Application of tortuosity resistivity model in low permeability reservoirs in G area of Daqing oilfield

It has been shown that reservoirs in P formation of Daqing G area have the characters of high shale content,low porosity and permeability.After further study on pore structure characters of reservoirs from mercury-injection data,we know that the reservoirs have bad sorting features,heterogeneous pore-throat distribution,and complex microscopic pore structure.Since the complex pore structure always leads to the complexity of conductivity pathway,it will change the electrical characteristics of reservoirs.However,Archie's equation is not suitable for the evaluation of water saturation in low porosity and permeability reservoirs,because the equation ignores the effect of variation of pore structure in low porosity and permeability reservoirs on rock resistivity.Thus,it becomes the most important issue to establish a resistivity model for low porosity and permeability reservoirs.Considering the P formation has more complicated pore structure,an equivalent tortuosity volume model is used.In the model,pore volume and the conductivity path of the rock are considered to be a tortuous tube.And the matrix and formation water is conducting current in parallel.By the equivalent volume model,a resistivity equation is derived in which the relationship between resistivity of the rock and pore structure is established.According to the resistivity equation,the more tortuous pore pathway is,the more complex pore structure is,then the greater pore tortuosity is,and the higher formation factor becomes.In order to make this equation be applied practically,a new parameter i.e.structure coefficient is introduced in the equation to describe the level of pore tortuosity,which also makes possible quantitative representation of the pore tortuosity.As a result,a new formation factor equation is established with structure coefficient.The analysis of experimental data of mercury-injection and core resistivity demonstrates that power function relationship between formation factor and structure coefficient does exist.Also it shows the more complex rock pore structure is,the greater structure coefficient is,and the higher formation factor becomes.Since P formation has characters of low salinity and high shale content,DOLL equation are chosen as a basic equation which can describe the curved relationship between water saturated formation resistivity and formation water resistivity.By introducing the new formation factor into DOLL equation,a new water saturation equation applied in shaly sand reservoirs with low porosity and permeability is established.A good consistency between calculated water saturation and analyzed water saturation in sealed coring well shows that the proposed model does contain effect of pore structure on rock resistivity in some degree,and the average relative error of calculated water saturation is less than 8%.Therefore the proposed model can meet the requirement of production and exploitation to the low porosity and permeability reservoirs.