|
Abstract:
|
Nuclear magnetic resonance (NMR ) has become an effective borehole measurement option to assess petrophysical and fluid properties of porous and permeable rocks . In the case of fluid typing , two -dimensional (2D ) NMR interpretation techniques have advantages over conventional one -dimensional (1D ) interpretation as they provide additional discriminatory information about saturating fluids and their properties . However , often there is ambiguity as to whether fluids detected with NMR measurements are mobile or residual . In some instances , rapid vertical variations of rock properties (e .g . across thinly -bedded formations ) can make it difficult to separate NMR fluid signatures from those due to pore -size distributions . There are also cases where conventional fluid identification methods based on resistivity and nuclear logs indicate dominant presence of water while NMR measurements indicate presence of water , hydrocarbon , and mud filtrate . In such cases , it is important to ascertain whether existing hydrocarbons are residual or mobile . The radial lengths of investigation of resistivity , nuclear , and NMR measurements are very different , with NMR measurements being the shallowest sensing . Even in the case of several radial zones of NMR response attributed to different acquisition frequencies and DC magnetic field gradients , the measured signal originates from a fairly shallow radial zone compared to that of nuclear and resistivity logs . Depending on drilling mud being used and the radial extent of mud -filtrate invasion , the NMR response of virgin reservoir fluids can be masked by mud filtrate because of fluid displacement and mixing . In order to separate those effects , it is important to reconcile NMR measurements with electrical and nuclear logs for improved assessment of porosity and mobile hydrocarbon saturation . Previously , Voss et al . (2009 ) and Gandhi et al . (2010 ) introduced the concept of Common Stratigraphic Framework (CSF ) to construct and validate multi -layer static and dynamic petrophysical models based on the numerical simulation of well logs . In this thesis , the concept of CSF is implemented to reconcile 2D NMR interpretations with multi -layer static and dynamic petrophysical models . It is found that quantifying the exact radial zone of response and corresponding fluid saturations can only be accomplished with studies of mud -filtrate invasion that honor available resistivity and nuclear logs . This thesis indicates that the two interpretation methods complement each other and when applied in conjunction , improve and refine the overall petrophysical understanding of permeable rock formations . Examples of successful application include field data acquired in thinly -bedded gas formations invaded with water -base mud , where bed -boundary effects are significant and residual hydrocarbon saturation is relatively high . In such cases , numerical simulation of mud -filtrate invasion and well logs acquired after invasion enables reliable interpretations of petrophysical and fluid properties . The interpretation procedure introduced in this thesis also provides an explicit way to determine the uncertainty of petrophysical and fluid interpretations . |