Pore System Characterization Of Carbonate Reservoir And Its Application To Petrophysical Evaluation

Huge hydrocarbon reserves lie in carbonate reservoirs,which makes carbonate rocks important targets for hydrocarbon expoloration and development.However,complex pore structure of carbonare reservoirs poses big challenges for productivity prediction,reservoir stimulation and remaining oil analysis.The objective of this paper is to establish a methodology of evaluating carbonate pore sturcutre to enhance the accuracy of petrophysical evaluation of carbonate reservoirs.This paper begins with the basic characteristics of carbonate pore structure,and simplifies the classification scheme of carbonate pore system based on a full absorbtion of the previous schemes.From the petrophysical evaluation perspective,the new scheme classifies carbonate pore system into five major groups,interparticle,intercrystalline,microporosity,vug and fracture,according pore connectivity and similarity of petrophysical properties.Vug is further subdivided into touching vug and separate vug according to whether individual vugs are interconnected directly.Pore throat is classified into nano,micro,meso,macro and mega according to pore throat size,and the new classification scheme is also compared with previous schemes.Secondly,pore geometric parameters such as equivalent diameter and shape factor,are derived from digital image ananlysis on core photos,thin sections and SEM images.Pore throat parameters such as the number of sub poresystems,pore throat distribution and sorting,maximum and effective(inflextion)pore throat radius,are extracted by fitting the raw MICP curves using a multi-hyperbolic tangent function.Finally,a new rock typing and petrophysical evaluation approach is proposed and applied to the Khasib reservoir in Ahdeb field,Iraq.Reservoirs of cored wells are classified into individual layers according to log responses,and the layers with similar lithofacies and petrophysical properties are merged into one rock type with distinguishing log responses,resulting in 27 rock types in the Khasib reservoir.Meanwhile,a cross plot of TVDSS versus ILD is created to identify rock types in uncored wells and the distribution of rock types is characterized through out the Ahdeb field.Pore structure of each rock type is characterized,and corresponding permeability and water saturatioin models are established.The accuracy of permeability is improved and the water saturation distribution on the edge of the structure is characterized,which provides important parameters for reservoir simulation.In addition,a new fluid typing approach is established by integrating relative permeability,absolute permeability,water saturation and fluid viscosity to calculate fluid mobility,and a criterion is determined for the Khasib reservoir based on production tests.The approach is proved successful in distinguishing what fluid a reservoir may produce.This study indicates that accurate evaluation of pore and thorat geometry that dominate fluid flow properties as well as the efficiency of electrical current flow is the basis of reliable petrophysical evaluation of reservoirs.The inflexion pore-throat radius correlates best with permeability and is the dominant factor that controls permeability.The cementation exponent(m)which is a function of the variation degree of cross section area that allows the flow of electrical current positively correlates with pore to throat ratio.The saturation exponent(n)which is a function of the variation degree of resistivity index with the variation of water saturation correlates negatively with average pore diameter.The relationship between permeability and porosity varies with pore types,which means that rocks with different pore types have different K-Phi relationships.The free water level of a reservoir is constant,but water saturations of different rock types vary differently with the height above free water level.Rocks with better pore structures have lower oil-water contacts and narrower transition zones,and the water saturation decreases quickly with the increase of height above free water level.Whether a reservoir can produce any fluids and what fluids it may produce are dominated by the mobilities of the reservoir fluids,which is related to fluid sarutations,viscosities,relative permeabilities and absolute permeability of the reservoir.The Kh2-1-2 layer with its permeability as high as 400 mD is identified as the high-permeability layer in the Khasib reservoir.It is distributed throughout the Ahdeb field except for some lower areas in the AD4 block,which is the key factor that leads to the fast break through of injecting water.In conclusion,the accuracy and reliability of petrophysical evaluation of the Khasib reservoir based on pore structure characteristics are greatly enhanced,which provides reliable reservoir parameters for reserve estimation and reservoir simulation.