Abstract

Directional drilling makes it possible to drill multilateral wells into different parts of a reservoir from a single wellbore. Many directional wells are drilled to reach reservoirs inaccessible from a point directly above because of surface obstacles or geologic obstruction. Wellbore sidetrack drilling operations with hard cement plugs have been used for years. Placing cement plug in the borehole and allowing the cement to develop high compressive strength perform sidetracking technique. The hardened cement plug when drilled deflects the bit away from the current borehole, starting another open hole section.

Conventional cement formulations for sidetrack kickoffs usually fail when the ROP (Rate of Penetration) for the cement plugs is much more than the ROP in the formation Sidetracking failures, in building up kickoff angles, results in operational delays and cost overrun. High sonic compressive strength cement system with slow ROP should be designed and developed specifically for side tracking operations.

A rate of penetration device was used to optimize cement formulations to determine the ROP through cement plugs. Different chemicals for building up sonic compressive strength were evaluated. Special types of cements were designed and evaluated for possible use for sidetrack kick-off plugs. The effect of inert particles on the sonic compressive strength and ROP were investigated. The cement slurry was cured for 24 hours in the Ultrasonic cell at bottom hole static temperatures of 260–290°F and a bottom hole pressure of 4,700 psi to represent gas well conditions and 200–250°F and 2,200 psi for oil wells. The outcome of this work was cement formulations with high potential (high sonic compressive strength and slow ROP) that can be used to drill horizontal and multilateral wells.

Introduction

Cement plugs are placed in oil and gas wells for various reasons, including:1 well abandonment, sidetracks, squeezing and zone isolation. Sidetrack cement plugs are the focus of this study. In sidetrack operations, an average of 2.4 attempts per sidetrack, with 24 hours with each attempt, has been reported and experienced in the field. Failures in sidetrack cement plugs can occur because of one or more of the following reasons:2

  1. Plug slippage

  2. Drilling out too soon without waiting for compressive strength development

  3. Inaccurate well data

  4. Insufficient slurry volume

  5. Slurry design

  6. Losses while reversing

  7. Poor mud removal (not using a proper spacer)

Previous studies were done to improve placement methods of cement formulations for sidetracks.3–5 Others focused on studying the effect of hole size, geometry and mud properties on cement plugs.6 It is an easy way out to blame placement methods in case of sidetracks failures. Still with proper placement methods, failure in sidetracks cements can occur.7

Currently, conventional cement formulations are used to prepare plugs used in sidetrack drilling. Many hours of rig time are lost to set plugs just to build up the angel in sidetrack drilling. Some times it is extremely difficult to sidetrack in some areas because the cement rate of penetration is much faster than that of the adjacent formation.

The sonic compressive strength of regular cement is much lower than the formation leading to this extremely fast cement ROP compared to formations. To address this problem, the difference in the rate of penetration between the cement plug and the adjacent formation must be minimized by increasing the sonic compressive strength of the cement.

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