An Innovative Approach to Real-Time NMR Permeability Calibration Utilizing Azimuthally Oriented Formation Testing While Drilling

Abstract

Evaluating lateral variations of permeability is usually attempted using conventional logging tools and relying on field-specific porosity-permeability relationships. These transforms are generally built on core analysis and previous reservoir knowledge. Many Carbonate reservoirs given their complex pore structure, however, exhibit poor relationship between porosity & permeability and would hence need other more sophisticated methodologies and workflows to evaluate permeability. Nuclear magnetic resonance (NMR) is a well-known measurement that can provide indications of pore size in heterogeneous reservoirs and can hence be used for permeability evaluation. Formation testing while drilling (FTWD) can be used for mobility estimation while drilling. This paper presents the benefits of integrating logging-while-drilling (LWD) azimuthally oriented FTWD and NMR to reliably evaluate lateral permeability variations in horizontal wells. NMR log-derived permeability is achieved by analyzing the T1 or T2 distributions and applying a cutoff differentiating pore size variation. The classification of the pore size distribution is input into established NMR permeability equations. These equations, however, have coefficients that need to be determined. The optimum irreducible water saturation cutoff as well as the permeability equation coefficients are ideally established with core analysis techniques. In horizontal wells across heterogeneous reservoirs having no representative core plugs formation testing can be used to help establish these parameters. Formation tester tools have been traditionally utilized to acquire a single pressure measurement and mobility calculation at any specified depth station. LWD testers, however, can be oriented around the wellbore to acquire several pressure and mobility values at different orientations. After investigating real time azimuthal images for bed boundaries and azimuthal variations, specific orientations of the formation tester can be selected to provide the most representative. LWD NMR real-time permeability indicators showed pore size variations at the same porosity range. FTWD mobility data has enabled normalization of NMR permeability curves by providing calibration points at the test stations. The azimuthal orientation of FTWD tests was selected using LWD real time density and resistivity images to ensure the FTWD probe is not pointing to a tight bed boundary. High-resolution resistivity images from memory data provided an understanding to the mechanism causing variation in the azimuthal permeability profile. Combination of oriented FTWD and NMR permeability indicators aided real-time well-placement decisions and helped placing the wells into the most permeable zones. This paper presents an innovative approach for evaluating real-time permeability utilizing oriented LWD formation-testing mobility data with NMR permeability indicators.