The main objective of this study is to devise a methodology, utilizing a wealth of information gained from NMR (Nuclear Magnetic Resonance) and resistivity logs, and to assess and to quantify formation damage due to filtrate invasion in the flushed zone. Quantification of the damage, in terms of damage-ratio within vertical resolution of the logging tools, provides the input for the calculation of economic implications of the problem. This, in turn, helps us decide either the need or the scope of the remedial action.
The quantification of the damage relies on the determination of the amount of shift in the relative permeability curves for the flushed zone against the undamaged zone. Depth of investigation for the NMR logs is limited to the flushed zone while the resistivity logs provide a deeper insight into the formation. The DWT (Dual Wait Time) mode of an NMR logging pass yields the movable and immovable parts of the porosity in addition to the type of the fluids contained within. Derivation of an absolute permeability profile from the Bound Water Model in conjunction with synthetic capillary pressure-based RBVI (bulk volume irreducible as capillary pressure approaches to infinity) or Purcell's technique along with water saturation profile from DWT NMR data yield the foundation for computing the relative permeability characteristics of the formation by using empirical equations. A water saturation profile for the uninvaded zone is computed from resistivity logs using the proper petrophysical model while corresponding relative permeability characteristics are obtained via the same set of equations. The superimposed relative permeability curves from NMR and resistivity-based data indicate the presence and/or amount of the shift of the curves between the flushed-zone versus the uninvaded zone. The amount of increase in effective permeability to water and drop in effective permeability to hydrocarbons are computed from the superimposed relative permeability curves. The radius of the damaged zone is determined from the 2D inversion of resistivity data and inserted into the DR (damage-ratio) equation together with the effective permeability of the undamaged and damaged zones. Skin factors can also be computed as a function of the available data. The annual revenue loss per well, formation damage dollars lost (FD$L)3, due to filtrate invasion-based damage can then be assessed by multiplying the damage-ratio with the current price of the hydrocarbon and the rate of production. To further evaluate the economics, the damage-ratio and FD$L can be plotted for various damaged radii to determine the threshold radius for economically justifying the cost of remedial action.