A practical integrated forecast method for estimated ultimate recovery (EUR) and well production performance after water breakthrough during waterflooding in naturally fractured reservoirs (NFRs)

Abstract

It is both an opportunity and a challenge to develop naturally fractured reservoirs (NFRs) effectively and efficiently, especially with waterflooding development measures. In order to make production performance analysis and estimated ultimate recovery (EUR) forecast for NFRs during waterflooding easier and more widely applicable, a practical integrated method was established in this study based on two-phase flow characteristics of oil and water in NFRs and classic reservoir engineering theories of non-piston like displacement of oil by water, which consists of several novel practical forecast models for EUR and well production performance. Firstly, the well-controlled original oil in place (OOIP) of an NFR can be quickly fitted and obtained from actual production data by the established relationship between cumulative oil production and cumulative water production. Then, the maximum cumulative oil production, namely, the EUR of the production well can be determined when water cut is infinitely close to 1, and the well production performance in the future can be conveniently forecasted by the derived generalized relationships between water cut and other production characteristic parameters including cumulative water (or oil) production and recovery factor of well-controlled OOIP (or EUR). A lab-scale and two field-scale NFR models with different well patterns were established and simulated so as to verify whether the practical forecast models proposed in this paper are reliable and accurate, and comparison results indicated that all these models are in good agreement with the simulated production data. Sensitivity analysis for the curve fitting model of well-controlled OOIP demonstrated that viscosity ratio of water phase to oil phase has an enormous influence on the model and residual oil saturation also has a greater influence on the model compared with connate water saturation. In addition, two actual production wells in the X oilfield were selected as case wells for field applications and it was found that the unified type-curve for recovery factor performance forecast can be determined and employed for different production wells in the same oilfield unless the viscosity ratio of water phase to oil phase, the connate water saturation and the residual oil saturation do not change comparatively. The authors believe that the practical integrated forecast method established in this study is of great help for field reservoir engineers to analyze production data and forecast EUR and well production performance in NFRs during waterflooding development processes more quickly and conveniently. • A practical model for EUR forecast during waterflooding in NFRs was proposed. • Well-controlled OOIP can be conveniently fitted by actual field production data. • Well production performance can be forecasted by the derived integrated models. • The novel method is of great help for field engineers to quickly perform PDA.