Image analysis in the study of theporous system of reservoir rocks

Abstract

The petrophysical properties of reservoir rocks are strongly controlled by the geometry of the porous space. Size, volume, and interconnection of the pores are parameters that may be semiquantitative and qualitatively evaluated through optical petrography. Detailed petrophysical analyses, such as capillary pressure, used to· interpret the size of the throats do not furnish
information about its shape and spatial distribution. Image analysis (AI) of thin sections, using computer systems, permits a fast
quantitative evaluation of the geometry and distribution of the porous space of the reservoir rocks. The image analysis system being used at the Geo/ogy of Reservoirs Sector (SEGRES) of the Geology and Engineering of ReservoirDivision (DIGER) aims to characterize the porous space of reservoir rocks, to optimize the petrographic measurements, correlating them to the petrophysical measurements, in order to provide a complementary system to study the porous space geometry. It is even possible to develop methodology capable of foreseeing values for porosity and permeability in intervals where the lack of core samples andlor plugs restrain petrophysical measurements. The presence of lateral andlor cuttings samples permits the preparation of thin sections which are processed by the analyzer, therefore allowing petrophysical measurements estimates. The methodology being used utilizes backscattered electrons (BSE) images obtained from thin sections analyzed by scanning electron microscopy (SEM). These images are digitized and sent to the analyzer in order to be processed. The analyzer consists of a computer, two high resolution color monitors, a printer and a unit tnet controls brightness and contrast of the acquired image. This equipment uses the IPA-85 software, of Tractor Northern Inc., that has several applicatives to process andanalyze images. The image analysis was used in the study of sandstones of the Pendência Formation that were cored from the outcropping portions of the Potiguar Basin, and calcarenites of the Guarujá Formation in Santos Basin. While stucJying the sandstones, measurements of porosity. size, shape andpore distribution were collected in thin sections from fluvial, deltaic and turbiditic sandstones which were compared with the respective values of lhe petrographic and petrophysical analyses. The size, distribution and pore connection patterns showed good correlation with the petrophysical values obtained in the laboratory, which permitted to explain the different permeability results of the reservoirs analyzed. In the carbonatic reservoirs of the Guarujá Formation, at TubarãoField, lhe pore slze, externai perimeter, average area andpore shape factor parameters obtained by image analyses presented good correlation with lhe petrophysical values of the laboratory, such as permeability. size of lhe pore throat and efficiency of the mercury recovered. The methodology thus permitted calculation of permeability values correia tive with the values measured by petrophysical methods.