ABSTRACT:

The sub-salt reservoirs of High Temperature and High Pressure (HTHP) are often drilled with Oil-Based Mud (OBM), for the OBM has better rheology and better keeping stability performance than Water-Based Mud (WBM) under HTHP condition. This paper aims to analysis the influence of OBM on salt formation creeping rate, based on the difference on interface tension of OBM. Through the experimental research, it get the relations between salt formation creeping with different exposing time and different interface tensions. An Electron Microscope Scanning Investigation (EMSI) has also been applied for saturated and unsaturated salt samples. Through EMSI, the micro change of salt samples has been testified. The research shows that the interface tension has a great impact on salt creeping. When interface tension increases, the infiltrate length of drilling fluid per unit time increases when quantity of micro-cracks increases, the infiltrate length of drilling fluid per unit time decreases. The experiments in this study were conducted under simulated high formation temperature. The OBM with different interface tension were obtained by adding surfactants into the OBM used in real drilling operations.

1 INTRODUCTION

The sub-salt reservoirs are often drilled with OBM, because the WBM's inherent shortage in rheology and in keeping wellbore stability under HTHP condition. On the other hand, the OBM has a better performance in keeping wellbore stability under high temperature. The creeping of salt under WBM has been investigated in variety of studies (Fabre 2006). Some scholars have proposed several different equations for expressing the relationship between stress creeping rate and temperature (Homand 2006). In studies concerned with the micro mechanics of salt creeping, Fredrich (2007) employed statistics analysis on experimental data obtained from the salt rock experiments of Gulf of Mexico. Fuenkajorn (2010) analyzed the changes of Young's modules, and Poison's ratio on salt rock under repeated load and unload procedures. Popp (2000) investigated the changes of rock's permeability and acoustic logging data under different stress load. In the studies concerned with the interaction between salt and drilling mud, Chen, X. analyzed the salt dissolving under WBM (Chen et al. 2011). De Meer, S. & C.J. Spiers studied the creeping rate of salt formation under WBM (De et a!. 1999). G & Baoping proposed a theoretical model incorporated the couple effects of thermal conduction and fluid infiltration on salt formation (Bao et al. 2008). To sum the above studies up, the previous studies have concluded that when the stress loading increases, the pre-existed internal micro cracks will expend (Chan et al. 2001).

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