The Influence Of Solid Bitumen On Reservoir Quality And Logging Parameters

Solid bitumen is similar to the cement and autogenic clay filled in reservoir pores or rocks fracture,occupied the megapores and blocked pore throats.It dramatically reduces reservoir porosity and permeability,even changes the oil/gas accumulation history at a later stage.Solid bitumen is an immobile hydrocarbon that can have significant effects on the resistivity log.Discriminating between producible oil and solid bitumen is critical for the physical property in log interpretation,recoverable hydrocarbon volume calculations,and production planning,yet the lack of resistivity contrast between the two makes it difficult,using conventional logs.Taking Jurassic and Triassic lower maturity bitumen from Santai-north Santai area in Junggar Basin and pyrobitumen from Longwangmiao formation in Moxi area of Sichuan Basin as an example,through the experiment of comparing reservoir properties before and after the core samples were soaked by organic solvent,and analyzed the impact of bitumen on reservoir properties.Meanwhile we discuss the effects of the presence of reservoir bitumen through extraction-cleaning process,helium-injection porosity and permeability,NMR experiments,acoustic wave velocity,resistivity,thin section images and so on.Then we use the experimental data to quantitative evaluate the effect of solid bitumen on reservoir properties and logging parameters,and establish a volumetric rock model to quantify the reservoir bitumen saturation in the reservoir.Finally,the methodologies for reservoir bitumen characterization with conventional loging and NMR loging parameters can be applied successfully in different reservoirs for more reliable and precise reservoir evaluation.The key technology of the experimental analysis is the dissolution of the solid bitumen in the rock.The lower maturity of bitumen is easily dissolved and the high maturity of the pyrobitumen is very difficult.The high extraction yields support the conclusion that most of the dissolved pyrobitumen are both carbon disulfide,Nmethyl-2-pyrrolidinone,chloroform,benzene mixed solvent and carbon disulfide,Nmethyl-2-pyrrolidinone,N.N-dimethyl-formamide,chloroform,benzene mixed solvent.Reservoir property experiment is pointed out that the property of the dissolved samples is much better than the pre-dissolved samples.The reservoir property of low-maturity bitumen sample contrast between before and after the organic solvent extraction indicates that the porosity adds 14.24%,the permeability adds 98.19 mD,and the pore size distribution mainly increase between 0.02-2?m.The reservoir property of pyrobitumen sample between before and after dissolved indicates that the porosity adds 11.3%,the permeability adds 0.037 mD,and the pore size distribution increase mainly between 20-200?m.An important observation in experimental analysis,resistivity became small and the acoustic wave velocity became greater after cleaning of the sample,but the variation of S wave velocity is more obvious than P wave velocity.Due to decrease volume of the low-maturity bitumen samples,P wave velocity increase average 20.71 ?s/ft,S wave velocity increased 48.76 ?s/ft,and the resistivity value is decrease by 196.1 Q.m.By cleaning the pyrobitumen samples,P wave velocity increase average 0.52 ?s/ft,S wave velocity increase 8.92 ?s/ft,and the resistivity value is decreased by 34.4 ?.m.Nuclear magnetic resonance(NMR)log can reflect the fluid properties in the reservoir.The NMR experimental results show that the low-maturity bitumen and pyrobitumen have different transverse relaxation times(T2),NMR response and T2 cut-off time.A low apparent fluid hydrogen index(Hlapp),providing an opportunity to identify the presence of reservoir bitumen.Therefore,NMR logging technology becomes crucial in the characterization of reservoirs in which the presence of bitumen may be of concern.Because the nuclear magnetic resonance response of pyrobitumen is very weak and the porosity such as NMR and He injection also underestimate the total porosity of the rock,a pyrobitumen-model for the interpretation of NMR porosity and total porosity is established.However the low-maturity bitumen has a short relaxation time and the spectra of 0.01?5ms,and displays a high peak in T2 maps.The signal detected by NMR of the low-maturity bitumen is about half of the signal in the same volume of water(HIapp?0.5).The total porosity of rock is larger than that of NMR porosity,so a volumetric rock model was used to reconcile core and log data as well as to calculate the saturation of low-maturity bitumen.The devised volumetric model is able to explain and quantify the results of the core analysis data when low-maturity bitumen is present in the reservoir.The NMR response in low-maturity bitumen or heavy crude oil zones is dominated by short transverse relaxation times(T2)and a low apparent fluid hydrogen index(Hlapp),and T1Lm/T2LM value can be used to estimate the fluid viscosity using empirical correlations.Presence of high-viscosity hydrocarbons reduces the estimated porosity from the NMR tool,and plotting the density porosity and NMR porosity on the same scale helps us identify the zones in which NMR shows a deficit in porosity.Therefore,NMR logging technology becomes crucial in the characterization of reservoirs in which the presence of bitumen may be of concern.However,the pyrobitumen-model has some limitations in Sichuan Basin,we use resistivity logs at different depths of investigation and identify pyrobitumen zones by mud invasion profile interpretation.