Summary
Objective
The integrity of a well is dependent upon the casing-cement contact and its capability to seal the annulus throughout the life of the well. Conditions in HPHT wells are particularly challenging, and little is known about the fatigue of cement and the performance of the casing -cement interface in HPHT wells. The objective of this project is to develop a better understanding of the performance of the casing cement bond under HPHT well conditions that can lead to best practices and a model to predict well life.
Approach
Conditions in HPHT wells are particularly challenging, and little is known about the fatigue of cement and the performance of the casing -cement interface in HPHT wells. Drill string-induced vibrations and changing pressures and temperatures cause damage at the casing-cement interface by forming micro annuli, and the accumulation of that damage can lead to a loss of well integrity and well failure.
Tubulars used in HPHT wells are tested under simulated borehole conditions prior to field application. Cement slurries and cement properties are also measured, but none of the known methods are able to evaluate the life of the well based on actual wellbore parameters. Even less information is known on the fatigue of cement under cyclic loading under HPHT conditions.
Understanding the mechanics of the casing-cement system will enable a new model to be developed to evaluate and predict the life of the well.
Deployment of Results
Provide best practices guideline and assess well completion options and methodologies for high pressure/high temperature/ and sour (H2S) wells.
Background
Fatigue of materials is a known failure mode. Metals and concrete for buildings and bridges are intensively investigated under fatigue aspects, however well cement behavior is still an unknown for the engineers. Although the latest advances in well cement mixtures are huge, well cementing practice shows many quality problems along the well. The investigation of the casing – cement – rock system is new for most of the oil industry. Some work has been carried out by Krusche (SPE 100390) and Schubert (IPTC Paper 10961) to investigate the casing- cement- rock behavior using FEM methods. Unfortunately cement properties and behavior is not easy to model with FEM. Well cement fatigue was considered as an unimportant load, since well location, depth and time have not induced excessive fatigue loads in the past. New types of cements (I.e. expandable cements) are now inducing a new stress-strain state into the casing-cement- rock system. A very first attempt to perform such investigations was done by Teodoriu at the Institute of Petroleum Engineering of TU Clausthal [1]. The investigations were stopped since Teodoriu moved to TAMU. SPE 98896 presents another way of investigating the effects of long time exposure to High temperature environments, but does not refer to fatigue resistance of tested cements.
Project Plan
Scope and Plan:
We propose to complete a focused literature review to assess the following areas:
- The state of the art in the fatigue of cement (particularly for well cements since no such studies are known).
- Casing cement interaction models currently being utilized by the operators and drilling contractors, (if any).
- Casing-Cement integrity under HPHT conditions and Cement Fatigue
From these learnings, we will develop an understanding of the mechanics of casing-cement systems under HPHT conditions, especially the long term integrity of the system.
Further work will concentrate on:
- Finite element investigations of casing-cement-rock interaction. Some modeling work in this domain has been performed by Dr. Schubert (reference 1).
- Developing a prototype long term/short term test cells for cement fatigue investigations under HPHT conditions based on FEM results (a prototype was developed by Dr. Teodoriu (reference 2)
Using Finite Element Analysis (FEM) combined with experimental stress analysis will help us to develop a fatigue life for the cement sheath under HPHT conditions. The resulting models together with other already developed models can be integrated in a simple to use computer software package capable of run by recording the history of the well and predicting its life.
Anticipated Results: The final report will document results, including
- Description of the importance cement fatigue to the life of HTHP wells
- Laboratory procedures and data from a prototype test cell that measures the performance of cement in cyclic loading under HPHT conditions.
- A new model to evaluate and predict the life of a HPHT well
- Recommendation for further work to complete a set of best practices and a model to predict well life.
