By DR. SUSHANT DUTTA AND DR. FEI LE, BAKER HUGHES
Baker Hughes scientists turn to COMSOL Multiphysics to develop a reservoir navigation service more efficient and precise than ever before, saving customers millions of dollars.
Baker Hughes engineers lower the AutoTrak™ LWD tool, fitted with an AziTrak™ module, into a well.
Headquartered in Houston, Texas, with operations in more than 90 countries, Baker Hughes is known around the world as a leading provider of products, services, and solutions for the petroleum and continuous-process industries. Formed in 1987 by the merging of Baker International Corp. (originally Baker Oil Tools, founded in 1907) and Hughes Tool Co. (founded in 1909), the company designs high-performance technologies aimed at creating value from oil and gas reservoirs for customers such as Shell, Exxon Mobil, Chevron, Marathon, Texaco, and Conoco.
Baker Hughes’ global operations are backed by three product-line groups, which develop, manufacture, and support their advanced technologies. One of the groups — Drilling and Evaluation — offers real-time reservoir navigation and formation evaluation services during drilling in the form of logging-while-drilling (LWD) tools, and a complete range of wireline logging tools for detailed postdrilling formation evaluation in every environment. These services are designed to help customers drill more efficiently, evaluate geologic formations, place wells in productive zones within the reservoir, and perform petrophysical and geophysical data acquisition.
The Importance of “Sight”
Reservoir navigation and formation evaluation are important for the simple reason that drill operators need to “see” exactly where to place the well. While reservoir navigation is the matching of geological and resistivity models to drill along and through bed boundaries to precisely place wells, formation evaluation is the process of interpreting a combination of measurements taken inside a wellbore to detect and quantify oil and gas reserves in the rock adjacent to the well. The data from the evaluation is organized and interpreted by depth and represented on a graph called a log. “Baker Hughes’ reservoir navigation tools routinely enable wells to be placed accurately by measuring and visualizing bed boundaries and oil-water contact zones and providing accurate geosteering information,” said Sushant M. Dutta, Ph.D., a scientist in the Strategic Technology and Advanced Research group for the Drilling and Evaluation product line at Baker Hughes.
“ Resistivity logging is the oldest systematic technique for formation evaluation, and is still the foremost technique in formation evaluation used for reservoir navigation.”
Figure 1. A simplified triaxial induction-logging tool located eccentrically in a deviated well drilled through an invaded, layered earth formation.
One such tool is Baker Hughes’ Azi-Trak™ Deep Azimuthal Resistivity induction- logging tool. Resistivity logging is a method of well logging that works by characterizing the rock or sediment near a borehole by measuring its electrical resistivity. “Resistivity logging is the oldest systematic technique for formation evaluation, and is still the foremost technique in formation evaluation used for reservoir navigation. It is based on the fact that oil and gas have a substantially higher electrical resistivity compared to (salt) water. Underground formations usually contain salt water in their pores. Hence, the bulk formation resistivity is higher if the pores contain oil or gas in addition to salt water,” said Dr. Dutta.
Figure 2. Imaginary magnetic fields, all three direct components, for frequency 20 kHz and when the tool is centered in the borehole. The lower frequency corresponds to a larger region of investigation. The gray shaded regions indicate the oil-bearing layers.
Until recently, oil and gas operators had been limited in the types of real-time resistivity logging measurements they could use for reservoir navigation. Although deep-reading measurements provide information about approaching boundaries and fluid contacts, the azimuth of these boundaries and contacts is unknown. The AziTrak solves these issues by providing a 360° view of the downhole environment. It is capable of detecting, measuring, and visualizing bed boundaries and oil/water contact zones hours before they can be “seen” with conventional sensors.
“ COMSOL simulations for sensor design reduce prototyping costs. Solving forward problems helps us characterize new tools and build confidence in fast forward models for inversion.”
Simulating the Tools
Dr. Dutta and his colleague, Dr. Fei Le, are using COMSOL Multiphysics to model induction logging tools, like the AziTrak, in a variety of situations. “Over the course of their continuous development, inductionlogging tools have become increasingly difficult to characterize by the use of simple models. Similarly, with the advent of directional drilling, it has become imperative to model fully 3D formations,” stated Dr. Dutta. And although Baker Hughes does possess in-house codes for true 3D models, there are limitations. “The lack of user interface and visualization capabilities motivated us to go for commercial packages. COMSOL Multiphysics is one of a suite of commercial FEM-based packages that we have been using for some time now.”
Figure 3. This image created with COMSOL Multiphysics shows a simulation result snapshot when the inductive logging tool’s Z-transmitter is active at a depth of 8 feet. The subdomain colormap shows the magnitude of induced current density in the formation, while the arrows show the direction of flow of the induced currents. The colormap is log10 scaled for clarity.
To illustrate his use of COMSOL, Dr. Dutta described a simulation for a full 3D formation model with a simplified induction tool in a borehole: The formation (Figure 1) consists of five horizontal layers, with anisotropic oil-bearing layers invaded by the borehole fluid. The borehole makes a high angle with the vertical, which represents a realistic directional drilling scenario. The induction tool may or may not be centered inside the borehole. Furthermore, the induction tool transmitters and receivers are triaxial, which makes them capable of transmitting and measuring magnetic fields in each of three orthogonal directions, although they are modeled as simple wire loops. The induction tool operates at multiple frequencies. The results show (Figure 2) the direct magnetic fields in all three directions (imaginary parts) logged by the tool as a function of true vertical depth. The imaginary magnetic fields represent the voltage signals generated in the receivers that are in-phase with the transmitter currents. A 3D simulation (Figure 3) shows the induced currents in the formation when the Z-transmitter is active.
Fei Le (left) and Sushant Dutta studying their simulation results.
Dr. Dutta continues to use COMSOL for sensor design, solving forward problems, validating experiments, and testing new design ideas. “COMSOL simulations for sensor design reduce prototyping costs. Solving forward problems helps us characterize new tools and build confidence in fast forward models for inversion,” he said. And since directional drilling, reservoir navigation, and formation evaluation are areas of the oil and gas industry that use some of the most advanced technology in the world, “Baker Hughes’ expertise in these areas often saves customers millions of dollars in improved productivity and time saved. The ability to accurately characterize induction-logging scenarios and to evaluate ideas for new and improved induction logging tools helps us keep that edge.”