Everest™

The primary goal of the Everest tool is to find optimal well planning and production strategies by utilizing an ensemble of reservoir models (e.g., an ensemble of geologically-consistent models). This will enable robust decisions about drilling schedule and well placement, in order to achieve results of significant practical value.

Requirements

Python version: Everest supports Python version 3.8, 3.9, 3.10, 3.11, 3.12

In order to build, test and run Everest, a number of Python libraries must be available.

First make sure to install the required and test dependencies

    pip install . "[test]"

In addition, the following libraries must be present in your environment For details about the build process, see the web pages of each individual library

• everest-models
• OPM (Optional)

Everest-models

Get source from everest-models

    git clone https://github.com/equinor/everest-models
    cd everest-models
    pip install . --upgrade

OPM (Optional)

Read documentation at https://github.com/OPM

Testing

Some tests require access to the /project/res folders in the Equinor internal network. If you don't have access to that folder, you have to export NO_PROJECT_RES=1 so that those tests will be skipped.

Tests can be executed with

python -m pytest -m <test_category>

where <test_category> must be replaced by one of the following categories:

• ui_test contains all the tests related to the user interface
• integration_test contains self-contained integration tests (the above installation should be sufficient for them to pass).
• simulation_test requires a more complete setup and among others requires the job SYMLINK to be known to everest as well as access to /project/res in Equinor (the later is to be changed soon).
• redundant_test test that covers portions of code already covered by other tests. They are regularly run on our CI system

The optimization process

The following is a brief explanation of how Everest works. It is inaccurate and incomplete, but it is hopefully a good starting point.

Everest is designed for finding local maxima of functions $y=f(x)$ where the variables (a.k.a. controls) $x$ are properties of wells (such as the date a well is drilled, the position of points along its path. etc.), while the objectives $y$ are typically quantities measured over a long period of time (such as the the Net Present Value, CO2 emissions, etc.).

An optimization problem must be specified using an Everest config file. The optimization part is handled by the ropt package, the computation of the objectives is handled by ert.

Optimization

The actual robust optimization workflow is implemented by the ropt package, which builds on standard optimization algorithms to implement robust optimization strategies. By default ropt provides only algorithms from the SciPy package as its optimizer backend, but Everest by default installs a plugin to support gradient-based optimization algorithms from the Dakota package, e.g. Newton and quasi-Newton methods.

Based on an initial assignment of the control variables $x_0$, ropt starts a backend optimization algorithm, which asks ropt to provide the values of the function $f(x_0)$ and its gradient $f'(x_0)$. Once the values are provided, the optimization algorithm determines a new set of control variables $x_1$ with a corresponding function value $f(x_1)$. If $f(x_1) > f(x_0)$, $x_1$ becomes the current maximum, and the entire process is repeated until certain exit conditions are encountered.

The role of ropt is to implemented robust optimization functionality on top of the backend algorithm, among others this includes:

• Maximization: Most standard optimization algorithms solves minimization problems. ropt take care of negating all the values so that a maximization problem is solved instead.
• Robust function evaluation: the function to evaluate usually derives from an ensemble of different geological models (realizations), in order to take uncertainty into account. ropt asks ert to evaluate each realization and calculates a single function from the set of calculated function values.
• Gradient estimation: in most of the cases we are targeting, it is impossible to provide the exact value of the function gradient. When a gradient is needed, ropt generates several perturbations of the controls $x_i = x + \delta_i$ and asks ert to compute all the functions $y_i=f(x_i)$. Once the results are available, ropt determines an estimate of the gradient by performing a linear regression of the values $(x_i, y_i)$.
• Multiple objectives: ropt supports optimizing several objectives at once.

Function computation

ert takes care of computing function values for the ensemble of geological models. ert knows nothing about the problem to be solved, its main responsibility is to execute a forward model, on all the geological realizations. The forward model is given by a sequence of jobs that the user must provide in the Everest configuration file.

NOTE: since the most expensive job in a forward model is usually a reservoir simulation, the execution of a forward model is often called a simulation, especially when talking about batches (see below).

Let's look at an example: assume the config file contains the specifications for a group of control variables called my_wells and two objectives called npv and co2_emissions. ert creates a dedicated folder for executing the forward model, and copy/links all the necessary files (the geological realization) into it. The folder is located deep down in the simulation folder specified in the config file. The current assignment of the control variables is written to the file my_wells.json located in that folder. A forward model typically begins with some setup operations based on my_wells.json, then runs a reservoir simulation (Eclipse or flow), followed by some post-processing and the eventual creation of the files npv_0 and co2_emissions_0. Each of those files must contain a single line of text, which is the desired function value.

Requests from ropt to ert are grouped into batches. A batch can include simulations related to evaluating the function, the gradient, or both. The number of simulations in a batch is often a good indication of what is going on. Let real_num be the number of geological realizations and pert_num the number of perturbations ropt generates for computing a gradient. Then:

• A batch executed for evaluating the function $f(x)$ has exactly real_num simulations
• A batch executed for evaluating the gradient $f'(x)$ has exactly real_num * pert_num simulations
• A batch that evaluates both the function, and the gradient has real_num + real_num * pert_num simulations

Notes:

• By setting the speculative option, all the batches have both function and gradient evaluations
• Simulations in a batch have an incremental id. If a batch has both function and gradient evaluations, the first real_num simulations are the ones used for the function evaluations