ABSTRACT

In the well test or production process, wellbore stability plays a decisive role in designing the reasonable test pressure difference and maintaining long-term stable production. Therefore, avoiding wellbore instability is a prerequisite for reducing development risks and improving economic efficiency. In this paper, the wellbore instability in the production process is regarded as a quasi-static mechanical problem under negative pressure conditions. A series of the laboratory experiments were carried out to simulate the process of wellbore destruction, in which the thick-walled cylinder used is artificially made by sandy conglomerate. Combined with the acoustic emission instrument, the stress condition of the wellbore failure was determined. In order to extend the experimental results to the real underground environment, the scale effect must be considered. Therefore, based on previous research conclusions and the results of laboratory experiments on sand production, the rock mechanical parameters were recalibrated. As an application example, according to the regional in-situ stress condition of the Bohai Oilfield in China, the test pressure difference of two wells in Bohai Oilfield were predicted. Finally, the method mentioned above was proved to be practical by field application.

1. INTRODUCTION

Wellbore stability has long been considered one of the most difficult problems in the drilling and completion process. During well test, if the test pressure difference is set unreasonably, it will cause not only the inaccuracy of the productivity evaluation, but also the wellbore instability or collapse.

Many scholars have used thick-walled cylinder theory or experiment to analyze the wellbore stability and achieved meaningful results, as discussed by Warlick, L.M. et al., 2009 and Li, Q.D. et al., 2011. Therefore, the stress state of rock failure is obtained through the experiment of thick-walled cylinder combined with acoustic emission equipment in this paper. In addition, considering the influence of failure criteria and scale effect on wellbore stability, the rock mechanical parameters were recalibrated to make the prediction results more realistic. It should be noted that the recalibrated mechanical parameters are not the same as the mechanical parameters measured by uniaxial or triaxial compression experiments, which will be explained in detail later.

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