What are the most effective formation evaluation techniques for sandstone reservoirs?

Gamma Ray/Neutron-Density/Resistivity are common open hole logging techniques for formation evaluation. Standardizing such logging tools across a reservoir/field should help cost optimization and areal and vertical correlations of the reservoir signatures. There is no free uncertainty logging technique. Simple Archie with common petrophysical parameters (a=1, n=m=2) should yield proper evaluation of saturations following necessary corrections. Imbedded errors in the logging techniques, acquisition and reservoir rock environmental uncertainties, mathematics, ... etc. all call to standardize the logging program and evaluation technique. This should result in minimum fixed error rather than adding uncertainties using additional tools.

From my old career (I’ve been out of oil & gas for 8 years now) it surprised me how undervalued the power of time lapse analysis in LWD was. Hope it has changed! Having measurements so close to the bit gives an extreme insight on the fluid movement and rock Alteration through time. From basic measurements such as resistivity, density/neutron to the marvelous Magnetic resonance where you can observe fluid movement & effect of mud filtrate in front of your eyes.

Even when sandstones reservoirs are assumed to be homogeneous in terms of grain density, this kind of reservoirs are usually interbedded by shales lamanae or are influenced by the presence of disperse clays. So, variations in grain density are obvious and may affect in a higher or lower degree (depending on the type and vulume of clay) to properties related to density porosity: total and effective porosity, and clay volume. Accounting for grain density variations, by calibrating logs and clay types and volumes with XRD analysis, can be very helpful when estimating these properties out of density logs, for example.

Dynamic methods like transient gamma ray are a new type of measurement Gamma ray can increase by a factor of 10 or more when a cooled well intersects rock containing condensable vapor. When the well reheats GR returns to normal and subsequent cooling can regenerate the high GR See the many papers I have published on this unique technology Other discoveries remain to be made

One thing I have found helpful is run spectral GR additional to the Triple combo. Depending of complexity of your reservoir you should considere additional logs as for example Magnetic Resonance, Dipole Sonic, spectroscopy, dielectric, etc. the more complex the more data to acquire.

The goal is fluid production, so understanding both the reservoir rock and its interaction with fluids is crucial. These factors influence how fluids move within the rock pores, shaping the development strategy. Take unconventional plays, for instance, where high-quality oil is trapped but faces challenges in flowing due to pore interconnectivity. Capillarity, interphase interaction, and flow direction are also pivotal. Core analysis is essential in any exploration campaign to comprehend these elements.

One thing I've found helpful when modeling core permeability out of a basic set of interpreted well logs, such as total and/or effective porosity, and clay volume, is the design of multiple regressions to predict core permeability. The first step in the methodology I like to use is to limit the cloud of dispersed points on a K vs PHI plot; then I logarithmically normalize core measurements with a higher limiting curve (K*=1) and a lower limiting curve (K*=0). This way, the predictions made by the multiple regressions are going to be more accurate. Finally, I denormalize the resulting curve and get a very well core-adjusted permeability curve as a function of porosities and clay conntent.

By experience core analysis are key to characterize your Reservoir and to calibrate any model (dynamic and static). Rock types, capillary pressure, SEM, digital coring are a MUST have for better understanding of your reservoir

Another very useful and helpful technique when geomodelling petrophysicial properties out of seismic 3D data is the seismic attributes analysis, in combinations with geoestatistical methods. This kind of analyses have to be firstly validated to well log data and prove if they can predict basic properties such as total porosity (the most likely to model with seismic information). Even when most of geomodelers do not like to model total porosity, this can be a the very first 3D property to use as an input in some other techniques such as collocated co-kriging to model more useful properties such as effective porosity, clay volume and even absolute permeability.

All analyses are objective driven and fit for purpose. Want formation lithology? Run GR, maybe density-neutron, even elemental spectroscopy. Want formation porosity? Run density-neutron, maybe add NMR or sonic. Want reservoir saturation? Run the above plus resistivity. Want mobility? Run Stoneley or more expensive FTS. Cores are used to calibrate all the above log derived formation properties.

It will depend not only on kind of reservoir but also in other factors like, mineral composition, fluid content, fluid salinity, sorting, etc. depending on these uncertainties or complexities we could consider from basic set of data(triple combo) to advanced set of data (Magnetic Resonance, Dielectric, Spectroscopy, micro resistivity images, cuttings analysis, CT scan, XRD, XRF, mud logging , fluid sampling). Yes it seems a lot however we could find complexities that will require not only these amount of data but more importantly experience interesting and interpreting all of them. In my opinion experience there is not easy reservoir or basic petrophysical analysis.

By experience I could say there is not “easy” reservoir all of them have a grade of complexity, integration of all available source of information (from drilling to production) will provide a better understanding of them.

Well, it depends on the objectives. Usually a triple combo is required a Neutron-Density-Resistivity and of course GR for lithology and calliper for logs corrections. This is the minimum. If other information are needed other logs can be run Sonic for the rock geomechanics, NMR for better understanding of porosity type and distribution, testing is to confirm productivity of potential layers and core data are used to calibrate logs.

For loosely unconsolidated sandstone formations in which conventional coring is not an option, not applicable, EL conveyed sidewall sampling (SWS). Sidewall cores may be taken using percussion or mechanical drilling. Percussion cores are taken by firing hollow bullets into the formation.