DFIT Based Empirical Approach to Predict Unconventional Tight Well Productivity Index and Post Closure Formation Related Properties

Diagnostic fracture injection test (DFIT) is a widely used tool to derive key reservoir properties for unconventional reservoirs. This paper proposes a novel and simplified approach to estimate the reservoir transmissibility/permeability from DFIT closure data at linear flow, and without the need for radial flow. The work was implemented in multiple tight sand formation wells. Results yielded an estimate of transmissibility/permeability, and optimize DFIT post-injection monitoring time. The proposed approach incorporates Closure Analysis and After Closure Analysis (ACA) results from previous DFIT tests to derive a correlation for estimating formation permeability. The approach begins by estimating injected fluid leak-off efficiency through the application of the G-function, followed by utilizing ACA to obtain reservoir transmissibility and pore pressure. The concept is built on fluid diffusivity mechanisms. During fracture closure, the fluid leak off mechanism into the formation is governed by the magnitude of Instantaneous Shut-in Pressure (ISIP), until closure pressure is reached. After closure, fluid diffusivity is influenced by another pressure mechanism that is the excess pressure as it diffuses into the formation and approach the reservoir pore pressure. Finally, the fluid efficiency from Closure Analysis is considered as a formation transmissibility index, which is then compiled with the actual transmissibility estimated from ACA to develop the prediction correlation. In this study, the approach was applied in multiple wells where DIFT was concluded before reaching the radial flow. Additionally, the built correlation was used to evaluate the transmissibility in other wells where only closure analysis was available. The correlation developed between the fluid leak-off efficiency and reservoir transmissibility from ACA showed an exponential trend with high accuracy, with a Coefficient of Determination higher than 95%. The result of the correlation was linked to well performance using productivity index as a proxy, yielding accurate result, indicating the critical role formation transmissibility has on fluid deliverability. Applying the productivity index and the predicted formation transmissibility from the correlation matched with high accuracy which validates the methodology. For extremely tight reservoirs, radial flow might be reached after several days, weeks, or months of monitoring pressure decline. The proposed approach saves significant DFIT post-injection time required to observe radial flow. Also, it can be used as validation tool for closure pressure picking where in some case the closure estimation utilizing G-function is selective.