Computation of the Formation Cementation Factor m and the Saturation Exponent n and Its Application to Shaly Sand Formation with Low Porosity and Low Permeability

When the Archie equation is applied to computation of the water saturation in shaly sand formation with low porosity and low permeability,problems in accurate prediction of the cement factor m and the saturation exponent n are commonly encountered,because of the limited experimental conditions and the parameter distribution from which the Archie equation was derived.In fact,the Archie equation only accommodates the clean sand formation which holds medium to high porosity and the fluid in it must be homogeneously distributed.For shaly sand formation of low porosity and low permeability,it holds complex and poorly connected pore space,and the fluid in it is usually saturated inhomogeneously,the coefficients m and n in Archie equation can not be evaluated accurately.Commonly,the coefficients m and n determined by conventional statistical methods vary with the formation porosity and the water saturation,therefore resulting in an inaccurate hydrocarbon saturation derived from the well logging resistivity.According to the math-physical boundary condition and the geological character of the shaly sand formation of low porosity and low permeability,analyzed are the characteristics of the variation of the coefficients m and n through the geological process and the petrophysical alteration of the reservoir rock.The percolation theory has been used to explain the varying process,and established are the calculation methods for the coefficients m and n.The saturation equation with newly determined coefficients m and n has been applied to two boreholes in one of the reservoirs in QL oilfield,West China.Comparison of the calculated result with the core experimental data shows that our new result is quite consistent with the core analysis data,it validates the efficiency of the newly developed method.