Evaluation of Laminated Formations Using Nuclear Magnetic Resonance and Resistivity Anisotropy Measurements

摘要

Laminated formations create two major evaluation problemsfor petrophysicists. First is the classic low resistivity payproblem as seen in vertical wells. Layers of fine-grained sand,silt, and clay distributed within a hydrocarbon bearing sandwill significantly reduce the apparent resistivity measured byan induction or propagation tool. The fine-grained layers holdhigh volumes of irreducible water, the sand will producewater-free oil or gas, yet the oil company may not evenattempt to complete the zone. Second is the high angle wellevaluation problem. The same laminated formation, whenmeasured by an induction or propagation tool at moderate-tohighrelative dip, will exhibit an increase in apparentresistivity beyond that which was measured in the verticalwell. Again, the accurate calculation of water saturation andhydrocarbon volume eludes the petrophysicist. Both of theseclassic problems can be solved with a common methodologythat combines nuclear magnetic resonance (NMR) andresistivity anisotropy measurements.Case study wells are used to demonstrate two versions of thismethod. In the first version, horizontal resistivity (Rh) andvertical resistivity (Rv) are the initial inputs. With anassumption of the resistivity value of the silt-clay layers, wesolve for the resistivity of the hydrocarbon bearing layers andthe volume fraction of each layer. Water saturation (Sw) ofeach layer is calculated independently. Bulk volumehydrocarbon (BVH) of the entire formation is the sum of theproducts of the layer volume fractions and their respectiveBVH. The second version of the method uses themeasurement of bound fluid volume from the NMR tool todetermine the volume fraction of each layer. We then solve forthe resistivities of both the silt-clay layers and the hydrocarbonbearing layers. The solution of BVH for the formation thenproceeds as in the first method.