Studying the changes of deep mudstone strength properties after hydration are vital for the design of well drilling and proper choice of drilling fluid for deep wellbore stability. However, the previous methods (Coupled with conventional immersion test and compressive test) for evaluating the effect of hydration on rock strength cannot simulate deep well downhole hydration environments (HPHT and drilling fluid flow). In addition, high requirements of sample preparation and destructive test make it difficult to apply the compressive test to study strength of mudstone after hydration. In this study, a new experimental apparatus for hydration of mudstone under deep well drilling conditions (high temperature: ~200°C, high pressure: ~60MPa and flushing action of drilling fluid) is designed. Furthermore, continuous scratch method, which can easily achieve repeated tests using the same core sample at different extent of hydration, is adopted to evaluate the strength properties (uniaxial compressive strength (UCS), cohesive strength and internal friction angle) of mudstone after hydration. Based on the above works, a new approach to study the effect of hydration under deep well drilling conditions on mudstone strength parameters is developed. The comparison experiments about evaluating the influence of hydration on mechanical properties of mudstone (depth 5500-7000m, Western field China) under deep well conditions and ambient conditions are conducted. Results show that the reductions of UCS and cohesive strength of mudstones under deep well downhole conditions and ambient conditions are about 70-80% and 20-40%, respectively. It reveals that deep well drilling conditions aggravate the degradation of mudstone strength and cannot be ignored in managing wellbore instability. This proposed approach can provide significant support to the performance optimization of drilling fluid and design of deep well drilling.

1. Introduction

The development potential of oil and gas resources from deep reservoir in Western China is drawing significant attention as time goes by. However, due to the complex conditions (drilling fluid flush, high temperature: ~200°C and high drilling differential pressure: ~40MPa) in deep well, wellbore instability in deep mudstone formation seriously hampers the productive time (Zhou et al., 2018; Liu and Zhou, 2019; Cui et al., 2020). In order to sustain wellbore stability in deep mudstone, it is necessary to study the weakening effect of hydration on rock strength properties under the real conditions in deep formation.

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