| In the past decade,the target with respect to oil exploration and development have gradually changes from distant-source conventional reservoirs to near-source or inner-source unconventional reservoirs.A large number of reservoirs with oil-wet features have appeared.There is a big difference of logging petrophysical properties between these oil-wet reservoirs and conventional water-wet reservoirs.It leads to the inaccuracy of logging interpretation and evaluation.On the other hand,the in-situ wettability of reservoirs,which affects the distribution of rock fluids,determines the development and implementation of oil recovery program.How to characterize and evaluate reservoir wettability has become an important scientific issue to be solved in petrophysics.Low-Field nuclear magnetic resonance?NMR?,which has been extensively applied to rock analyses in both laboratory and downhole settings,is the one that can be adequate for in-situ wettability identification and evaluation.At present,NMR theory and methods applied to evaluate petrophysics such as porosity,permeability,pore size distribution,fluid distribution and saturation are quite mature,but the characterization and evaluation of reservoir rock wettability is still in the process.This thesis focused on the NMR characterization and evaluation methods in reservoir wettability,and provided some results and discussions.At first,the definition,the influencing factors,and the effects on permeability,capillary pressure and fluid distribution about the wettability of reservoir rocks are described.The theoretical relationship between wettability and NMR relaxation were introduced based on the discussions of the interface characteristics and the relaxation mechanism of solid-liquid interface.The researches on rock wettability using Low-Field NMR technology in the past 60 years were reviewed.This thesis presented a systematical NMR study,integrating with Amott tests,X-ray diffraction,and scanning electron microscope?SEM?experiments,on wettability characterization of five typical core samples from different low-permeability reservoirs.NMR relaxation mechanisms and the effect of the aging were analyzed in detail.The study showed that the core samples were divided into two types according to the shift characteristics of T2 spectra between before and after aging.The first type is that the wettability alteration relied on the aging and the T2 spectra after the aging shifted toward to the left.This type of formation is generated in distant-source reservoirs,which mainly involve fluid transport through buoyancy.Another type is that the wettability alteration was attributed to the natural oil-wet property of the rock matrix without T2shift before and after aging.This formation is mainly found in inner-source or near-source reservoirs,which may generate abundant oil-wet materials.This thesis presented the discussion on the theory that the interaction strength between pore surface and fluid molecules could be obtained through T1–T2 correlation spectra.Then,the relaxation correlation map of fluid molecules on pore surface has been forward simulated by the first order approximation method.And the basic method for distinguishing the wettability was established using T1/T2 ratio.Finally,glass beads and reservoir sandstone samples were selected for preliminary NMR experiments to verify the validity of the method.The results showed that the T1/T2 ratio of the wetting-phase fluid was higher than that of the non-wetting phase fluid when rocks were saturated with two-phase fluids.Rock wettability was analyzed by observing the changes of T1–T2 correlation maps in different saturation states.Conventional diffusion and relaxation?D-T2?measurements using the pulsed field gradient?PFG?-Carr-Purcell-Meiboom-Gill?CPMG?pulse sequence with unipolar gradients are influenced by internal gradients.D-T2 maps obtained from this method add uncertainty to the evaluation of rock wettability.The bipolar PFG-STE pulse sequence,which can eliminate the influence of the internal magnetic field gradient,was used to acquire D-T2 maps to analyze the influence of wettability for tight sandstones with strong internal gradients.Based on the obtained D-T2 correlation maps,an applicable solution for wettability evaluation was provided.In addition,T1-T2 plots were presented as a complement for qualitative wettability analyses.For the missing signal,i.e.,the bound fluids with a T2 less than 10 ms in the D-T2map,a T1/T2 ratio could be used to interpret the wettability.A workflow was also proposed to implement the wettability-determination technique based on two-dimensional NMR,and this workflow can be used even in downhole applications.The aging process is offen used to restore the wettability of the reservoir rocks in the laboratory,but the aging method affects wettability alteration.The thesis proposed the dynamic aging method with resistivity monitoring,which was compared with the conventional static aging method.The advantages and limitations were discussed.A resistivity wettability index was provided,but more samples are required to verify this method. |
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