Executive Summary

This issue of the SPE Journal includes 16 articles reflecting the current interest in various areas related to the upstream petroleum industry. The themes covered in this issue, respectively, are heavy oil and enhanced-oil-recovery methods, reservoir simulation, transport in porous media, unconventional hydrocarbon recovery, and drilling and completions. The following is a brief overview of the included articles.

Heavy Oil and Enhanced-Oil-Recovery Methods. The first paper in this section includes discussions on solvent dissolution in heavy oils and reports new viscosity measurements of Athabasca bitumen/pentane mixtures at temperatures up to 200°C and at pressures up to 10 MPa. These conditions are applicable for both in-situ-recovery methods and the pipeline transportation of heavy oil. The measurement data for the mixtures are then evaluated with predictive schemes and known correlation models. The influences of pressure, temperature, and solvent weight fraction on the viscosity of mixtures are considered in the models, and the paper concludes that the power-law model and the Cragoe (1933) model represent the experimental data better.

The next paper stems from the need to predict the field performance of a class of heavy-foamy-oil reservoirs, and describes a pseudobubblepoint pressure/volume/temperature (PVT) model and a methodology that can be used to compute foamy-oil fluid properties from conventional laboratory PVT data. The techniques developed are then used to study foamy oil in the Orinoco Belt, Venezuela.

The third paper includes experiments that were conducted for the compressibility of waxy crude oil at temperatures lower than pour point. Results show that the compressibility of gelled crude oil is much higher than that of the liquid oil at temperatures above pour point. The compressibility gradually decreases during the compression process until the gel structure is destroyed.

The fourth paper in this section discusses a special mode of alkaline/polymer core flooding. The authors argue that addition of cosolvents to alkaline/polymer solutions dramatically improves the performance of the core flooding. This new chemical-enhanced-oil-recovery technology has been termed alkali/cosolvent/polymer flooding.

Finally, I included a paper on the challenging area of carbon dioxide injection and sequestration.

Reservoir Simulation. This theme opens up with a paper on a newly-developed regional-scale multiphase upscaling method, in which the dynamic effects of sub-grid scale reservoir elements (e.g., channels, lobes, sand bars, and shale drapes) are captured. Unlike the conventional upscaling methods, this method does not require fine-resolution simulations at the reservoir scale; rather, it relies on an intermediate spatial scale, also referred to as the regional scale, defined as the scale at which the global multiphase flow effects of nonlocal stratigraphic elements are approximated by fine-resolution flow simulations.

The next paper is a review article involved with the computationally intensive phase-equilibrium calculations of the compositional simulation. This becomes a critical issue in simulation as the number of components and phases increases. Reduced methods were developed to address this problem. In this paper, the authors compare the reduced methods published to date.

The final paper in this section introduces fast computational flow-diagnostic measures to evaluate, rank, and compare realizations or strategies.

Transport in Porous Media. In this theme, a paper is included on the so-called free-fall gravity drainage (FFGD), which is the main production mechanism in the gas-invaded zone of a fractured reservoir. Gravity force acts as a driving force to remove oil from the matrix block, whereas the resistive capillary force tends to keep oil inside the matrix. The authors report on a series of micromodel experiments showing the effects of fracture geometry and of the matrix on the oil-production rate under an FFGD mechanism. The results show that different flow regimes occur during the production life of a single matrix block. The flow is controlled by a capillary-dominated regime at the early stage, whereas at other times, it exhibits a stabilized bulk flow behavior under a gravity-dominated regime. Experimental results revealed that in a heterogeneous layered matrix block, both the drainage rate and the recovery factor decrease in comparison with a homogeneous matrix.

Unconventional Hydrocarbon Recovery. Creating multiple-stage hydraulic fractures in horizontal wells and their optimization is critical for economical unconventional gas production. The authors of the first paper in this section argue that the optimization requires reliable models that can account for the simultaneous propagation of multiple, potentially nonplanar fractures. In the paper, a novel fracture-propagation model is described that can simulate multiple-hydraulic-fracture propagation from a horizontal wellbore. The model couples fracture deformation with fluid flow in the fractures and the horizontal wellbore. The displacement discontinuity method is used to represent the mechanics of the fractures and their opening, including interaction effects between closely spaced fractures.

In the second paper, the authors present a novel approach to calculate drainage volume and well performance in shale-gas reservoirs by use of the fast marching method (FMM) combined with a geometric pressure approximation. The approach fully accounts for complex fracture-network geometries associated with multistage hydraulic fractures and their impact on the well pressure and rates. A geometric approximation of the drainage volume is used to compute the production rates and the reservoir pressure.

The final paper in this section focuses on the transient-pressure behavior of multiwing fractures connected to a vertical wellbore. The vertical well is fractured with multiple-fracture wings with varied intersection angle, length, and asymmetry factor. For an anisotropic formation, the fracture may grow at a specific azimuth, and a fracture cluster may develop.

Drilling and Completions. Logging while drilling offers real-time geological information near the bottomhole assembly during the drilling process. These data provide information around and possibly ahead of the drilling path, and offer a huge potential for improved recovery through continuous optimization of the well path. The authors of the first paper in this section show an automated workflow for proactive geosteering through continuous updating of the estimates and robust optimization of the remaining well path under uncertainty. The estimates of the depths of the reservoir surfaces and the depth of the oil/water contact and their associated uncertainty are obtained through the ensemble Kalman filter by use of directional-resistivity measurements.

The next paper is on the generation of hydraulic fractures and the challenges posed by stress shadowing and reservoir variability. In this paper, the authors use a newly-developed, fully coupled, parallel-planar 3D fracturing model to investigate the impact of stress shadowing. The model confirms the phenomenon of inner-fracture suppression because of stress shadowing when the perforation clusters are uniformly distributed.

The third paper in this section discusses the requirement for intervention operations in long-reach lateral wells. It is common to run coiled tubing in 10,000-ft laterals. To reach such a depth, however, metal-on-metal lubricants will be required to work in conjunction with the other systems. A set of laboratory measurements was carried out with a linear-friction measuring device to understand and quantify the mechanical, chemical, and thermal metal-on-metal wet frictional effects. Other tests, such as regained-permeability and aging tests were also performed.

The final paper of the April issue includes a new buckling equation in horizontal wells, which has been derived on the basis of the general bending and twisting theory of rods. Buckling solutions under different boundary conditions are obtained by solving the new buckling model. The results indicate that buckling behaviors depend on both the axial force and the boundary conditions. Compared with previous results, the new buckling solutions provide a more comprehensive description of the tubular-buckling behavior.

I hope you enjoy this issue.

I. Yucel Akkutlu, Texas A&M University, College Station