| The types of carbonate reservoirs are various,and the pore structure of carbonate reservoirs is very complex and highly heterogeneous.Quantitative characterization of different types of pore structure lays a foundation for reservoir calculation and evaluation of oil and gas reserves.It is of great significance for oil and gas exploration to study the formation and evolution of pore in carbonate reservoirs.The mechanism of dissolution kinetics of carbonate rocks is the key point to predict the spatial and temporal distribution of pore in carbonate rocks.The dissolution and pore evolution under different conditions are the key scientific problems in carbonate reservoir evaluation.In this paper,the pore of carbonate rock is studied in three aspects.Firstly,the real pore of carbonate rock was obtained by CT scanning under different diameters and scanning resolutions.It mainly includes four typical pore types:intercrystalline pores?ZG9 fine-grained dolomite?,intergranular pores?LJ1-1 oolitic dolomite?,moldic pores?LJ5 oolitic dolomite?and mixed pore?LJ2-2 oolitic dolomite?.Secondly,the pore evolution of three carbonate rock samples,including yellowish-gray muddy dolomite?TL-6?,gray sandstone limestone?TL-8?and oolitic dolomite?NSP-4?,under high temperature and high pressure chemical dissolution was studied.Finally,the dynamic evolution process of carbonate pore is simulated by three mathematical pore models.The main achievements of this paper are as follows:1.The influence of different CT scanning resolutions on the micro-pore analysis of carbonate rocks is discussed.The best scanning resolution is 2 microns or less.In this paper,the quantitative characterization and heterogeneity characterization of pore with different scanning resolutions,such as 9 and 2 microns,are studied.It is found that when the scanning resolution is greater than 9 microns,micropore can hardly be extracted.When the scanning resolution is less than 2 microns,the multifractal spectrum can effectively reflect the pore characteristics of carbonate rocks.2.The multifractal parameters of pore characterization are discussed.?1?The relationship between porosity and box dimension extracted from two-dimensional CT image reflects the pore structure characteristics.The pore structure of CT images will change continuously,which is corresponded to the pore structure.Porosity is generally positively correlated with box dimension Db,information dimension D1 and correlation dimension D2.Slopes of porosity and box dimension are as follows:intercrystalline pore[2.205,3.744]>intergranular pores[0.478,0.772]>moldic pores[0.347,0.863],while mixed pore does not satisfy positive correlation at low scanning resolution.?2?The asymmetric index R is the best parameter for evaluating the pore structure of carbonate rocks,which shows that fractal and multifractal have advantages in quantitatively characterizing the pore structure of carbonate rocks.The asymmetric index R of two-dimensional CT images of each sample can best represent the pore structure characteristics.The absolute value of asymmetric index R is intercrystalline pore[-0.32,-0.44]>intergranular pores[-0.20,-0.22]>moldic pores[-0.13,-0.15]>mixed pore[0.08,0.12].Three-dimensional and two-dimensional results are similar.The asymmetric index R mainly reflects the pore structure characteristics.The asymmetric index R in multifractal spectra is stable and repeatable,and has the same rules:intercrystalline pore[-0.55,-0.52]>intergranular pores[-0.46,-0.45)>moldic pores?-0.35?>mixed pore[-0.19,-0.16].?3?The width of multifractal spectrum reflects the heterogeneity of pore,which is related to permeability.The permeability of different pore types is as follows:intergranular pores(258.42×10-3?m2)>intercrystalline pores(24.86×10-3?m2)>mixed pores?1.94×10-3?m2?>moldic pores(0.24×10-3?m2),corresponding to the three-dimensional multifractal spectral width,intergranular pores[5.09,5.88]>intercrystalline pores[1.89,2.01]>mixed pores[0.94,1.86]>moldic pores[0.98,1.32],which illustrates multi-fractal spectrum width is related to the permeability.3.Validation of "dissolution window" based on dissolution kinetics experiments and thermodynamic simulation of carbonate rocks is studied,and the pore evolution characteristics are also explored.?1?The dissolution experiment temperature is 40-160?;pressure is 10-50 Mpa in0.2%acetic acid solution.Based on the hydro geochemical software Phreeqc,three different acidic media,sulfuric acid solution,hydrochloric acid solution and brine containing CO2,are selected to simulate the precipitation-dissolution mechanism of carbonate rocks.It is known that the concentration of Ca2+and Mg2+conforms to“dissolution window”.It showed a pattern of first rising and then decreasing.Corresponding"dissolution window"ranges from 150 to 220 degrees,which is higher than that of NSP-4?oolitic dolomite?and TL-8?yellowish-gray mudstone dolomite?in the range of 70 to 120 degrees.?2?The dissolution of carbonate rocks is a complex problem.In addition to mineral composition and fluid medium,pore structure is also an important factor to control the dissolution of carbonate rocks.Sample TL-8?sandy limestone?has no"dissolution window",the difference between dissolution experiment and thermodynamic simulation shows that pore structure is also an important factor to control carbonate dissolution.?3?For two-dimensional pore structure,fractal and multifractal methods show that porosities p and box dimension Db,multifractal parameters??R and??R are positively correlated,and are negatively correlated with??,??L,??L and R,indicating that pore morphology changes under dissolution have self-similar characteristics in two-dimensional structure.For three-dimensional pore structure,the multi-fractal parameters also show the characteristics of decreasing??and increasing??,it shows that the spatial differentiation of large and small pore increases.Multifractal can quantitatively reflect the change characteristics of pore structure.4.Based on computational simulation,the dynamic evolution process of micro-pore is discussed.?1?The RBBDM model simulates the primary pore structure.The pore structure is simple and the porosity is high.The fractal and multifractal characteristics of the pore are mainly controlled by the occurrence probability of two basic elements,rectangular lattice and cubic lattice.With the increase of the proportion of rectangular lattice,the pore formed in the accumulation process will be narrower and longer,and the asymmetric index R presents a more uniform change,which can best reflect the pore structure.?2?The quartet structure generation set method mainly simulates intergranular pores,which simulates the gradual growth of particles?oolitic grains,crystals,etc.?and the filling of pore space.With the shrinkage of pore,the macropore is continuously divided into small pore,the connectivity of pore is poor,and the heterogeneity is enhanced.The values of multifractal parameters??,??,??L and??R become larger,reflecting the heterogeneity of the pore structure.?3?The Siepinski carpet model shows that the multifractal parameters of the pore reflect the spatial heterogeneity of the pore.Pore space with large asymmetric index and left-deviated multifractal spectrum may be a better reservoir.Because of the strong heterogeneity of pore structure in carbonate rocks,it is difficult to quantitatively characterize the pore structure of carbonate rocks.In this paper,fractal and multifractal are used to study pore structure.With the Scanning resolution less than2 microns,multifractal parameters have good indicative significance for quantitative evolution and characterization of carbonate pore,especially the width of multifractal spectrum and the asymmetric index R,which can distinguish different pore types.At the same time,the results of pore simulation also show that the changes of multifractal parameters correspond to the evolution of pore.These results can provide new technical support for the evaluation of carbonate reservoirs. |
PDF Full Text Request