Establishment And Application Of Saturation Model Of Altered Volcanic Rocks

Volcanic reservoir occupies an important position in China's oil and gas resources.However,because of its complex pore structure,diverse composition,relatively low permeability and porosity,it is difficult to evaluate the reservoir by logging,especially for the evaluation of saturation.Sandstone reservoir saturation formula has been used for most time.When volcanic rocks are altered,it is even more difficult to accurately calculate reservoir parameters.Now Studying on altered volcanic rocks has become a hot topic in the field of geophysical exploration.For conventional saturation calculation,Archie formula is usually used,but Archie formula can only be used for pure sandstone formation,but can no longer be applied to shaly sandstone formation.There is a big gap between the saturation calculated by Archie formula in volcanic reservoir and the actual oil and gas test results.The alteration of volcanic rocks will produce clay minerals,which have special conductivity,that is,cation exchange capacity(CEC).Therefore,it is necessary to establish the conductivity model and saturation evaluation method of volcanic rocks based on rock electricity and CEC measurement.At present,there are many studies on conductivity model of shaly sandstone,but few studies on establishing saturation model based on rock resistivity and cation exchange capacity for volcanic rocks.In this paper,the basic altered volcanic rocks in Longfengshan area are studied.Rock electrical measurement and X-ray diffraction analysis are carried out on 33 volcanic rock samples,and CEC measurement is carried out on 10 rock samples.Based on the above experimental data,the saturation model is established and evaluated in Longfengshan area.The change of rock resistivity with weight during the process from completely drying to dripping to saturation at normal temperature and pressure,and the change of rock resistivity with water decrease during the process from saturation to centrifugal to irreducible water under high temperature and high pressure(reservoir condition)were measured experimentally.Through the comparative analysis of the measured results under different temperature and pressure conditions,it is obtained that the rock resistivity under the same rock sample approximate saturation state is often lower than that under normal temperature and pressure;The n value of rock electrical parameters measured under normal temperature and pressure is less than that under high temperature and high pressure.This shows that the conductivity of rock under temperature and pressure is different from that under normal temperature and pressure.By recording the change of rock resistivity under different pressure conditions,when the pressure reaches a certain critical value,the resistance tends to a fixed value.Therefore,the research data of this paper adopts the rock electrical measurement data under the condition of reservoir temperature and pressure.Volcanic rocks in the study area are seriously altered to clay minerals,especially in chloritization.In order to verify the influence of altered clay types on rock conductivity,clay-containing artificial cores(including chlorite artificial cores,illite artificial cores,montmorillonite artificial cores and kaolinite artificial cores)are made to study the conductivity of various clays.It is found that the influence of four clay minerals on rock conductivity is chlorite > montmorillonite > kaolinite > illite.The conductivity of chlorite is special.According to the analysis and calculation of Hossin model,Simandoux model,Doll model,Indonesia model and W-S model,the W-S model and Doll model are optimized.when the existing data have CEC measurement data,the W-S model is preferred,and when there is no CEC measurement data,the Doll model is selected.The original model is improved,and the improved new model has a good application effect in wells,which is consistent with the data of oil and gas testing.The full application of the data of rock electricity experiment and CEC experiment makes up for the shortcomings and difficulties encountered in conventional saturation evaluation,and provides a new idea for comprehensive evaluation of volcanic reservoirs.