Challenges of Determining Petrophysical Reservoir Characterizations In Complex Low Porosity Fractured Carbonates – Integrating Pore Geometry,Deterministic Petrophysical Rock Types and Saturation Height Models Yields Positive Results – PEMEX Case Study Offshore Mexico

摘要

Offshore Mexico Cretaceous and Jurassic carbonatesare typically brecciated, dolomitized, low porosityfractured reservoirs. These reservoirs are veryimportant resources in Mexico and are petrophysicallyextremely difficult to interpret using common analysismethods and traditional data.Reservoir porosity sometimes exceeds 10 percent; butoften reservoir pay zones are less than 7 percent due tothe different types of intrinsic pore geometries. “Archiem” may vary from 1.3 to greater than 4. Resistivities innon-productive and productive sections can be verysimilar due to the low porosities. Nuclear magneticresonance (NMR) logs are considered questionable inmany areas due to the low porosity and otherconditions. Compounding the challenges withinterpretation, this region of the Gulf of Mexico isconsidered a HPHT (high pressure and hightemperature) environment.Core-log integration requires a working knowledge ofhow the borehole tools respond to lithology, reservoirfluids and the limits of vertical resolution and depth ofinvestigation.These challenges require PEMEX petrophysicists tounderstand geology, pore geometry, mineralogy,diagenesis and stress information before developing anintegrated porosity model. Delivering a simplelithology interpretation is a starting point, but thecomplexity of these reservoirs requires a rock-basedintegrated lithology model that is calibrated to a coredescription, lab based mineralogy measurements andpetrographic analysis. Permeability predictions are nolonger simple linear regressions, but should include aminimum of three components (lithology, porosity andstress) and are often extended to advanced statisticalmethods. Log based saturations for reservoir models arerarely based on simple electrical rock properties andconstant log parameters. Saturation determination hasevolved into considering various pore geometry models(1-4 systems), complex wettability, capillary propertiesand integrated deterministic petrophysical rock types.Results are confirmed by using customized saturationheight models.Results from three fields show uncertainty is reducedby using deterministic Petrophysical Rock Typing(PRT) methods and that the key to success isdetermining if there is a relationship between initialwater saturation obtained from wellbore analysis andpredicted pore throat size using various techniques.The method requires access to high quality wellboredata and pore geometry based special core analysis(high pressure mercury injection porosimetry, thinsections,mineralogy, petrographic analysis, electricalrock properties and wettability data). Neverthelesssome wells do not have core data available. In thatsituation the use of cuttings provides key reservoircalibration data.Through the development of these petrophysicalcharacterizations, several traditional methods have beenmodified and used on several fields. These “newpractical methods” will be highlighted and presented.PEMEX is using these methods to identify value addedopportunities in exploitation, exploration andproduction.