Abstract
Laminated formations create two major evaluation problems for petrophysicists. First is the classic low resistivity pay problem as seen in vertical wells. Layers of fine-grained sand, silt, and clay distributed within a hydrocarbon bearing sand will significantly reduce the apparent resistivity measured by an induction or propagation tool. The fine-grained layers hold high volumes of irreducible water, the sand will produce water-free oil or gas, yet the oil company may not even attempt to complete the zone. Second is the high angle well evaluation problem. The same laminated formation, when measured by an induction or propagation tool at moderate-to-high relative dip, will exhibit an increase in apparent resistivity beyond that which was measured in the vertical well. Again, the accurate calculation of water saturation and hydrocarbon volume eludes the petrophysicist. Both of these classic problems can be solved with a common methodology that combines nuclear magnetic resonance (NMR) and resistivity anisotropy measurements.
Case study wells are used to demonstrate two versions of this method. In the first version, horizontal resistivity (Rh) and vertical resistivity (Rv) are the initial inputs. With an assumption of the resistivity value of the silt-clay layers, we solve for the resistivity of the hydrocarbon bearing layers and the volume fraction of each layer. Water saturation (Sw) of each layer is calculated independently. Bulk volume hydrocarbon (BVH) of the entire formation is the sum of the products of the layer volume fractions and their respective BVH. The second version of the method uses the measurement of bound fluid volume from the NMR tool to determine the volume fraction of each layer. We then solve for the resistivities of both the silt-clay layers and the hydrocarbon bearing layers. The solution of BVH for the formation then proceeds as in the first method.
