Improved Experimental Characterisation of Cement/Rubber Zonal Isolation Materials

Abstract

Recently, various companies published sophisticated design methodologies to engineer the ‘cement' sheath in a oil/gas well completion such that the zonal isolation would remain intact (i.e. pressure tight) during its projected lifetime. Loading within a prescribed design envelope (e.g. pressure and/or thermal cycling or mechanical compaction processes) are the main considerations.

Improved performance of a candidate zonal isolation material is achieved by adjusting its material properties, especially the Young's modulus and the compressibility.

These earlier studies indicated that apart from a proper mechanical characterisation of the ‘cement sheath' (preferably described by non-linear material models), the in-situ stresses in the ‘cement' play a prominent role as well. The latter are influenced by the volumetric behaviour of the slurry during setting and the thermal expansion coefficient of the set ‘cement'.

However, often these parameters are not known for ‘real life' well construction materials.

Therefore, Shell International E & P B.V., the Netherlands, decided to design and construct dedicated laboratory equipment to determine those parameters for commercial ‘cement' formulations at conditions encountered in typical well completions.

A first feature of this unique system (which can operate at pressures up to 1500 Bar and temperatures up to 300°C) is to monitor i) the progress of the ‘setting' reaction of Oil Well Cements and or thermosetting resins, ii) their reaction behaviour (from the onset of gelling to the ‘final set') and iii) the softening or swelling phenomena encountered in thermoplastic and thermoset materials.

The apparatus has inter alia the same functionality as conventional API Cement Consistometers or Ultrasonic Cement Analysers. However, it can also be used in combination with self-vulcanising rubbers, that cannot be used in the conventional API devices.

A second aspect of the apparatus is the determination of the volume changes (at constant pressure) or alternatively the pressure changes (at constant volume), during setting of the cement (or resin) system.

All measurements can be performed at either isothermal conditions or for prescribed temperature sweeps over time. Either volumetric properties (shrinkage or expansion) or compressibilities of cements / resin systems can be measured in a time frame ranging from the onset of gelling to far beyond ‘final set'.

In yet another operational mode, the volume change of set materials as a function of temperature (i.e. their volumetric thermal expansion coefficient) can be quantified.

In addition to a review of the principles of this novel apparatus, this paper also presents a typical application of the data in a Plug and Abandonment design (i.e. the Finite Element Engineering design of a thermally pre-stressed ‘rubber' abandonment plug).

Introduction

The main purpose of primary cementing and well abandonment operations is to provide effective zonal isolation during the entire life span of the well. To achieve this objective the sealant should meet both the short-term and long-term requirements dictated by the well's operational regime.

Traditionally, petroleum engineers have only concentrated on the compressive strength as a quality indicator; long-term properties such as resistance to downhole chemical attack were only occasionally considered. Until recently, no attention has been paid to other mechanical properties such as tensile strength, Young's modulus, etc. The conventional approach was acceptable if the sealant would not be subjected to a "large" change in stress level.