Where are the Fractures? Incorporating Pressure Transient Analysis in Naturally Fractured Carbonate Reservoirs

Abstract Naturally fractured carbonate reservoirs (NFCRs) are among the most challenging hydrocarbon systems to characterize and develop due to their dual-porosity nature, complex fracture networks, and highly heterogeneous properties. Pressure transient analysis (PTA) has emerged as an indispensable tool for understanding the unique flow dynamics in these reservoirs, offering critical insights into fracture-matrix interactions, reservoir connectivity, and fluid flow behavior. This paper presents a comprehensive framework for incorporating PTA into the evaluation and management of NFCRs, with a focus on maximizing hydrocarbon recovery and reducing development uncertainties. By leveraging diagnostic plots, specialized type curves, and advanced analytical methods, PTA is used to unravel complex flow regimes that evolve over time, including early-time fracture-dominated flow, transition periods governed by matrix-fracture exchange, and late-time boundary-dominated behavior. These interpretations are essential for accurately defining reservoir properties and ensuring optimal field development strategies. The tool is used for identification and quantification of essential reservoir characteristics, including fracture permeability, matrix porosity, and key dual-porosity parameters such as the inter-porosity flow coefficient (lambda, λ) and the storativity ratio (omega, ω). The Field applications of PTA in NFCRs highlight its capability to address critical challenges such as identifying reservoir compartmentalization, estimating effective drainage areas, and delineating fracture connectivity. Additionally, this analysis sheds light on key operational hurdles, including fluid losses, significant wellbore storage effects, and unstable pressure responses. Advanced test designs, such as multi-rate or pulse testing, combined with numerical simulations, are proposed to enhance the reliability of PTA results in complex carbonate settings. The integration of PTA with complementary methods such as rate transient analysis (RTA), material balance, and full-field reservoir simulation is emphasized as a best practice. The presence of the V-shaped feature in the derivative plot of the crestal "well A" but its absence in the flank wells "B and C", despite all wells being located in the same dry gas fractured carbonate reservoir, can be attributed to several factors. Variation in fracture density and orientation across the reservoir can lead to differences in permeability, with the crestal area potentially having a higher fracture density or more favorably oriented fractures. Reservoir heterogeneity also plays a role, as fractured carbonate reservoirs are inherently heterogeneous, and the crestal well might be located in a more heterogeneous part of the reservoir. This multidisciplinary approach offers a holistic understanding of NFCR behavior, improving decision-making for well placement, stimulation design, and production optimization. This paper underscores the transformative potential of PTA in naturally fractured carbonate reservoirs, advocating for its inclusion as a core element of reservoir evaluation workflows.