Babadagli

I took over the Co-Executive Editor position of SPE Res Eval & Eng from Cosan Ayan effective after the 2010 SPE Annual Technical Conference and Exhibition. I am honored to provide this service to SPE next 3 years and it is my pleasure to present the Executive Summary of the April issue.

In this issue of SPE Res Eval & Eng, a mixture of reservoir engineering topics--from field and core scale enhanced oil recovery to formation damage, shale gas development to different scale reservoir characterization--are covered.

Field Applications and Simulation of EOR

In the first paper, Steamflood With Vertical Injectors and Horizontal Producers in Multiple Zones, the authors report the results of a simulation study on the Mukhaizna heavy oil field in Oman that they performed to evaluate the steamflood performance of a three zone using different horizontal and vertical well configurations. After showing a good agreement with 2-year history of the field application, they tested an optimal design with vertical injectors and horizontal producers. They showed that the combination of vertical injectors and horizontal producers prevented the early breakthrough of steam in comparison to horizontal injectors and yielded a better performance. In another EOR performance analysis paper, Improved Polymer-Flood Management Using Streamlines, the authors used streamline-simulation techniques to model polymer injection and reported an optimization workflow. In addition to its computational efficiency, the streamline technique was observed do be more advantageous when compared to finite-difference modeling in optimization exercises because the polymer floods can be optimized on a well-pattern basis. The authors tested a suggested workflow for optimal design of polymer flooding on a 50-year old field in Romania with nearly 150 wells and showed that an additional 5% recovery can be obtained in compared to waterflooding.

Waterflooding

An evaluation of low-salinity water injection through coreflooding experiments and a field pilot was the subject of the paper Snorre Low-Salinity-Water Injection—Coreflooding Experiments and Single-Well Field Pilot. Remaining oil saturation from the core tests showed a good agreement with the field pilot but, interestingly, both measurements indicated only low or no effect from low-salinity-water injection. Their suggestion was that low-salinity-water injection has a potential for improved oil recovery in all types of clayey sandstones.

Formation Damage

This issue of SPE Res Eval & Eng contains one paper on formation damage. In the paper Formation-Damage Evaluation From Nonlinear Skin Growth During Coreflooding, the authors introduce a new semianalytical model for axisymmetric suspension filtration to predict well-injectivity decline from the linear coreflood data with pressure measurements in three core points. They validated the model using laboratory core tests with additional (fourth and fifth) points. They showed that their linear model is a practical way to obtain well-injectivity parameters for homogeneous systems.

Reservoir Characterization--Direct and Inverse Approaches

A group of papers under the category of reservoir characterization deals with the detection of different reservoir parameters and characteristics at different scales. Starting with well-scale characterization, Method To Generate Full-Bore Images Using Borehole Images And Multipoint Statistics proposes an approach to generate 360° views of the borehole. One of the problems in imaging wellbores is incomplete images because of unmeasured areas caused by the gap between pads. The method presented in this paper uses the Filtersim algorithm of multipoint statistics to generate realizations to fill the gap between the measured areas. The results were conditioned to the logs and they perfectly matched the measured data. A few case studies also showed that full-bore images generated from one pass of an imaging log show only subtle variations when compared to multipass logs. The authors concluded that the Filtersim algorithm successfully models layers, vugs, fractures, and other features commonly seen in borehole images. 

In a larger scale reservoir-characterization attempt, Generating 3D Permeability Map of Fracture Networks Using Well, Outcrop, and Pressure Transient Data, an approach to estimate fracture network permeabilities and to generate 3D permeability map using data obtained at different scales is introduced. The authors correlated 1D (well), 2D (outcrop), and 3D (well-test) data to effective fracture network permeabilities (EFNP) and proposed several empirical correlations to determine the EFNP. The eight-variable equation honors all these types of data and yielded the best match to the actual EFNP. The accuracy of the three-variable case (1D well data and 3D well-test data) was surprisingly comparable to the six-variable correlation (five 2D fractal and statistical parameters from outcrop studies and the permeability obtained from single well pressure-transient data). Hence, 1D single well data could be useful for mapping fracture network permeability when it is used along with the permeability obtained from a single well pressure-transient test. In another field-scale reservoir characterization study, the authors of Simultaneous Interpretation of Pressure, Temperature, and Flow-Rate Data Using Bayesian Inversion Methods, introduce an inverse method (i.e., using permanent downhole pressure, temperature, and flow-rate measurements). The main focus of the paper was to add temperature for a better interpretation and characterization of reservoir in case of insufficient pressure and flow rate data. Supported by matched field data, the authors showed that the inversion technique they applied was successful in extracting signals even with noise levels as high as 10%. This new approach that incorporates the temperature as data to the cointerpretation of pressure and flow-rate from permanent downhole measurements not only allows the identification of reservoir behavior, but also provides a way to determine the rate from temperature measurements.

Determination of accurate formation mobility (permeability to viscosity ratio) at early stages of development is highly critical in further decision making practices. In the paper A Robust Method for Calculating Formation Mobility with a Formation Tester, a practical but rigorous formula was derived to determine the drawdown mobility associated with formation testers. The model assumes the presence of both tool storage and formation compressibility, which makes it appropriate for fully unsteady-state flow from the formation without requiring formation compressibility to determine mobility. The usefulness of this novel method of determining drawdown mobility is that it could be used under many diverse situations such as nonconstant pretest piston speed, the adherence of mud to the wellbore wall, large probe orifice, and anisotropic formation.

Horizontal Wells in Tight Reservoirs

The horizontal-well technology and unconventional resources were developed in parallel over the last three decades but studies on the applications horizontal wells in tight sands and shale reservoirs are very limited. The last paper of this issue, Comparison of Fractured-Horizontal-Well Performance in Tight Sand and Shale Reservoirs, reports a comparative discussion of horizontal-well performances for a wide variety of permeabilities, from milli- to nano-Darcy. Applying a trilinear model, it was shown that high hydraulic or natural fracture conductivity may not improve productivity in tight shale gas reservoirs. The authors concluded that attention should be given to the ways to improve the density of natural fractures for this type of reservoirs for incremental recovery. They finally noted that hydraulic fracture conductivities should be optimized on the basis of the flow capacities of the matrix and natural fracture network as decreasing fracture spacing may increase well productivity but the incremental gain for each additional fracture decreases.

Tayfun Babadagli
University of Alberta