Executive Summary

Cosan Ayan, Schlumberger

It seems that enhanced recovery, be it for oil, coalbed methane, or for other unconventional resources, continues to be a major field for research and engineering. This is not surprising given the current levels of oil pricing and the state of our industry. I tried a few search engines with "EOR" as the only keyword and had hits ranging from 1.2 to 3.3 million sites. Though some of them were not relevant--including hits about a well known Winnie the Pooh character--the existing interest in, as well as a new focus on, EOR is evident. Even within the intranet at Schlumberger, my company, a search for "steam"” produced 10,400 hits, which is a considerable number. I also checked the SPE eLibrary, searching with simple keywords like "EOR, IOR, steam, CO2, WAG, polymer".

From 2003 onward, papers related to "Improved Oil Recovery/Enhanced Oil Recovery" are on the rise. Though all search categories show an increase, polymer- and CO2-related papers show significant growth. Polymer-related papers also cover applications such as water shutoff, and the latest CO2 projects now have a bigger scope with sequestration as an application, which is becoming the main goal.

This issue of SPE Reservoir Evaluation & Engineering is no exception, as almost half of the papers are related to some form of enhanced recovery of hydrocarbons. It was quite interesting to see an update on the Bati Raman immiscible CO2 flood in this issue of SPEREE, considering that I personally heard about its concept, feasibility, and initial application stages during my early years in the profession. The article"Seawater in Chalk: An EOR and Compaction Fluid" unexpectedly classifies seawater as an EOR and compaction fluid. In the Ekofisk chalk reservoir, seawater was changing the wettability to more "water-wet" and even though injection repressurized the reservoir, compaction did not stop because of seawater weakening of chalk at high temperatures. The authors outline the chemical mechanisms of seawater/chalk interactions, which increase the recovery favorably. A method to better characterize fluids for processes involving condensing-vaporizing drives is outlined in the paper "Improved Fluid Characterization for Miscible Gas Floods." The method tunes the EOS using the calculated MMP and reduces the number of pseudocomponents significantly, resulting in computational efficiency without losing accuracy. In the paper"Predicting Openhole Horizontal Completion Success on the North Slope of Alaska," the authors study the reasons of successful openhole horizontal completions in a particular field and identify geological properties, rock mechanics, and fluid-flow control as the three key governing factors. Extending their findings, they try to identify other reservoirs in which similar completion techniques can be applied. Condensate or water blocking is a well known phenomena, which has a severe impact on the productivity of some gas-condensate wells. In the paper "Wettability Alteration in Gas-Condensate Reservoirs to Mitigate Well Deliverability Loss by Water Blocking," by using core samples, the authors propose chemical methods to permanently alter the near-wellbore wettability from liquid-wet to preferentially gas-wet to arrest productivity decline. In the paper "Seismic Geomorphology and Connectivity of Deep-Water Reservoirs," the authors outline a seismic-volumetric-visualization method that combines reflectivity and impedance seismic volumes to assess reservoir connectivity and permeability heterogeneity. Complemented with analog outcrop data, the method significantly improves the characterization of a deepwater turbidite reservoir in the Campos basin. The paper "A Hybrid Approach To Improve Reserve Estimates in Waterdrive Gas Reservoirs," outlines the methods used for gas-in-place estimation in water-drive gas reservoirs. An approach that uses available methods is presented, narrowing the reserve-range estimates currently faced with each individual technique. With EOR methods becoming prominent in the current environment, the paper, "The Use of Fractional Flow Theory for Foam Displacement in Presence of Oil" investigates mechanisms of foam displacement in porous media using three-phase fractional-flow theory and ternary diagrams. Oil can be detrimental to foam stability; the strength of foam in presence of oil was accounted using a mobility reduction factor that can be measured in the laboratory core floods. In the paper "P-Wave Azimuthal AVO in a Carbonate Reservoir: An Integrated Seismic Anisotropy Study," the authors outline seismic anisotropy, which is a field-wide fracture-detection technique sensitive to major open fractures. In the proposed workflow, azimuthal anisotropy of amplitude vs. offset inversion results are combined with rock physics, forward seismic modeling, seismic-scale fault interpretation, image logs, and core data to study the intensity and orientation of fractures in a carbonate reservoir. In the never-ending quest for permeability determination, "An Integrated Approach to Obtain Reliable Permeability Profiles From Logs in a Carbonate Reservoir" uses Stoneley data in combination with formation images to determine permeability in the Karachaganak field of Kazakhstan. When compared with transient well tests, the high permeability streaks were found to be missed by the log derived matrix permeability in some wells. The paper "Thermal and Hydraulic Matrix-Fracture Interaction in Dual-Permeability Simulation" outlines the shape factors that govern the interaction between two different continuums given in the literature and show that a transient shape factor that also considers the physical recovery mechanism is the suitable approach. In a closely related paper, "Efficient Field-Scale Simulation of Black Oil in a Naturally Fractured Reservoir Via Discrete Fracture Networks and Homogenized Media," the authors outline a hybrid method that combines a discrete fracture network and dual-continuum approach with effective properties. The fractures are classified as short, medium, and long on the basis of their lengths relative to the gridblock size. While the short and medium fractures are "homogenized," networks of long fractures were modeled explicitly within this "effective matrix-blocks." "Lessons Learned From Energy Models: Iraq's South Rumaila Case Study" shows that classical material balance techniques are key to understanding reservoir behavior before applying numerical simulation. Via material balance, streamline simulation, and finite-difference prediction runs, the field case shows strong and uneven aquifer support, suggests stopping water injection and identifies attic oil in a mature asset. In another case history, the paper "Improved Understanding of Reservoir Fluid Dynamics in the North Sea Snorre Field by Combining Tracers, 4D Seismic, and Production Data," interwell tracer test data was analyzed along with three different seismic surveys and production data in the Snorre field during its WAG program. The integrated analysis showed that seismic dimming correlated well with water fronts and seismic brightening outlined gas accumulations, both also compared well with water- and gas-tracer results. Another EOR-related paper, "Bati Raman Field Immiscible CO2 Application, Status Quo, and Future Plans," the authors give the current status of the CO2 flood in this heavy oil field after more than 25 years of implementation. The challenges of immiscible CO2 flood in a heterogeneous fractured limestone formation are outlined from gas source, transportation, injection, and performance evaluation. The recovery from 1.85 billion barrel OIP asset increased from less than 2% to approximately 5% with CO2 injection. From CO2 injection for heavy oil, we visit CO2 injection for enhanced-coalbed-methane recovery with the paper titled "Sorption-Induced Permeability Change of Coal during Gas-Injection Processes." The authors outline the well-known problem of permeability reduction with CO2 introduction into the CBM asset. However, introducing binary mixtures of gases can help preserve permeability; 10 to 20 mol% of nitrogen was found to be enough to preserve permeability.

Cosan Ayan
Schlumberger