects) must be taken into account. Additionally, variations in formation water salinity "Rw" value, being a primary input to log analysis, must be determined when using conventional log interpretation. Accordingly, in tight gas sands, operators usually resort to well testing for characterization of tight gas sands. Effective well log evaluation will offer a first approximation as to the selections of best zones to be tested.Magnetic Resonance Imaging "MRI" technology contributes significantly in the evaluation of complex tight shaly-sands, because it offers in-situ evaluation helpful in determining fluid producibility and collaborates the conventional resistivity based log interpretation. All resistivity based log analyses require formation water salinity "Rw" information. However, MRI logs offer a "non-resistivity" based fluid identification and "stand alone" interpretation. In the case of tight gas sands, where effective porosities are low, sensitivity of log interpretation to Rw becomes more importantThis paper illustrates such a case and offers the MRI as a mean to circumvent the issue of "Rw" determination as well as to verify porosity and provide a permeability indicator. Such an indicator can be calibrated when integrated with the results from core analysis.The ability of MRI to detect the type of fluid in the formation is intended to influence selection of zones to be tested in an exploration well.This article demonstrates the application of the MRI logs in characterizing tight gas shaly-sand formations in a South Texas exploration well where the "Rw" from produced water samples was different from the assumed "Rw" from an analogous field. It will also argue for the value of MRI technology in minimizing the cost and associated risk of well testing. This risk is minimized by using the MRI log to provide information on the fluid type (water or gas) present in each zone.

The deep reservoir sandstones of the El Furrial and Pirital oilfields, located in northern Monagas state, eastern Venezuela, are famous for their high degree of geological complexity and heterogeneity. Typical well completions in this area consist of natural flow from multiple intervals through perforations in cemented casing. However, conventional perforating guns used in these completions cause additional near-wellbore formation damage that adversely affects well productivity and results in reduced productivity indexes in these wells.There have been significant efforts in recent years to implement new methods and technologies to produce cleaner perforations and better production rates. One of these technologies is a new dynamic underbalance perforation system which allows precise management of the underbalance differential pressures to produce a relatively clean perforated tunnel while minimizing formation damage. Application of this system results in improved connectivity between the reservoir and wellbore which allows substantial improvement in oil production rates.Formation petrophysical properties are used to determine whether an interval is a suitable candidate for application of this technology. A separate nodal analysis model is created for every reservoir interval in each well. These models are designed to represent the actual and expected production conditions and account for reservoir heterogeneity. In the El Furrial and Pirital fields, reservoir properties can vary widely: permeabilities from 0.1 to 1,000 mD; porosities from 9 to 18%; oil gravities from 22º to 32º API; static pressures from 6,000 to 8,000 psi; and confined compressive strengths from 7,000 to 20,000 psi. This paper presents case histories from wells in eastern Venezuela to illustrate the production benefits derived from this advanced perforating system technology. In three wells, supporting evidence demonstrates that near-wellbore damage caused by this new innovative underbalance perforation system is close to zero, resulting in over 100 % increase in the productivity index. In the fourth case history, we discuss the reasons why the actual well production rates did not achieve the rates projected by nodal analysis. IntroductionDiscovered in 1986, the El Furrial field is currently considered to be one of the largest oil producing fields in Venezuela, with a current production rate of approximately 425,000 BOPD. Located on the north flank of the eastern Venezuela basin (Fig. 1), the field is currently produced and operated by Petroleos De Venezuela S.A. (PDVSA). Since its discovery, it has produced over 2 billion bbl of approximately 25 o API oil. Proven reserves total over 4.1 billion bbl and estimated oil in place is more than 8.6 billion bbl.

fax 01-972-952-9435. AbstractEvaluating the complex clastic reservoirs in El Tordillo field of the San Jorge Basin in Argentina using conventional logs is greatly 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 wide variations in oil viscosity. All of these factors affect most of the conventional logs responses in such a way that traditional log analysis methods 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.To address uncertainties in conventional log evaluations, operators may resort to excessive well testing for reservoir characterization and production verification. However, well testing is known to be costly (considering the rig time and the frac jobs used). On the other hand, if the operator does not proceed with well testing, productive zones can be bypassed. That is why an effective log evaluation would be the optimum, cost effective method when it demonstrates agreement with production results.The latest acquisition and interpretation techniques of the magnetic resonance imaging logs (MRIL) have demonstrated promising results in the complex shaly-sand reservoirs of El Tordillo field. The magnetic resonance imaging (MRI) technology run on wireline provides an in-situ evaluation of the reservoir properties vital for producibility predictions. 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 determining the need for well testing and in enhancing the effectiveness of any particular well test.This article discusses the effectiveness of applying the MRI logging technology to the characterization of the shalysands of El Tordillo field in both the Comodoro Rivadavia and Mina El Carmen formations. It 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 necessary "prior-to-testing" information on the fluid type (i.e., water, light oil, heavy oil or gas) present in a zone and rock properties, such as porosity and permeability.

Nuclear Magnetic Resonance (NMR) logging has gained its position in the industry as stand-alone interpretation or as an additional source of information to conventional log data. There are a wide variety of examples where NMR logging has disclosed secrets of the well that were not discovered by conventional logs, e.g. low resistivity pay zones identified by NMR logs but completely overlooked or viewed as not economically viable by conventional log interpretation. In some complex environments, NMR logging may even be the only alternative available to assess critical formation parameters. The interplay of rock and fluid properties, reservoir parameters, and NMR tool characteristics and settings shape the recorded NMR signals. Pre-job planning is critical for the success of the logging run especially now that NMR technology has gained more general acceptance and is being applied in complex environments. Integration of known parameters and estimated in situ properties may reduce the acquisition time, demonstrate the feasibility of a proposed logging run, and provide a handle to optimize the acquisition cycle. A priori knowledge will ultimately lead to better data and results in less time. We have built a state of the art, multi-fluid, forward model to simulate NMR responses under given conditions. The latest understanding of NMR physics is incorporated in the model, including free and restricted diffusion. The model is a versatile planning tool, linked to a very user-friendly graphical user interface, which offers easy input and great flexibility. Specifics of all currently available MRIL® tools and a broad range of rock and fluid properties are directly accessible through embedded database files. The user, however, has the flexibility to alter these values or to incorporate specific, regional knowledge. Polarization curves, composite transverse relaxation time (T2) spectra, and differential spectra are displayed. Computed fluid volumes, saturation levels, and relevant, mean T2 values are tabulated. The Planner is also being used more and more as an interpretation aid: geologists and petroleum engineers are testing their hypotheses by comparing computed NMR signals to actual logging data. Introduction Technology continues to drive our industry toward investigating reservoirs with grater complexity. Either the formation is more complex in it's lithologic variation or the setting calls for more complicated drilling methods, and often both are the case. Conventional logging methods are being used to provide quantitative information in mixed-lithology reservoirs, low-resistivity / low-contrast pay zones, low porosity / low permeability formations, medium-to-heavy oil reservoirs, when dealing with fresh formation water, etc. It has been well documented that these logging and interpretation methods have failed to delineate critical reservoir properties in these complex-logging environments. NMR logging may be the differentiating technique that can provide both a quantitative and qualitative assessment of these reservoirs. For NMR the logging environment is primarily defined by the formation being logged, it's temperature and pressure, and the drilling fluid that is being used. The industry offers a wide variety of drilling fluid products and each product can have an impact on the NMR data acquisition and/or interpretation. Thus today's NMR logging techniques must contend with a wide variety of environments and still provide the parameters needed to assess the reservoirs productivity. Thus, careful planning prior to data acquisition is critical for the success of the NMR logging run.