Abstract:

This paper applied the mercury capillary-pressure test,pore casts and dynamic test to study the pore geometry of the Middle Carboniferous car-bonate reservoir in eastern Sichuan and attempted to explain its pore geo-metry chracteristics and porosity parameter.On the basis of the observation of core-cast-sections of Xiang-16,Wo-49 and Zhang-2 wells,the pores of the reservoir may be classified into threetypes,i.e.intercrystal pore,fracture and irregular corrosion pore.In orderto precisely evaluate the role of every basic pore type in capacity and per-meability,the relationship of pore combination have been further studied and divided into eight combination forms.The optimum pore combination isthe“coarse intercrystal pore-corrosion pore-corrosion fracture”,the sec-ond,will be“coarse intercrystal pore-fine intercrystal pore”and“inter-granular pore-intergranular intercrystal pore”,the others,belong to lowcapacity-permeability.In short,the goodness of the capacity-permeability of the reservoir depends on the development of corrosion pore and coarseintercrystal pores.The authors propose that the lower limit of width of effective(hydro-carbon bearing)porosity in reservoir may be defined as 0.1μ.This value,according to the characteristics of flaky throats of acrbonate rocks,corre-sponds to the pore-throat width of mercury injection at 75 atm.Applyingthe mercury capillary-press ure method,the effective porosity of more than 0.1μ of each sample may be determined.With respect to the Middle Car-boniferous,the effective porosity is one of the main parameters in evalua-ting reservoirs.The pore-size distribution of corroded dolomite is caused by various origins during the diagenetic and deuterogenic processes.In statistics,they wouldbe characterized by the numeral characteristic parameter of the geologicalmixed experimental distribution.By using the matrix operation method,wecan calculate four important characteristic parameters expressing the poredistri bution of reservoir rocks,i.e.mean value(X),standard deviation(σ)variable coefficient(C)and skewness(Sk).In order to describe completelythe capacity of carbonate reservoir,the authors suggest a synthetical param-eter of eflective porosity(M_g)and pore geometry,i.e.M_g×C or M_g/X.Consequently,on the basis of main pore combination,effective porosity(>0.1μ)and the syntheitcal parameter of porosity and pore geometry,we can classify and evaluate the Middle Carboniferous rersevior.On thesegrounds,the reservoir of Xiang-16 well may be divided into seven members,the thicksess of effective reservoir of each member and its effective meanporosity have been listed in table 2.For the sake of studying fracture porosity,the data of dynamic test hasbeen used to analyse the combination relationship of pore-fracture in reser-voir.The main characteristics of actual build-up pressure curve of shut-ingas wells in Middle Carboniferous apper to be multistep-shaped.This is be-cause of that the matrixes of reservoir rocks are not homogeneous,there-fore,their original pores are multigrade,and inevitably there exist multi-grade pore throats.They indicat that the reservoir must be a medium comp-osed of multipore systems-a multiporous medium.In this paper,we have sug-gested a reservoir model of multiporous medium,and deals with the methodwhich applied multistep build-up curve to calculate the total porosity.Finally,some actual calculations of porosity in various parts of Xing-guosi gas field have been made,they reflect that the fracture porosity pred-ominates over this gas field,especially in the structural high.