Comprehensive Research On Generalized Resistivity Models In Laminated And Dispersed Shaly Sands

Some complex hydrocarbon-bearing shaly sand reservoirs have been found out with development ofoil exploration. These complex reservoirs include shale-rich reservoirs, interbedded sand and shale, shalysands with conducting matrix grains et al. Some of them have commercially produced oil. Now theybecome one of important oil sources for increase of oil reserves and oil production in old oil field. Sinceconductance mechanism in complex hydrocarbon-bearing reservoirs is different from that in conventionalhydrocarbon-bearing reservoirs, resistivity models in current use can not completely describe conductancemechanism in the complex hydrocarbon-bearing reservoirs. Especially when dispersed clay, laminatedshale, conducting matrix coexist in hydrocarbon-bearing shaly sand reservoir, conductance mechanism inthe complex hydrocarbon-bearing reservoir becomes more complicated and a generalized resistivity modelhas not been proposed to describe the conductance mechanism. Therefore, it is very important andvaluable to propose a generalized resistivity model with extensive application to enhance evaluationaccuracy for complex hydrocarbon-bearing reservoirs.Based on conductance theory and rock sample experiment, shale content resistivity models, doublelayer resistivity models, and effective medium resistivity models are studied in detail for complexhydrocarbon-bearing reservoirs. Then generalized shale content resistivity model, double layer resistivitymodel, and effective medium resistivity model in laminated and dispersed shaly sands are proposed, andapplied to complex shaly sand reservoir evaluation. Evaluation accuracy for the complex reservoirs isimproved with proposed models.In theory, the following work has been done in the paper.First, based on the parallel conductance assumption between laminated shale pattern and the dispersedshaly sand pattern, and the assumption under which the dispersed shaly sand is conductively equivalent tothe clean sand with dispersed clay and water taken as an conductively equivalent fluid, and dispersed clayconductivity increases as water conductivity increases in low water conductivity, a generalized shalecontent resistivity model is established. It is pointed out that shale distribution largely affects watersaturation predicted by the shale content resistivity model. The curvatures of the curves between formationconductivity and effective water saturation are about the same when only dispersed clay conductivityvaries. As effective water saturation increases, the effect of cementation exponent on the relation betweenformation conductivity and effective water saturation is increased while the effect of saturation exponenton that is decreased.Second, based on the parallel conductance assumption between laminated shale and the dispersed shalysand, and the assumption under which dispersed shaly sands described with three double layerconductivity models, the generalized double layer conductivity models are established. Completetheoretical comparison shows that the theory of the S-B model is best, although the D-W model is verygood. Experimental comparisons of dispersed shaly sand core samples show that the error betweenmeasured values and values predicted by the S-B model is the minimum, and the error from the W-SIImodel is the largest. Also the effects of cementation exponent and saturation exponent in the three modelson applications are studied. When saturation exponent is fixed and cementation exponent is changed, theresults from the three models are similar, but large differences occur when cementation exponent is fixedand saturation exponent is changed.Third, based on the parallel conductance assumption between laminated shale and the dispersed shalysand, and the assumption under which dispersed shaly sands described with dispersed clay particles, sandgrains, and oil as resistors-in-parallel, and HB resistivity model, the generalized asymmetrical effectivemedium resistivity model in laminated and dispersed shaly sands is established. In the derivation of themodel we assume that clay-bound water fraction is included in total pores, clay-bound water andformation water have the same resistivity, and yet the difference of electrical properties between the twowaters is incorporated into clay grain conductivity. By analyzing parameters of the model, we find out thatshale distribution largely affects water saturation calculated by the model. The less the resistivities of sandgrains or clay grains, the more largely the resistivities of grains affect the relation between formationconductivity and total water saturation. The effect of cementation exponent on the relation betweenformation conductivity and total water saturation is increased with total water saturation increased whilecementation exponent and saturation exponent are the same.Forth, based on the parallel conductance assumption between laminated shale and dispersed shalysands, and the assumption under which dispersed shaly sands described with symmetrical anisotropictheory of resistivity interpretation, the generalized symmetrical effective medium resistivity model isproposed. The dispersed shaly sand contains conducting rock matrix grains, nonconductive hydrocarbons,dispersed clay particles and water. It is pointed out that shale distribution largely affects water saturationpredicted by this model. The curvatures of the curves between formation conductivity and total watersaturation are greatly affected by clay conductivity, while it is less affected by formation matrixconductivity. The curvatures of the curves between formation conductivity and total water saturation areonly affected by water percolation rate or percolation exponent, while is not much affected by matrixpercolation rate. When formation conductivity is kept to be constant, total water saturation increases asmatrix percolation rate or percolation exponent increases, and decreases as water percolation rateincreases.Fifth, with the ordinal addition of dispersed clay particles, oil drops, conducting matrix grains,laminated shale to water and successive integration over the addition of a single inclusion, generalizedmatrix-conducting pore combination model in laminated and dispersed shaly sands is proposed. Byanalyzing effects of parameters of the model, we find out that shale distribution largely affects watersaturation calculated by the model. Difference between the corresponding formation conductivities for thetwo matrix grain resistivities or the two clay particle resistivities hardly varies with total water saturation,and difference between the corresponding formation conductivities for the two laminated shale resistivitiesincreases as total water saturation increases. The curves between formation conductivity and total watersaturation are most affected by cementation exponent of matrix grains, while they are least affected bycementation exponent of dispersed clay particles. The effect of saturation exponent on the curves betweenformation conductivity and total water saturation decreases as total water saturation increases.In experiment, 16 shaly sand samples with variation of dispersed clay content and laminated shalecontent have been designed and manufactured for the first time. Core sample resistivity is measured indifferent water salinity and oil saturation. Effects of shale distribution and shale content on conductancemechanism in shaly sands are studied with measured data. The experimental result shows that whenporosity and resistivity of dispersed shaly sand in laminated and dispersed shaly sand samples are keptconstant, or laminated shale content and laminated shale resistivity of the samples are kept constant, orlaminated shale content is equal to dispersed clay content in the samples, core sample conductivitydecreases as laminated shale content or dispersed clay content or total shale content increases in the samewater conductivity if water conductivity is greater than some critical conductivity value, and core sampleconductivity increases as laminated shale content or dispersed clay content or total shale content increasesin the same water conductivity if water conductivity is less than the critical value. Although shale contentis kept constant, conductivity of laminated and dispersed shaly sand samples is different with shaledistribution different. It proves that conductance mechanism in laminated and dispersed shaly sandsfollows parallel conductance between laminated shale and dispersed shaly sand or clean sand.Experimental result on artificial samples with conducting rock grains shows that the three effectivemedium resistivity model can be applied in clay-free porous rocks with conducting grains, with acondition of formation water resistivity being less than rock grain resistivity. Rock sample data andlogging data show that shale content resistivity models, double layer conductivity model, effectivemedium resistivity models proposed in the paper can be applied to laminated and dispersed shaly sands,and taken as a generalized resistivity model to be used in complex shaly sand formation evaluationeffectively. Theory of double layer conductivity models in laminated and dispersed shaly sands is betterthan that of shale content resistivity model. However, the two kinds of resistivity models are all empirical,and applied in shaly sands with non-conducting matrix grains. The three effective medium resistivitymodels proposed in the paper can be applied in shaly sands with conducting matrix grains, and are ofextensive application, and theory of the effective medium resistivity models are better than that of abovetwo kinds of resistivity models.