Babadagli

The December 2011 issue of SPE Res Eval & Eng contains 10 papers categorized under three headings--pressure-transient-test applications, chemical enhanced oil recovery (EOR), and reservoir characterization.

Pressure-Transient-Test Applications

Four papers under this category deal with different types of pressure-transient-test applications for different reservoir types. Starting with a theoretical approach, Pressure-Pressure Deconvolution Analysis of Multiwell Interference and Interval Pressure-Transient Tests presents an investigation into the use of pressure-pressure (p–p) deconvolution for interpretation of interference tests. The authors showed that deconvolution algorithms developed for pressure-rate (p–r) deconvolution based on a single source/sink well system can be used for (p–p) deconvolution by simply replacing the rate data in the p–r deconvolution algorithms. After providing applications of (p–p) deconvolution to synthetic- and real-field data from wireline-formation-tester interval pressure-transient and interference tests, the paper concluded that the (p–p) deconvolution could be used for model identification purposes in a similar way based on p–r deconvolution without any flow-rate information.

The next paper, Practical Solutions for Pressure-Transient Responses of Fractured Horizontal Wells in Unconventional Shale Reservoirs, introduces an analytical trilinear-flow solution to simulate the pressure-transient and production behaviors of fractured horizontal wells in unconventional shale reservoirs. The model presented includes the intrinsic properties of matrix and natural fractures, as well as fluid exchange between these media. The trilinear solution is highly practical and an alternative to rigorous numerical or semianalytical models because of reduced computational time.

Another practical application of well performance analysis is presented in Semisteady-State Productivity of a Well in a Rectangular Reservoir Producing at Constant Rate or Constant Pressure. Two exact analytical formulas for the semisteady-state productivity index of a closed-rectangular reservoir, based on exact analytical solutions for the long-time well-pressure and well-rate responses, were provided. The formulas presented are simple and can be implemented in any spreadsheet program. The major outcome of the work is that the constant-pressure productivity indices are always lower than constant-rate ones.

Interpretation of Immiscible WAG Repeat Pressure-Falloff Tests reports an analysis of repeat falloff tests acquired in two vertical pattern injectors in a carbonate reservoir. The authors quantified the performance of an immiscible water-alternating-gas (WAG) mechanism using this analysis and were able to detect creation of an effective mixing zone of injected gas and water through changes in the pressure-derivative slope. Interpretations indicated that the two pattern vertical injectors behaved differently, and this was attributed to different reservoir characteristics and perforation intervals. Preferential fluid conduits were determined at the lower subzone as well as at a strike-slip fault corridor, inducing preferential lateral fluid movement through the interpretation of the falloff tests.

Chemical EOR

A number of papers report laboratory and field pilot applications of chemical EOR. A design process for a field-scale alkaline/surfactant-polymer (ASP) -flood application is introduced in Alkaline/Surfactant/Polymer Flood: From the Laboratory to the Field. Starting with molecular-scale experiments to identify surfactant/oil/rock interaction and continuing with core-scale experimentation, suitable ASP formulation was found. Next, the results of a series of single-well chemical-tracer tests, performed in a total of five wells in three different fields, were presented to assess the remaining oil saturation. The tracer-test observations were in agreement with those of the corefloods. Finally, the paper presented a design of a pattern-flood ASP pilot for which the following were considered as criteria for which: the maximization of data acquisition, such as injectivity, desaturation, and recovery factor; the robustness against well or equipment failure; the quantification and mitigation of emulsion and scale formation; a representative geological setting; and a feasible pilot duration.

An evaluation of low-salinity-water injection through coreflooding experiments is the subject of Wettability Studies Using Low-Salinity Water in Sandstone Reservoirs. Fluid/rock interactions were studied at different salinity levels and elevated-temperature conditions in terms of wettability and surface charge. After evaluating the wettability at high-pressure/high-temperature with the contact-angle method and zeta-potential technique using different synthetic brines, aquifer water, and seawater, the authors observed a wettability alteration to more water-wet with low-salinity aquifer water, which will eventually lead to improved waterflooding recovery.

Rheology of a New Sulfonic Associative Polymer in Porous Media investigates the behavior of a new associative polymer in porous media. This hydrophobically associative polymer (tetra-polymer), which has low hydrophobic-monomer content and a molecular weight of 12–17 million g/mol, was compared with a conventional polymer (hydrolyzed polyacrylamide, 3820S) with 18–20 million molecular weight and higher anionic content. Although both have similar rheological properties and excellent filterability at lower concentrations yielding no faceplugging at any concentration, experiments on long cores (up to 1.5 m) suggested that the new polymer has a higher level of shear thinning at low fluxes and a lower degree of shear thickening at high fluxes.

Reservoir Characterization

Different aspects of reservoir characterization using different tools are covered in the last three papers of this issue of SPE Res Eval & Eng. Intelligent Production Modeling Using Full-Field Pattern Recognition presents a new approach to fieldwide production-data analysis using artificial intelligence for future production performance and field recovery. Synthetic cases were used for validation purposes, and future production performance analyses were performed for two giant fields in the Middle East. After training a series of neural networks using a back-propagation algorithm, the networks were fused together to form the "intelligent time-successive production modeling" (ITSPM) system. It was shown that ITSPM can be a promising technique, especially in field cases with a long history of production data. The possibility of success increased as more data became available.

A new method was presented in Predicting Effective Permeability to Oil in Sandstone and Carbonate Reservoirs From Well Logging Data to predict effective permeability to oil in sandstones and carbonates. The method is based on using model-independent mapping functions, which replace the empirical equations in the conventional approach, constructed from Gaussian radial basis functions, which are derivable from laboratory measurements on partially saturated core samples. Using a worldwide database of measurements on 79 sandstone and 25 carbonate core samples including irreducible water saturation, effective permeability to oil, porosity, and nuclear-magnetic-resonance (NMR) T₂-distributions, mapping functions were derived. Finally, the method was applied to log data from both sandstone and carbonate formations in three wells from different fields, and consistent results with oil mobilities measured in the formations by fluid-sampling tools were observed. Advantages of using mapping functions over the conventional empirical-equation approach to predict reservoir properties from well logging measurements were explained in detail.

The last paper of this issue, A Simple Formula To Estimate 2D Fracture Connectivity, introduces a simple expression for average number of intersections per fracture as a product of fracture density, average length, and angular standard deviation in 2D fracture networks. It was shown that the average intersection per fracture term can be used as a good estimator of fracture connectivity. The paper concluded that, because the validation was performed only for stochastic fracture models rather than considering all rock-mechanical aspects and crack growth, limitations might exist.

Tayfun Babadagli
Co-Executive Editor of SPE Res Eval & Eng