Finite Element Analysis Shows Screenout Development and Cement Bond Destruction in Horizontal Wells
Authors: Surjaatmadja Jim B. 
Published: December 2007

Premature screenouts during proppant placement have often interfered with fracturing treatments of cased horizontal wells. Fractures and tortuosities are believed to cause reduced fracture widths, which in turn are believed to be the primary causes of screenouts. No one can be certain which (or if either) of these two phenomena is the primary cause of screenout. This paper investigates a theory that could a furnish third factor causing screenouts, even after treatment engineers take extreme precautions to avoid tortuosities and multiple fractures. This investigation focuses on the complex interaction of the fracturing fluid flow and the mechanics of the fracture using finite element analysis.

The modeling primarily addresses the radial fracture occurring perpendicular to the borehole of a cased horizontal well. The Darcy-Weisbach relationship is used to analyze the fracturing fluid that flows radially in the fracture while it drains into the producing formation. From this analysis, pressure distribution across the fracture can be computed and used as inputs to the finite element model. By using this technique, fracture-width development may be evaluated, so that screenout predictions may be made before the job. Possible solutions to avoid such screenouts are also evaluated using the Finite Element Analysis (FEA) technique. Finally, different tools and solutions, such as expansion joints and hydrajet perforating are also analyzed using this approach and are presented in this paper.

Introduction

For many years, service companies have successfully fractured vertical wells. Although many of these wells may not show the intended results, the fracturing process itself can be claimed as mostly successful. Here, we loosely define "fracturing" as the process of creating the fracture, followed by opening the fracture and propping it with natural or man-made proppants. Documentation of successful fracture placements, written as early as 1865, has been found. Fracturing at the time was accomplished using explosives; the hydraulic fracturing era did not begin until 1948. Since 1948, major improvements have been made in the fracturing process.

With the advent of horizontal well completions, many new problems appeared. Decisions had to be made: open hole vs. cased hole, which drilling process to use, which direction to drill, how to initiate fractures. Terms such as "tortuosity" became commonly used. Many of the technologies currently used demonstrate consistently good results, while many others show only sporadic success. To resolve problems presented by horizontal-well fracturing requirements, planners must consider how the well is placed relative to parameters defined by nature; for example, its position relative to the surrounding stress fields, relative to gravity, or the hole interval length that is exposed to the targeted producing formation. Understanding these factors and considering them in planning and execution, are the keys to successful placement, treatment, and even abandonment of horizontal (or deviated) wells.

Among the predominant solutions, it is generally recommended that horizontal wells, when possible, are drilled in the approximate direction of least minimum stress. Although openhole completions are the most-selected choice of completions because of early-stage economical considerations, there are also compelling reasons in favor of cased completions. Proponents of cased-hole completions are more concerned about their ability to control the well through its lifespan and for proper future abandonment. In this paper, discussions are limited to fracture development in horizontal wellbores. In particular, cased completions drilled in the direction of least minimum stress are addressed in this paper.

P. 465