An NMR-based model for determining irreducible water saturation in carbonate gas reservoirs

Abstract Unambiguously determining irreducible water saturation $$\left({S}_{\rm{wirr}}\right)$$ S wirr poses a formidable challenge, given the availability of multiple independent methods. Traditional approaches often depend on semi-experimental relationships derived from simplified assumptions. These methods, originally designed for oil sandstone reservoirs, result in varying $${S}_{{\text{wirr}}}$$ S wirr values when employed in carbonate gas reservoirs. Nuclear magnetic resonance (NMR) is the most advanced technique for determining $${S}_{{\text{wirr}}}$$ S wirr . While highly accurate, the NMR-based method necessitates the laboratory measurement of the transverse relaxation time $$\left({T}_{2}\right)$$ T 2 cutoff. Laboratory-based $${T}_{2}$$ T 2 cutoff determination is resource-intensive and time-consuming. This research aims to develop a robust model for determining $${S}_{{\text{wirr}}}$$ S wirr in carbonate gas reservoirs by utilizing NMR well logging measurements and special core analysis (SCAL) tests. Various $${T}_{2}$$ T 2 cutoff values were initially employed to compute bound water saturation $$\left({S}_{{\text{bw}}}\right)$$ S bw at different depths to achieve this. Subsequently, the data points $$\left({T}_{2}, {S}_{{\text{bw}}}\right)$$ T 2 , S bw were graphed on a scatter plot to unveil the relationship between $${S}_{{\text{bw}}}$$ S bw and $${T}_{2}$$ T 2 . The scatter plot illustrates an exponential decrease in $${S}_{bw}$$ S bw with increasing $${T}_{2}$$ T 2 , forming the basis for the $${S}_{{\text{wirr}}}$$ S wirr model derived from this relationship. Finally, the parameters of the $${S}_{{\text{wirr}}}$$ S wirr model were fine-tuned using SCAL tests. Notably, this $${S}_{{\text{wirr}}}$$ S wirr model not only accurately yields $${S}_{{\text{wirr}}}$$ S wirr at each depth but also offers a dependable determination of the optimal $${T}_{2}$$ T 2 cutoff for the reservoir interval.