Whether through advances in parallel computing and greater simulation performance or enhanced history matching capabilities and a more integrated workflow from geosciences through to production, reservoir simulation and the technologies and tools around it have advanced dramatically over the last few years.

Reservoir simulation stands as a crucial element of the reservoir management workflow—mitigating risk, improving decisions and ensuring more productive and profitable fields. Yet for all its advances, one area that reservoir engineers have struggled to incorporate into their workflows is that of fractures.

Importance of fractures

Fractures have a direct influence over fluid flow behavior, greatly enhancing the permeability of rocks and changing the distribution of flow in the reservoir.

Decades before fracturing became prevalent in unconventional reservoirs, inducing fractures around wells to enhance fluid flow was a common technique. This was achieved by injecting a mixture of fracturing liquid and proppant into the well at high pressures to create a set of (usually) vertical fractures propagating from the wellbore into the surrounding rock.

Because of the proppant, these fractures remained open even after the injection ceased, effectively expanding the influence region of the well and enabling increased production without the need to drill new wells.

Yet, despite the importance, influence and popularity of fractures, existing reservoir simulation workflows have struggled to incorporate well fractures either efficiently or accurately into the calculation of reservoir fluid flows.

Reservoir simulation advances

When trying to incorporate fractures into the workflow, reservoir simulation software has tended to be either overly simplistic—limiting the sureness of its predictions—or has tried to gain accuracy by using a fine-grained discretization of the reservoir around the fractures. However, fractures are regions of small volumes with high flows. This makes the modeling of fractures through conventional numerical techniques a challenge, impacting reservoir simulation performance.

It is with these issues in mind that Emerson has developed a new method to quickly and accurately model well fractures within its Roxar Tempest MORE reservoir engineering software.

The method forms part of a workflow, from well fracture planning to results visualization, and is designed to let reservoir engineers gain deeper insights into the effect of well fractures on their reservoirs.

How it works

The new fracture modeling feature handles the physics of the fracture-to-well connection and the multiphase flow within the fracture. It enhances previous tools that model fractures as a set of two-point flows as depicted in the left-hand sketch in Figure 1.

Although simple and fast, the approach ignores cell pressure variations along fractures, tends to overestimate inflow and does not allow direct crossflow between cells through the fracture. To address these issues, an enhanced approach has been developed with fractures modeled separately as 2-D grids intersecting the 3-D reservoir grid (Figure 2).

The simulator solves the pressure field inside each fracture and subsequently calculates the component flows from the reservoir into and along the fracture to the wellbore. This requires the solving of only a small linear system of equations, thereby avoiding separate fracture solver iterations inside the well solver and thus adding to the overall robustness and performance of the model.

How it looks

Setting up the fractures is straightforward using simple data entry forms that allow the user to define the fracture properties, including height, length, conductivity and geologically consistent direction.

High-quality visualization and plotting capabilities then offer a quick and clear understanding of the fracturing effect as shown in Figure 2. The figure shows a 3-D visualization of a reservoir with two types of well fractures: along vertical trajectories and across horizontal trajectories. The fracture grid coloring illustrates the pressure inside the fractures.

Fluid flows also are reported for each fracture rather than for the individual fractured grid cells (Figure 3). This makes it simple to see the effect of individual fractures and keeps data volumes manageable.

In summary

Hydraulically fracturing wells is a common engineering technique to enhance well productivity but has been awkward to simulate in the past and has led to slow or inaccurate predictions. This new technique scales to large reservoir simulations with hundreds of wells and provides easy setup and intuitive analysis tools. In turn, this allows more effective risk mitigation and decision support, especially when embedded in Big Loop uncertainty analysis workflows.

Following the recent Emerson acquisition of Paradigm, the combination of Emerson and Paradigm technologies will accelerate the launch of exciting geoscience and reservoir engineering solutions over the coming months.

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