Executive Summary

This issue of SPE Journal includes 18 papers that reflect the current interest in various areas related to the upstream petroleum industry. The themes covered in this issue, respectively, are transport in porous media, heavy oil and enhanced oil recovery, unconventional resources, reservoir engineering, and flow in pipes. The following is a brief overview of the included articles.

Transport in Porous Media

The first paper in this section focuses on nanoparticle adsorption during transport in porous media. Particles with diameter less than 50 nm easily pass through typical pore throats, but the molecular forces between the particles and the pore walls may still lead to retention. A series of flow experiments are presented that use core plugs and columns packed with crushed rocks. Results with a variety of nanoparticles yield a wide range of adsorption capacities. The capacity is typically much smaller than that with the monolayer coverage.

The second paper is on developing a procedure for visualization of acid treatments of carbonates by use of nondestructive 3D nuclear-magnetic-resonance imaging and microfocus-computed-tomography technology. The results show high-quality imaging achievements for the extracted wormholes created during the treatment.

The last paper in this section is on dynamic modeling of channel formation during water injection into poorly cemented granular formations. Injection of water leads to formation of preferential flow paths and channelization. Hence, porosity and permeability of the medium could evolve along the induced flow paths. A model based on a multiphase-volume-fraction concept is used to study characteristic behavior during fluid injection and the possible effects of injection parameters on the evolution of porosity and permeability.

Heavy Oil and Enhanced Oil Recovery

The first paper in this section focuses on optimizing the workflow of the in-situ upgrading process for extraheavy-oil fields. This is a highly complex process, and it is difficult to conduct an automated optimization. In this paper, the authors developed an efficient optimization work flow by addressing three technical challenges for the field-scale developments. The proposed approach requires only a few dozen simulation cases as training data, compared with thousands of simulation runs required by the conventional methods. This work flow can perhaps also be applied to other kinds of pattern-based field developments of oil shale and thermal processes such as steamdrive or steam-assisted gravity drainage.

The next paper presents a new graphical-estimation method to predict the parameters related to vapor solvent dissolution and diffusion in heavy oils. The method uses the late-time pressure-transient data of the pulse-decay experiment to directly estimate Henry’s constant and solvent-diffusion coefficient.

The third paper in this section focuses on improving production from gas condensate carbonate reservoirs by use of wettability modifiers. A reduction in well productivity occurs in these reservoirs because of reduced gas mobility caused by the presence of high-saturation liquid phases around the wellbore. Certain fluorinated surfactants were capable of delivering a good level of oil and water repellency to the rock surface, making it intermediate gas-wet and alleviating liquid blockages. Screening tests with brine are performed in the presence of some ionic and nonionic surfactants. Positively charged carbonate surfaces with the anionic surfactant is effective to repel the liquid phase, whereas the nonionic surfactant promotes stability of the aqueous phase. A combination solution is proposed.

The next paper is centered on mitigating formation damage by use of nanoparticles during waterflooding. During water injection, the released clays and other formation fines can accumulate and plug the pore throats, causing lower sweep efficiency. A common solution is the use of additives that can stabilize the fine particles at their sources during the injection operation. Some inorganic nanoparticles have previously been proposed to control the fines migration. The particles with high surface forces attach themselves to the surfaces and allow adsorption of the migrating fines. This paper provides a detailed laboratory evaluation on the use of nanoparticles to stabilize formation clays and fines during the waterflooding operations.

In the next paper, the authors present the least-squares Monte Carlo method as a decision-evaluation method, providing us with some insight into the effect of uncertainty on decision making and to help us capture the upside potential or mitigate the downside effects for chemical enhanced-oil-recovery (EOR) processes. The study enhances the performance of the method by improving the sampling used to generate the technical uncertainties in production and to extend its application to different chemical-EOR processes.

The final paper in this section presents a mechanistic model for wettability alteration during chemical flooding in carbonate reservoirs. Wettability alteration of rock from oil-wet or mixed-wet to more-water-wet conditions has previously been proposed as one of the oil-recovery mechanisms occurring during chemical flooding. Modeling of this alteration, however, is challenging because of the complex fluid/solid interactions. In this paper, the authors developed a multiphase and multicomponent reactive transport in porous media model that explicitly takes into account wettability alteration from geochemical interactions in carbonate reservoirs. One widely accepted mechanism is that sulfate ions replace the adsorbed carboxylic group from the rock surface, whereas calcium and magnesium ions decrease the oil-surface potential. In the proposed mechanistic model, a reaction network is used to capture the competitive surface reactions among the carboxylic groups, the cations, and sulfate. These reactions control the wetting fractions and contact angles, which subsequently determine the capillary pressure, relative permeability, and residual oil saturations. The results show that an increase in sulfate concentration could increase oil recovery significantly, whereas cations have a relatively minor effect on recovery.

Unconventional Resources

This issue presents a wide variety of papers in the unconventional-resources theme. The first paper presents a mathematical-integration process for a multistage hydraulic-fracturing operation through which the important information related to stimulated rock volume is extracted so that the total effect of the stimulation is presented properly by the microseismic data collected during the hydraulic-fracturing process. The approach is based on chaining the stage results one-by-one. At each stage, the stimulated volume is constructed as a 3D body on the basis of its observed microseismic events with an enhanced convex-hull approach. More-detailed geometric characteristics (i.e., length, width) are calculated further from the ellipsoid that best fits the constructed volume. This forms the basis for calculating the overall stimulated volume stage-by-stage. In the second part, the proposed algorithms offer characteristics related to the interaction between multiple stages, such as overlapping volume between the stages. These volume-overlapping quantities reveal the potential communication between the stages, indicating the efficiency of the multistage hydraulic fracturing.

The second paper in this section presents a new experimental approach to measure the so-called excess sorption amount of shale over a wide range of pressures and temperatures by use of a thermally-controlled manometric apparatus with a sample cell heated separately. A mass-balance is used to quantify the existing temperature gradients and to account for thermal expansion of the sample cell in the calculation of the excess sorption.

The next paper includes image analysis to characterize calcite-filled fractures and pore volumes in Barnett shale samples. The analysis uses multiscale laboratory imaging techniques on the basis of X-ray computed tomography, transmission X-ray microscopy, and focused ion beam scanning electron microscopy.

The fourth paper includes an analytical approach to determine pore-size distributions for organic shale from nuclear-magnetic-resonance spectra combined with adsorption measurements. The results show that approximately 20–40% of the pore volume of the used samples was in pores smaller than 10 nm.

The final paper in this section is on predicting the depth of investigation and pressure response in fractured unconventional reservoirs by use of an asymptotic solution of the diffusion equation in heterogeneous reservoirs. The leading-order terms give the necessary formulation for the study.

Reservoir Engineering

There are two papers included in this section. The first presents a finite wellbore-radius solution for gas wells by use of Green’s functions. The diffusivity equation describing reservoir flow of gases includes nonlinearity inherent to pressure-dependence of the gas properties, namely viscosity and compressibility. A new methodology that is based on solutions from Green’s function theory has been proposed recently to handle the nonlinearity, and it was applied successfully to solve several gas-well-test problems. In those problems, however, the wellbore is always represented by a line-source. This paper extends the theory by considering a finite wellbore radius as a boundary condition for a single vertical well producing at constant rate from an isotropic homogeneous and infinite gas reservoir.

The second paper proposes a formulation to solve the reservoir-control-optimization problem with the direct multiple-shooting method. This method divides the optimal-control problem prediction horizon into smaller intervals that can be evaluated in parallel. This allows the authors to include state constraints on a much broader scale than is common in reservoir optimization today. To demonstrate the capabilities of the method, the optimization algorithm is interfaced to an open-source reservoir simulator, and the performance of the proposed method is compared with the conventional method.

Flow in Pipes

The first paper in this section presents computational models for the investigation of guided particle transport for healing of damaged piping systems by use of electro-magneto-mechanical methods. The purpose of the study is to heal industrial-piping systems noninvasively, from the exterior of the system during operation. The particle accumulation at a target location is controlled by external electromagnetic/mechanical means. The detailed particle behavior is discussed in detail.

Finally, the last paper includes a new simple algorithm for rapid screening of hydrate-plug-formation risk by use of experimental models of gas-hydrate-plug formation.

I hope you enjoy this issue.

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