Ambastha

Papers in this issue of SPE Res Eval & Eng focus primarily on carbonate reservoirs, diatomites, coalbed methane, gas hydrates (GHs), waterflooding, pressure/volume/temperature (PVT) calculations, and reservoir simulation. The following is a brief outline of the papers in this issue.

Carbonite Reservoirs

Upscaling Saturation-Height Technology for Arab Carbonates for Improved Transition-Zone Characterization describes upscaling of the capillary pressure data based on the availability of a large amount of mercury-injection data and statistical analysis thereof, obtained by fitting hundreds of individual core plugs to Thomeer functions. For the Arab-D limestone, and similar carbonates, the author derives a closed-form analytic expression for the upscaled capillary pressure function. A key result of the upscaled formalism is that reservoir cells--consisting of a large variation of pore systems--start to fill with hydrocarbons much closer to the free-water level when using saturation-height functions based on simple averaged pore system parameter values. Therefore, transition zones for upscaled reservoir elements (and well-log volumes) are thicker than those indicated by simple calculations based on data from single core plugs. The Challenges for Carbonate Petrophysics in Petroleum Resource Estimation uses a zonal discrimination based upon the distribution of microporosity and its connectivity with macroporosity and fractures. The discrimination scheme uses downhole technologies such as high-resolution imaging and magnetic resonance logs, supported by advanced core analysis. On this basis, a value-adding workflow is proposed to increase confidence in petrophysical deliverables used in volumetric estimates of petroleum resources. Using an Artificial-Neural-Network Method To Predict Carbonate Well Log Facies Successfully develops a workflow including core description preprocessing, log and core data cleanup, and probabilistic neural network (PNN) facies analysis. PNN is shown to outperform multivariate statistic algorithms and in this study gave good prediction accuracy (above 70%). The prediction uncertainty has been quantified by two probabilistic logs—discriminant ability and overall confidence. Lithofacies were predicted for 15 key wells in the Wafra Maastrichtian reservoir and effectively used to extend the understanding of the Maastrichtian stratigraphy, depositional setting, and facies distribution.

Diatomites

Temperature-Induced Fracture Reconsolidation of Diatomaceous Rock During Forced Water Imbibition presents results from a series of forced water-imbibition experiments to study the role of solution pH, temperature, salinity, and various divalent metal ions on silica dissolution and fracture reconsolidation of outcrop diatomite core. The experimental results suggest that temperature and pH are the two of the most important factors that have an impact on silica dissolution of diatomite. Tests also suggest that both silica dissolution and confining stress are necessary for fracture reconsolidation.

Coalbed Methane

Coalbed Methane: Current Field-Based Evaluation Methods reviews the state of the art in field-based techniques for coalbed-methane reservoir-property and stimulation-efficiency evaluation. Advances in production and pressure-transient analysis, gas content determination, and material balance methods made in the past 2 decades have been summarized. The impact of these new methods on the evaluation of key reservoir properties, such as absolute/relative permeability, gas content/gas-in-place, and completion/stimulation properties has been discussed.

GHs

Challenges, Uncertainties, and Issues Facing Gas Production From Gas-Hydrate Deposits explains the concept of the GH petroleum system; discusses advances, requirements, and suggested practices in GH prospecting and GH deposit characterization; and reviews the associated technical, economic, and environmental challenges and uncertainties, including the accurate assessment of producible fractions of the GH resource, the development of methods for identifying suitable production targets, the sampling of hydrate-bearing sediments and sample analysis, the analysis and interpretation of geophysical surveys of GH reservoirs, well testing methods and interpretation of the results, geomechanical and reservoir/well stability concerns, well design, operation and installation, field operations and extending production beyond sand-dominated GH reservoirs, laboratory investigations, fundamental knowledge of hydrate behavior, the economics of commercial gas production from hydrates, and the associated environmental concerns.

Waterflooding

Identification and Characterization of High-Conductive Layers in Waterfloods expands upon the use of modified-Hall analysis. The modified-Hall plot uses three curves involving improved Hall-integral and the two derivatives--analytic and numeric. The distance separating the integral and derivative curves provides the required information to quantify channel properties. A simple analytical solution is presented for transforming the separation distance into channel permeability-thickness product. Several simulated examples verified the channel-property-estimation algorithm and identified the distinctive derivative signatures for channeling and fracturing situations. New Gel Aggregates To Improve Sweep Efficiency During Waterflooding shows that microspheres (5–30 μm) synthesized using acrylamide monomers crosslinked with an organic crosslinker can plug some of the pore throats, and, thus, force water to change its direction and increase the sweep efficiency. A high-pressure/high-temperature rheometer was used to measure G` (elastic modulus) and G`` (viscous modulus) of these aggregates. Field data using microspheres showed significant improvements in the injection profile and productivity enhancement.

PVT Calculations

Using a Committee Machine With Artificial Neural Networks To Predict PVT Properties of Iran Crude Oil uses a new artificial-neural-network (ANN) model using component mole percent, solution gas/oil ratio, bubblepoint pressure, reservoir pressure, API oil gravity, and temperature as input data. The used ANN model is from the committee-machine type. The designed model processes its inputs using two parallel multilayer perceptron networks and then recombines their results. The results obtained show that the committee-machine model is a dependable network for prediction of PVT properties in reservoirs among the other ANNs and empirical correlations.

Reservoir Simulation

A Method To Implement Permeability Anisotropy Associated With Fault Damage Zones in Reservoir Simulation starts by using geomechanically-constrained discrete fracture models of damage zones. Then, the orientation and size of the faults with reference to the grid axes is used to incorporate the effect of permeability anisotropy in the simulation grid. In this case study, in which faults are formed in an extensional regime, damage zones show increased permeability along the strike of the fault and in the vertical direction, but there is no significant change in the permeability perpendicular to the faults. Inclusion of damage zones in the simulation model shows significant improvement in the history matching with respect to a base reservoir simulation model with no damage zones. Further, the uncertainty of the damage zone modeling in the reservoir simulation is analyzed by simulating multiple equiprobable models.

As you study your favorite paper(s) to enhance your own knowledge and/or apply in your work activities, please recognize that SPE welcomes further "Discussion" of any of the papers published in any SPE journal, including this one. Therefore, please feel free to submit discussion of a paper either online or by mail to SPE.

Sincerely,
Anil Ambastha, Chevron