Executive Summary

Cosan Ayan, Schlumberger

In this issue, we have 15 papers focusing on transient well and formation testing, enhanced oil recovery, formation evaluation, reservoir simulation, and instrumented wells for fluid-flood-movement monitoring. A brief synopsis of the papers follows, grouped in broad discipline topics.

Instrumented Wells, Multiphase Flow Properties and Optimization

This issue of SPE Res Eval & Eng starts with a novel technique to determine multiphase flow properties through downhole fluid movement monitoring. The paper "Determination of In-Situ Two-Phase Flow Properties Through Downhole Fluid Movement Monitoring" presents an integrated evaluation of time-lapse downhole pressure, resistivity array, and flow-rate data. The joint inversion of multiscale and multiphysics data enables determination of two-phase relative permeabilities within tens of feet from the instrumented well. A different type of multiphase flow problem is discussed for waterflood optimization in the St. Joseph field, which had ongoing crestal gas injection. The paper "Optimization of Smart Wells in the St. Joseph Field" studies horizontal redevelopment wells completed with multiple inflow control valves. By using optimal control theory, the number of valves, configuration of perforation zones, and optimal operational strategies were determined.

Transient Well and Wireline Formation Testing

Swabbing is a common practice in many parts of the world to clean up wells, understand the nature of the reservoir fluids, and to evaluate productivity. Often, the tests are performed without downhole shutin and the rates are not measured. In the paper "Swabbing Test Interpretation Using Nonlinear Regression in San Jorge Gulf Basin," the authors analyze swabbing tests with techniques used mainly for formation tester transients, also treating flow rates within the optimization loop along with reservoir parameters. In the paper "A New Pressure/Rate-Deconvolution Algorithm To Analyze Wireline-Formation-Tester and Well-Test Data," the authors describe a deconvolution method in which the analyst can define variable error levels for any range of measured pressures and rates. The technique was applied to drillstem and interval-pressure-transient tests (DST and IPTT), also including some drawdown pressure data in one of the examples. Continuing with pressure-transient testing, the paper "Significant Improvement in the Accuracy of Pressure Transient Analysis Using Total Least Squares" considers the errors in pressure as well as in time while analyzing pressure transients with the nonlinear total least squares (TLS) approach. The paper compares least squares (LS) and TLS for synthetic and field data in which TLS was found to perform better than LS in presence of both time and pressure errors.

Reservoir Simulation

Uncertainty estimation and probabilistic forecasting were studied in the paper, "Continuous Reservoir Simulation Model Updating and Forecasting Improves Uncertainty Quantification." The authors used Markov Chain Monte Carlo (MCMC) methods in real time in a continuous process of data acquisition, model calibration, forecasting, and uncertainty quantification. This approach allows generation of a reasonable probabilistic forecast with fewer models than the traditional application of the MCMC method. Shale gas is now the focus in several geographic regions over the world with increasing importance. Though microseismic monitoring/mapping has provided valuable insights for stimulated reservoir volume (SRV), to better design stimulation treatments and study the production performance, reservoir modeling of shale-gas reservoirs is suggested and discussed in the paper "Reservoir Modeling in Shale-Gas Reservoirs." The paper presents simulations that were conducted using a detailed numerical grid that represents the complex fracture network, primary fracture and tight shale matrix, also including gas desorption and stress-dependent fracture permeability. The paper suggests that gas desorption may not be a significant component of production for many deep to medium depth shale-gas reservoirs.

Formation Evaluation

In the paper "Magnetic Resonance in Chalk Horizontal Well Logged With LWD," the authors outline geological and petrophysical evaluation of a horizontal well drilled in the chalk layers of the Oseberg-B area using several logging-while-drilling (LWD) measurements, including low-gradient magnetic resonance (MR). Two mud systems were used in different sections of the horizontal well; in the absence of water-based mud-filtrate invasion, the MR data showed good agreement with saturations from core. Continuing with LWD formation evaluation, the paper "Determination of Intrinsic Dip and Azimuth From LWD Azimuthal Propagation Resistivity Measurements in Anisotropic Formations"presents a method to determine formation anisotropy and associated dip using a simple homogeneous anisotropy model. Boundary effects are reduced by using the inverse compensation scheme. The authors describe its applicable conditions to calculate resistivity, anisotropy, and associated structural dip and azimuth angles from both azimuthal and standard propagation resistivity measurements. Fracture-corridor characterization was the focus of the paper "Use of Exclusion Zones in Mapping and Modeling Fracture Corridors." An exclusion zone was defined as an area through which no fluid conductive fault/fracture corridor can pass, though fractures can still exist in such zones. The author describes how the concept of exclusion zones can be used to constrain fracture-corridor occurrence and to estimate orientation, length, and intra-connectivity. The author suggests that deterministic mapping of fracture corridors is a more efficient approach than characterizing and discrete-fracture-network (DFN) modeling of an entire fracture system in a field.

Geomechanics

Production-induced stress change is an important factor to study and predict several important well/reservoir related issues. Citing a few are reservoir compaction, wellbore stability, casing deformation, sand production and compaction-induced permeability change. In the paper, "On the Wellbore Stress Change Because of Drawdown and Depletion: An Analytical Model for a Vertical Well in a Thin Reservoir," the authors introduce a model describing the stress change at the wall of a vertical wellbore following drawdown (pressure drop near the wellbore) and reservoir depletion (pressure drop at reservoir scale). They predict a lower effective tangential stress and higher effective axial stress at a given drawdown than the widely used model since their method is developed for a finite reservoir thickness as opposed to the assumption of an infinitely thick reservoir.

Enhanced Oil Recovery (EOR)

To reduce total mobility and delay/reduce gravity segregation between injected gas and water, water-alternating-gas (WAG) and simultaneous water and gas (SWAG) schemes are used. In the paper"Injection of Water Above Gas for Improved Sweep in Gas EOR: Nonuniform Injection and Sweep in 3D," the authors study the "modified SWAG" process in 3D in which water is injected from an upper well and gas from the lower, using two parallel horizontal wells. They found that existing 2D solutions for sweep efficiency when water and gas are injected from parallel horizontal wells, with the water well higher in the formation, may be optimistic. We revisit chalk formations with a laboratory EOR study presented in the paper "Transport of CO2-Foaming Agents During CO2-Foam Processes in Fractured Chalk Rock." For static experiments, the authors use cores of different diameters packed in a steel container with the annular space between them filled with the CO2 foaming agent. Numerical modeling of the static experiments was conducted to determine the transport rate for the foaming agent. In the flow-through experiments, the model was created by drilling a concentric hole through the center of the plug, simulating an artificial fracture. This was filled with glass beads of different dimensions. Results from dynamic experiments show that the transport of CO2-foaming agent into the matrix is slower in fractured models than in homogeneous models. We continue with fractured reservoirs and CO2 EOR with the paper,"CO2 EOR Potential in Naturally-Fractured Haft Kel Field, Iran." In this study, the authors investigated the EOR potential for CO2 injection in the naturally-fractured Haft Kel field using compositional simulations of a homogeneous single matrix block surrounded by fractures. At lower pressures, CO2 is less dense than the reservoir oil and traditional gas/oil gravity segregation was observed. At higher pressures, CO2 density is greater than reservoir oil density, which causes an unusual gravity drainage mechanism in which CO2 enters from the bottom of the matrix block and pushes oil out from the sides and the top. In our last paper, "Potential for Polymer Flooding Reservoirs with Viscous Oils," the author investigated possible polymer flooding applications for the Alaskan North-Slope viscous oil fields in which thermal methods may not be applicable because of extremely cold weather and severe heat losses. The screening criteria revealed that higher oil and modest polymer prices, increased use of horizontal wells, and controlled injection above the formation parting pressure all help to extend the applicability of polymer flooding to reservoirs with viscous oils. High mobile oil saturation, degree of heterogeneity, and the ease of attaining crossflow for the North Slope reservoirs also increase the potential for polymer flooding.

Cosan Ayan
Schlumberger