Simulation approaches reality for modeling well inflow

One of the great challenges facing the oil and gas industry is accurate prediction of well inflow. Conventional methods have been too cumbersome, imprecise, and suffered from a lack of accuracy and clarity.

Yet accuracy is vital to appraising development prospects, well planning, and reliable prediction of true well and field value. Knowing the outcome of both our actions and well design allows informed choices to be made on formation damage impact and mitigation as well as fit-for-purpose well drainage architecture strategy.

The quantity and quality of laboratory formation damage testing has increased in recent years with the North Sea a particular focus for testing. However, while our understanding of formation damage mechanisms has improved, translating from pore scale damage mechanisms to reservoir scale wells is very difficult.

This is because much of the detailed information available from core testing, logs, well testing, and PLTs is not adequately captured in conventional well inflow models, while the value is greatly diminished by trying to fit the detail into general analytical solutions.

It is not surprising, therefore, that we have to develop boost, fudge, and skin factors in order to match predicted analytical well performance to real well performance.

By losing the detail of the reservoir, well, completion, and formation damage, the potential is lost for well performance prediction and predicted productivity indices to be accurate. There is a clear requirement for a flexible, detailed, numerical model to predict well inflow.

Hydraulic fracturing is a technique commonly used to mitigate against formation damage and improve well productivity. However, the prediction of the production performance of hydraulically-fractured wells – an essential step during hydraulic fracture designs – uses analytical solutions that cannot reproduce the complexities associated with flow from the near-wellbore area into the well.

Despite some recent improvements, the basis of the hydraulic fracture design is contradictory to the real objective. The objective should be to develop a model to evaluate the fracture performance based on its location and size, then try to create that optimum fracture using the reservoir and rock mechanical properties. The objective of the hydraulic fracture is to improve productivity (or injectivity) and this must be prioritized. What is required is a model that can capture the geometry of the fracture, or fractures, and position them within the reservoir model together with the conductivity of the fractures and the reservoir around the fractures.

A 3D numerical model, based on Computational Fluid Dynamics (CFD), has been used to estimate the inflow performance of horizontal, deviated, and vertical wells under various formation damage and hydraulic fracturing scenarios.

Computational fluid dynamics is a technology that enables study of the dynamics of things that flow. Using CFD, it is possible to build a computational model that represents a system or device. This could be a Formula 1 car where the impact of fluid airflow over the surfaces of the car are converted into downforce or for well flow prediction, the flow of fluid in porous media and pipes and its restrictions are calculated and compared for different drilling and completion options. The fluid flow physics and chemistry are applied to this virtual prototype, and the software will output a prediction of the fluid dynamics and related physical phenomena.

Very complex, non-linear mathematical expressions that define the fundamental equations of fluid flow, heat, and materials transport are incorporated. These equations are solved iteratively using complex computer algorithms embedded within CFD software. In typical near-wellbore inflow prediction models, between one and 10 million discrete volumes are used to capture the well and completion geometry and the near wellbore part of the reservoir. The flow of fluid from the reservoir through each of these volumes, in to the wellbore is calculated simultaneously and often 200 iterations of the calculation are required to reach a converging solution.

In recent SPE publication SPE122361, different completion geometries and solutions for a multi-layered reservoir are evaluated. Using CFD modeling of potential productivity, options on perforation and hydraulic fracture placement are evaluated. The individual perforations and hydraulic fracture elliptical shape are included in the model producing a detail and reality far in advance of any previous modeling capability. Some observations include the productivity improvement into perforations (shot after fracture placement) even in planes where the fracture does not have any direct connection with the wellbore. This process never has been captured previously.

Vertical and deviated well options with multiple fractures were evaluated and the vertical well option gave the optimum performance. Interference between different fractures, potential breakthrough of pressure profile into unwelcome water or gas legs, and the limitation of deviated well hydraulic fractures were captured. Some previous simple well inflow models of hydraulic fractures have relied on analytical solutions to “view” the fracture as a bigger wellbore or have modeled the fracture as a planar structure. The CFD model captures the true fracture geometry and in the case presented had thousands of cells on the fracture face (some previous models have just one cell to represent the fracture face).

This is the first time that computational fluid dynamics has been employed to predict well performance based on high quality laboratory testing and the first time it has been used as a practical tool to assess the well drainage architecture design of hydraulic fractures.

It is anticipated that the modeling of well inflow in great detail and in three dimensions using CFD will become routine in the next few years.

Michael Byrne

Principal Formation Damage Consultant, Senergy Ltd.

Editor’s note: The author of this article is an SPE Distinguished Lecturer 2009-2010.

This page reflects viewpoints on the political, economic, cultural, technological, and environmental issues that shape the future of the petroleum industry. Offshore Magazine invites you to share your thoughts. Email your Beyond the Horizon manuscript to Eldon Ball at[email protected].