Executive Summary

Tayfun Babadagli, University of Alberta

Nine papers were selected for the February 2013 issue of SPE Res Eval & Eng in the areas of reservoir characterization and EOR.

Reservoir Characterization

The first paper under this category, NMR Facies Definition for Permo-Triassic Kangan/Dalan Carbonate Formation by Use of Core/Log and Pore-Scale Measurements, presents an integrated approach for reservoir-quality (more specifically, pore-network system) determination of carbonate rocks. A gas field was taken as a case study and nuclear magnetic resonance (NMR) data (the relaxivity constant and specific-surface-volume data) on 28 samples were used to define seven facies using core/log NMR data, thin sections, and mercury injection experiments. For reservoir-quality description, the pore-network characteristics for this complex carbonate system were defined rather than grain property characteristics, which would not have been possible with conventional analysis approaches.

The next paper in this category, On Some Characteristic Features of Fractured-Horizontal Wells and Conclusions Drawn Thereof, examines the characteristics of horizontal wells with multiple vertical fractures using a previous model developed by the authors. After providing a comprehensive and comparative discussion on the behavior of horizontal wells, the authors concluded that scaling is needed to ensure that volumes drained by approximate models are reliable. The extension of the concepts introduced and discussed can be applied to the cases where the fracture and formation properties are different and interference effects are negligible.

In the last paper of this category, Methods To Obtain Quick Estimates of Formation Parameters in Interference Tests Derived From Features of the Line-Source Solution--Theory and Application, the authors present a practical method to design and interpret interference tests. They demonstrate that early-time features of the exponential integral functions occur much later in the interference test than in the single-well tests. These features can be used to estimate the storativity and transmissibility ratios through the intersection of the log-derivative and the pressure on the log-log plot and the inflection point of the pressure derivative. A field case was included to demonstrate the practical use of these new approaches in interference well-test interpretation. This exercise also indicated that the gauge resolution is critically important in multiwell testing because the response in the observation well is significantly smaller than in the active well.

Enhanced Oil Recovery

The first paper under this category entitled Anomalous Foam-Fractional-Flow Solutions at High-Injection Foam Quality presents fractional flow solutions which yielded significant deviations from the conventional solutions for high-quality-foam injection. The authors show that three regions exist in the simulations based on coreflood experiments. For the first region (Region A), which has high water cut, the new and conventional solutions are consistent. Low water cut injection conditions (Region C) yielded remarkable deviation from the conventional solutions, whereas the region in between (Region B) resulted in unstable solutions caused by a negative dfw/dSw slope. These discrepancies were attributed to the trapped gas saturation and if there is no gas entrapment, no deviations are observed between the conventional and proposed-mechanistic-simulation approaches. Therefore, estimation of the trapped gas saturation in advance is critical in foam-injection modeling.

Optimization of Foam Enhanced Oil Recovery: Balancing Sweep and Injectivity presents a case study on optimizing oil recovery by foam injection considering the sweep efficiency and injectivity using a homogeneous 3D model. Specific attention was paid to the surfactant-alternating-gas (SAG) process where the surfactant-slug size is optimized. The authors observed that maximum oil recovery can be obtained when the reservoir was partially unswept by foam at optimum surfactant slug size. At larger slug sizes, the foam breaks through quickly, resulting in less than the optimum oil recovery. The results presented in this paper would be highly useful in the estimation of the optimum balance among sweep efficiency, injectivity, and project economics.

Water-alternating-CO2 process in thin heavy-oil reservoirs for pressure maintenance and improving oil recovery is investigated in Pressure Maintenance and Improving Oil Recovery by Means of Immiscible Water-Alternating-CO2 Processes in Thin Heavy-Oil Reservoirs. Operating parameters (e.g., slug size and water/CO2 ratio) were optimized through a numerical model validated by 3D physical model experiments, consisting of three horizontal wells and five vertical wells. The case with three horizontal wells and five vertical wells yielded an incremental oil recovery of 12.4% and 8.9% through three water-alternating-CO2 cycles, respectively. The optimal WAG ratios of 0.75 and 1.00 were obtained for the aforementioned well configurations, respectively.

Continuous monitoring is essential to maintain the efficiency of EOR applications. The paper Pulsed-Neutron Monitoring of the First CO2 Enhanced-Oil-Recovery Pilot in the Middle East presents pulsed neutron data recorded in the injector, producer, and observed wells of a CO2 injection application, which started at original reservoir saturation in a field in the Middle East. The steps followed in this process, limitations because of measurement environments, and how different modes of the pulsed neutron data complimented each other are presented. Chief contributors to production and potential paths for breakthrough were identified through a combined analysis of openhole and pulsed neutron data.

Investigation of Anisotropic Mixing in Miscible Displacements  investigates longitudinal and transverse local mixing using a finite difference compositional simulator coupled with a 2D convection-dispersion model at fine and coarse scales and provides a quantitative and systematic procedure to estimate the degree of transverse mixing (dispersivity). The authors show that transverse mixing is significant when the flow direction changes because of heterogeneity and the dispersion increases with increasing heterogeneity. Therefore, local mixing, including transverse mixing, should be considered in the upscaling of miscible displacement. It is also reported that the transverse dispersion could be more effective than the longitudinal one.

In the last paper of this category, Probabilistic Forecasting and Model Validation for the First-Eocene Large-Scale Pilot Steamflood, Partitioned Zone, Saudi Arabia and Kuwait, the authors report a probabilistic approach in the assessment of a heterogeneous carbonate steamflood in the Middle East. For facility design and early decision analyses, numerical simulations were conducted to generate probabilistic forecasts and the model was validated using the outcome of a pilot started in 2009. The authors observe that the modeling approach was able to capture the heterogeneity and steamflood performance in the pilot area and conclude that the oil recovery forecast is comparable to the observed in steamfloods in sandstones despite heterogeneity. Although the work performed increased the confidence in forecasting the early time steamflood response, it was too early to evaluate the breakthrough forecasts because more pilot data were needed to achieve this.

Tayfun Babadagli
University of Alberta