This issue brings you 22 papers divided into four sections.
Fracturing and Unconventional Resources. The first section consists of seven papers discussing various aspects of hydraulic fracturing and recovery of unconventional resources. Liang et al. study how surfactants may reduce the adverse effect that loss of fracturing fluid has on hydrocarbon permeability of shale and other tight reservoirs. The authors present an experimental investigation consisting of a coreflood sequence-simulating fluid invasion, flowback, and hydrocarbon production, and use this to identify the mechanisms responsible for permeability reduction in the matrix.
Hull et al. use a scanning electron microscope coupled with a focused ion beam and a special nanoindenter to study tensile failure of organic-rich shale samples on the micro- and nanoscale. The authors show that large tensile moduli of rupture were associated with high content of organic matter.
In the third paper, Tang et al. propose to improve neutron porosity logs that are used to characterize fracture density by injecting electrically charged contrast agents and apply an external electrical field to enhance their propagation into the secondary fracture network in the unpropped region.
Siripatrachai et al. develop a new compositional reservoir simulator for embedded-discrete fracture models incorporating the effect of capillary pressure on phase behavior. By studying Bakken and Eagle Ford fluids, the authors demonstrate that capillary forces are important for both flow and flash calculations. In particular, inclusion of capillary pressure gives larger original oil in place and causes bubblepoint pressure to be suppressed, allowing production to remain in the single-phase region for a longer period.
Li et al. develop a fully-coupled thermal/hydromechanical simulator and use this to study hydraulic fracturing in naturally fractured reservoirs. Through a series of numerical experiments, the authors demonstrate that the smaller the stress anisotropy and/or the larger the injection rate, the more complex the fracture network. High tensile strength can increase the occurrence of shear fractures, and large permeability of fracture filling combined with small cohesive strength and friction coefficient, could cause shear slip to be the dominant generating mechanism.
The interaction of hydraulic and natural fractures is also studied in the sixth paper by Ouchi et al., who develop a fully coupled hydraulic-fracturing model from a nonlocal continuum theory of peridynamics as an alternative to finite-element and finite-volume methods for solving poroelastic fracture propagation.
In the last paper, Sun et al. develop analytical solutions for pseudosteady flow in a three-layered system consisting of a hydraulically fractured mid-layer that is fed by crossflow from an upper and lower layer. The authors show that additional pressure drawdown reduces the productivity index compared with the case without crossflow. However, the drawdown is slower and hence gives a longer psesudosteady period and larger accumulative production.
Pressure and Temperature Analysis. Chen et al. propose a new model for pressure-transient analysis in multiple-fractured horizontal wells that divides the entire formation into three parts: reservoir, fracture, and wellbore. The model incorporates frictional and acceleration pressure drop. The authors show that the contribution of wellbore hydraulics to the total pressure drop first increases up to a peak value and then decreases, and that neglecting this contribution will lead to erroneous well-performance forecasts.
Analysis of bottomhole temperatures is increasingly being used in production operations, in part because of improvements in accuracy and resolution in downhole sensors, and in part because temperature contains information to assess reservoir characterization and completion efficiency. Therefore, this section also contains three papers on this topic.
The first paper by App gives a nice introduction to downhole temperature measurements and demonstrates how multiple-rate temperature traces from production-logging tools can be used to estimate the production-inflow profile and layer permeability and skin by use of a transient couple reservoir/wellbore model.
The second paper by Onur and Cinar presents new methods for interpreting and analyzing sandface-temperature data from constant-rate drawdown and buildup tests in infinitely-acting reservoirs containing slightly compressible fluids of constant compressibility and viscosity. The solutions are shown to exhibit three semilog periods, and by analyzing these periods, one can estimate permeability and radius of the near-well skin zone, as well as nonskin-zone permeability.
In the last paper, Onur et al. extend this analysis to nonisothermal conditions and demonstrate that drawdown- and buildup sandface-temperature data sampled near the production horizon exhibit two semilog straight lines—one at early times reflecting the adiabatic fluid expansion in the near-well skin zone, and one at later times, reflecting Joule-Thompson effects and properties from the nonskin zone. The authors also show that buildup-wellbore temperature is dominated by wellbore-heat losses compared with drawdown-wellbore temperature data, and that the skin-zone properties are difficult to estimate if the gauge is positioned far from the producing zone.
Wells and Pipelines. The third section consists of seven papers discussing wellbore stability, multiphase flow in wellbore and pipelines, and transport and deposition of solid particles.
In the first paper, Mehrabian and Abousleiman study tensile failure as a consequence of drilling with excessive mud weight. The authors extend conventional wellbore-stability analysis to the case of fractured wellbore with lost-circulation materials (LCMs), and take an analytical approach to solve the three-way mechanical interaction between the wellbore, fracture, and the LCM aggregates.
Song et al. present a full-scale flow-loop experiment designed to study how drill cuttings can be more efficiently removed from the wellbore during microhole-horizontal-well drilling. In particular, the authors present a model for estimating volumetric concentration and bed height of cuttings and use this model to compare and discuss major factors influencing the cuttings transport.
In the third paper, Li et al. investigate how gas-hydrate cuttings transported upward during drilling can affect the temperature of the drilling fluids along the wellbore. Release of methane and other hydrocarbons as free gas can potentially lower the bottomhole pressure and disturb the balance to the formation pressure, thereby increasing the risk of blowout.
The next paper by Sun et al. develops a new simulator to study the transportation of cuttings in an inclined-slimhole annulus. The simulator uses standard computational fluid dynamics to describe the liquid phase. Collisions among particles, between particles and wall, and between particles and drillstring are modeled as a spring-damping system using a discrete-element method.
Abouie et al. present a new compositional wellbore simulator developed for mechanistic modeling of scale deposition in surface and subsurface production equipment during oil production. The simulator is integrated with a module (Iphreeqc) for modeling comprehensive geochemical reactions and incorporates several mechanisms for the transportation, entrainment, and deposition of solid particles in the wellbore.
In the next paper, Raimondi discusses the development of a compositional approach for dynamic simulation of multiphase flow in pipelines. The approach accounts for the chemical composition of the vapor and liquid phase and instead of relying on property tables, computes the required physical properties in each timestep.
The last paper by Johansson et al. studies flow-assurance uncertainty and presents a new and simplified method for estimating uncertainty coming from approximations and simplifications made in the flow model used to provide multiphase-flow predictions.
History Matching and Optimization. The last section consists of three papers on history matching and one paper on production optimization. The first paper by Watanabe et al. presents an efficient history-matching approach that simultaneously integrates 4D seismic data, which are areally dense but sparse in time, and well-production data, which are dense in time but spatially averaged. The approach relies on a new and computationally efficient method for computing streamline-based, semianalytical sensitivities that account for both saturation and pressure effects.
In the next paper, Alpak et al. present a method for history-matching object-based geomodels in which the uncertain locations and attributes of the preraster objects are gradually and directly modified, without resorting to reparameterization techniques. The history-matching work flow contains both dynamic and static conditioning operations driven by separate objective functions. The static conditioning step can, in particular, both add and remove objects in the geomodel, which changes the number of active matching parameters over the massively parallel and iterative data-integration algorithm.
The third paper by Hutahaean et al. considers the problem of multiobjective history matching and presents a novel technique to group the objective components. The authors propose to compute potential conflicts between objective components a priori and use the results to identify which grouping scheme will result in a better performance.
In the last paper, Siraj et al. discuss how to develop optimization strategies that balance short-term and long-term objectives in the economic optimization of oil production. Their starting point is a robust and hierarchical method for optimizing net present value (NPV) while honoring geological uncertainty. Economic uncertainty is introduced through an ensemble of varying oil prices with average NPV as robust objective. The authors also discuss methods to quantify and reduce the effect of uncertainty.
Acknowledgments. I would like to welcome
Istvan Lakatos, University of Miskolc, to the editorial board. I also wish to thank all those who have contributed to write and review the 22 papers in this issue.
SPE Journal Executive Editor
SINTEF Digital / NTNU