A Recovery Curve Method Based Workflow for Reserves Estimation of Naturally Fractured Reservoirs – A Case Study
Authors: F. Justin Brand , Mittermeir Georg M. , Heinemann Gabor F. 
Published: November 2017

Naturally fractured carbonate reservoirs (NFRs) account for a majority of the world’s proven oil (~60%) and gas reserves (~40%). Many operators either significantly over-predict production and reserves, or non-optimize NFRs lacking suitable methods for modelling the fracture-matrix fluid interactions. Even complex functional shape factor based formulations fail. Only numerical fine-scale single-matrix-block (SMB) calculations capture the matrix-fracture fluid exchange in its complexity. A workflow and a successful case study (exceeding 80 years history) are presented.

Heinemann (2004) proposed a concept (Recovery Curve Method, RCM) utilizing predefined recovery curves instead of functional descriptions to model the fracture-matrix interaction. Meanwhile, the necessary methods and tools have been researched, developed and proven by field applications.

Our iterative multi-step workflow reduces uncertainty gradually. At first, matrix oil recovery is investigated on SMB models (Steiner and Mittermeir (2017)). The resulting recovery curves are verified and tuned with Mittermeir’s (2015) Dual Porosity Material Balance method. The link to full field simulation is established by a numerical dual porosity column model. Finally, the full field is calculated and matched by the RCM.

Based on a first conceptual reservoir model, burdened with uncertainty, the matrix and fracture rock system is examined gradually. The reservoir volume, time frame and complexity covered is stepwise extended. Using the finely gridded SMB, the physics of matrix-fracture interaction are investigated under different drive mechanisms including single phase expansion, solution gas drive, capillary imbibition, gravity drainage, viscous and diffusive forces. Effects of these mechanisms can be studied in an isolated or combined way, including its time dependency. The resulting recovery curves give a good estimate of reservoir behavior under different operating conditions. Dynamic data, such as pressure, production and phase contact history are matched subsequently by dual porosity material balance calculations. Results are water influx requirements (analytical aquifer models) and verified recovery curves. The latter are used to estimate the missing or highly uncertain data used to build the conceptual model by reversing the SMB calculation procedure. Consistency of the obtained improved data set is verified by a numerical dual porosity column model. It resembles the twin-barrels of the material balance model, however accounts for vertical transmissibility distribution. Individual well performance is matched in the full field model, which in addition to the material balance is used for forecasting.

The recovery curve based workflow provides a comprehensive concept and a consistent closed loop modelling of NFR exploitation. The shortcomings of the commonly applied shape factor concept (Kazemi et al. (1976)), which are well documented in the literature, are eliminated by omitting an analytical transfer function. Instead, diligently evaluated recovery curves are used directly to model the complex matrix-fracture interaction under different recovery mechanisms at each stage of the field life.