Method For Determination Of Permeability Of 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. According to the data of core analysis, the relationship of conventional reservoir parameters and efficient flow porosity is determined, and the calculation formula of efficient flow porosity can be obtained by multiple regression. 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 can be equivalent to the curved capillary pore, the straight capillary pore and circular truncated cone pore in the rock with the same volume and formation factor. The relations of formation factor and efficient flow porosity can be derived by three types of capillary models. Finally, the expression of efficient flow porosity can be obtained by combining the formula of the equivalent rock element model. The values of parameters of the expression are optimized with the experimental data, and the calculation formula of efficient flow porosity is determined and is verified theoretically and experimentally. 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 wells in low porosity and permeability reservoirs are processed with the proposed methods. Comparison of calculated permeability from measured permeability shows that the method can improve the precision of permeability of low porosity and permeability reservoirs.