Method for determination of permeability of shaly sand formation with low porosity and permeability based on efficient flow pore

Since pore structure has a great effect on permeability of low porosity and permeability reservoirs,the permeability prediction with conventional parameters such as porosity and irreducible water saturation is of low accuracy. Based on the characteristic of fluid percolation in low porosity and permeability rock, the largest contribution of pores on fluid percolation is discussed and the concept of efficient flow pore introduced to improve accuracy of permeability. Samples with different shale content,porosity and permeability are selected, and experimental measurements are designed for the samples. The correlation coefficient of the permeability and the mercury-injection porosity corresponding to different pore radius is calculated from the mercury-injection experiment data. By making the plot of correlation coefficient and pore radius, radius cutoff of efficient flow pore is determined corresponding to a certain value of correlation coefficient( 0. 8),and the corresponding mercury-injection porosity as efficient flow porosity experimentally. On the basis of similarity of water and electric flow,the formula of efficient flow porosity is derived from conduction theory. Firstly,for water-bearing shaly sand with low porosity and permeability,the irreducible water and clay bound water are assumed as non-conducting matrix,and formation factor formula is derived with equivalent rock element model describing pore-throat ratio. Secondly,according to the definition,the efficient flow pore of the above-assumed rock with movable water pore only can be equivalent to curved capillary pore in the rock with the same volume and formation factor. The relation of formation factor and efficient flow porosity can be derived by curved capillary model.Finally,the expression of efficient flow porosity can be obtained by combining the two formulas. The values of parameters of the expression are optimized with the experimental data, and the calculation formula of efficient flow porosity is determined. The calculation formula of permeability of shaly rock with low porosity and permeability can be regressed with efficient flow porosity and permeability of the samples. The log data of well X1 and well X2 in low porosity and permeability reservoirs are processed with the proposed method. Comparison of calculated permeability from measuredpermeability shows that the method can improve the precision of permeability of low porosity and permeability reservoirs.