ABSTRACT

The compression strength of set cement at ultra-high temperature conditions (200°C or greater) degrades seriously. Consequently, the strength and anti-strength retrogression of set cement at high temperature are studied using different compression strength experimental techniques including XRD, SEM-EDS, and MIP. The XRD and SEM-EDS analysis showed that the crystal phase transformation from Tobermorite to Xonotlite is an important reason for the long-term mechanical properties' retrogression of set cement. A new type of strength enhancer material, which ensures the stability of the crystal phase of the Tobermorite under ultra-high temperatures of 200∼240°C, has been developed and is studied based on the principle of crystal phase structure control. The compression strength test showed that when 10% strength enhancer and 60% silica flour were used, the compression strength of set cement does not degrade at 240°C and in fact resulted in an increase in strength from 36 MPa in two days to 42 MPa in 28 days. At the same time, the permeability cement paste remained unchanged. The cement mixed with the enhancer material has stable long-term mechanical properties, which can effectively solve the problem of long-term sealing integrity of ultra-high temperature deep wells in Tarim basin and deep-water oilfields in South China Sea.

INTRODUCTION

More than 50 oil wells above 8000m have been drilled in Tarim and Sichuan basins of China in recent years. Some ultra-deep onshore and deep-water offshore wells with well depths more than 10000 meters are planned in the next few years. Ultra-deep drilling has several challenges including complex geological conditions, ultra-high temperature and ultra-high pressure. The bottom hole temperature often exceeds 200°C and the pressure exceeds 150 MPa. The harsh working conditions of HT-HP require cement sheath has good long-term sealing integrity. Poor well bore sealing integrity will affect oil-gas exploration efficiency and even threaten well bore safety.

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