Executive Summary                                                                                                                             I. Yucel Akkutlu, Texas A&M University

This December issue of SPE Journal includes 18 articles reflecting the current interest in various areas related to the upstream petroleum industry. The themes covered in this issue, respectively, are Improved Oil Recovery/Enhanced Oil Recovery; Geomechanics; Fluids, Phase Behavior, and Compositional Simulation; Resource Shale; and Drilling. The following is a brief overview of the included articles.

Improved Oil Recovery/Enhanced Oil Recovery                                                                                                                        The first paper in this theme discusses the development of a comprehensive nonisothermal wellbore-flow-simulation tool, integrating horizontal and vertical flow with application to wax precipitation. The approach accommodates the complexities of advanced completions together with near-wellbore behavior. A significant difference in temperature profile is noted between a previously developed and widely used analytical model.

The second paper introduces a model that predicts water-injectivity decline as a result of the reinjection of produced water and presents its experimental validation by use of CT-scanned experimental corefloods and scanning-electron-microscopy photographs. The model takes into account deep-bed filtration and buildup of external filter cake. A distinct feature of the model is that it describes particle-retention kinetics responsible for internal filtration.

The third paper in this theme discusses geochemical interpretation of low-salinity water injection into carbonates to understand the oil-recovery mechanisms of this process. The paper compares results from two simulators and, unfortunately, it is found that the low-salinity waterflooding is case-dependent, and hence, the findings cannot be easily generalized.

The next paper is a laboratory and numerical-simulation study of co-optimizing carbon dioxide storage and oil recovery. A co-optimization function for the storage and incremental oil is defined and calculated using the measured data for each coreflooding experiment. A compositional reservoir simulator is then used to examine gravity effects on displacements and to derive relative permeability values. The approach suggests that near-miscible displacement yields the highest storage efficiency and displays the best performance for coupling storage to enhance oil recovery.

The next paper focuses on predicting the impact of pore-scale heterogeneities on the performance of acid-stimulation treatments in limestone using nondestructive tracer tests. The authors define the pore structure contributing to flow as a fractional parameter, with the objective of determining the acid pore volume to breakthrough for each pore class. Application to the design of matrix acid treatments in carbonate rocks is discussed.

We move on to a paper that discusses biotreatment of produced waters, including organic hydrate inhibitor. Tests were conducted in batch and continuous reactors under aerobic mixed-culture conditions without pH control during a 31-week period. The results indicate that one could remove more than 80% of the chemical oxygen demand and total organic carbon through biological treatment of produced water using small concentrations of thermodynamic hydrate inhibitor monoethylene glycol as additive.

In the last paper in this theme, a mathematical model is proposed for wettability alteration predictions with application in fractured reservoirs. Flow in the reservoir is modeled by looking at a single fracture surrounded by matrix on both sides. The predicted model behavior and comparisons with a commercial simulator indicate that an improved understanding of the wettability alteration controlled by water/rock chemistry and fracture/matrix flow is important for gaining insight into recovery from naturally fractured reservoirs.

Geomechanics                                                                                                                                                                             The first paper in this category introduces a semianalytical thermo-poro-elastic model for investigation of wellbore strengthening to help reduce mud loss during drilling in depleted reservoirs. By use of field data, the model is considered as a geomechanical tool to analyze stress around the wellbore for strengthening applications.

The next paper focuses on the geomechanical aspects of pulse fracturing in shale reservoirs and the creation of extensive fracture networks by use of an advanced constitutive model implemented within ANSYS software. Pulse fracturing is an alternative fracturing technique that could overcome some of the fracturing limitations by customizing the pulse-fracturing rates and peak loads to lie in between hydraulic and explosive fracturing. This study shows that, for a certain combination of reservoir, geomechanical, and pulse-loading parameters, induced fractures can propagate in multiple directions.

In the next paper, geomechanics of lost circulation and wellbore strengthening are discussed through an analytical solution to a solid-mechanics model of the wellbore and its adjacent fractures. The underlying mechanisms through which a wellbore is strengthened are not yet fully understood. The findings indicate that the induced hoop stress is significant at some regions near the wellbore, especially in the vicinity of the fracture. The study also examines the phenomenon of tip isolation.

The final paper in this category considers time-dependent initiation (or delayed failure) of multiple hydraulic fractures in a formation under varying stresses. Rocks are known to exhibit static fatigue; that is, delayed failure at stresses below the tensile strength. In this paper, the authors explore the consequences of delayed failure on initiation of multiple hydraulic fractures during the fracturing operation.

Fluids, Phase Behavior, and Compositional Simulation                                                                                                         The authors of the first paper in this theme introduce a new method to estimate reservoir-fluid composition by use of downhole optical spectrometry for reservoir-fluid characterization. The method includes fluid-type (gas, condensate, or oil) estimation and computation of fluid composition. As a measure of uncertainty, confidence intervals are computed for the predicted components of the composition and gas/oil ratio.

The next paper presents a multiple mixing-cell method for three-hydrocarbon-phase displacement observed during low-temperature oil displacements by carbon dioxide. Previous computational minimum miscibility pressure estimation methods do not attempt to estimate the pressure for mixtures exhibiting three hydrocarbon phases because of a lack of robust three-phase equilibrium algorithms. In this paper, the authors present a multiple three-phase mixing-cell method that is based on a robust three-phase flash by mixing injected gas with oil in place through multiple contacts at given reservoir temperature. They show that the minimum miscibility pressure does not exist for three-phase displacements.

The next paper presents a multiphase isenthalpic flash calculation for multiphase water/hydrocarbon mixtures during steam injection. Here, the challenge is that total enthalpy can be substantially nonlinear, or even discontinuous, with respect to temperature. This type of phase behavior is known as narrow-boiling behavior in the literature. The algorithm developed is based on direct substitution. A detailed analysis is given for narrow-boiling behavior and its effects on the direct-substitution algorithm. A new method is also presented for K-value estimates for three phases for water/hydrocarbon mixtures.

The final paper in this category presents a fully-implicit coupled multiphase thermal-compositional flow and geomechanics simulation using finite-volume and Galerkin finite-element methods, respectively.

Resource Shale                                                                                                                                                                           Two papers are included under this theme. The first focuses on modeling organic-rich shale permeability. We perform accurate measurement of shale permeability in the laboratory, but often we do not know what this measured permeability really means in the reservoir. In this work, the authors consider that the shale matrix consists of multiple continua with organic and inorganic pores. Stress-dependency of the permeability comes along with slit-shaped inorganic pores, whereas the sorption effects are associated with nanoscale organic capillaries. A simple conceptual flow model with an apparent shale permeability is developed that couples the molecular transport effects of the sorbed phase with the stress-dependence of the inorganic matrix. Sensitivity analysis on the new permeability model shows that the stress-dependence of the overall transport is significant at high pore pressure, when the effective stress is relatively low. Diffusive molecular transport of the sorbed phase becomes important as the stress gets larger and, hence, the inorganic pores close.

The second paper introduces a new analytical method for estimation of gas content in shale core samples by use of gas-evolution data of the canister test. The current analysis method, which was originally developed for coalbed methane, accounts for the sorbed gas only, and underestimates gas content of the shale core samples. In this study, the authors present a new model accounting for expansion of free gas in addition to desorption.

Drilling                                                                                                                                                                                           The final paper in this issue analyzes the effects of the two kinds of boundary conditions on tubular-string buckling. To clarify the quantitative relation between boundary conditions and tubular-string buckling, a general-packer model is used. With the general-packer model, the general potential energy of the tubular string is deduced. According to the minimum-potential-energy principle, the existence and stability of full helical-buckling solutions are given. The bending moment, shear force, and contact force on the tubular string caused by buckling are also analyzed. The results show that boundary conditions, especially the second kind of boundary conditions, are an important factor that makes the tubular-string buckling problem complex.

I hope you enjoy this issue.

I. Yucel Akkutlu