Formation with complex pressure system are those in which there are abnormally high or low pore pressures, unusually high collapse pressure and low leakage pressure in the stratigraphic section, and drilling this kind of formation will bring many challenges. In the analogous areas with abnormal pressure system, it is necessary to strictly prevent the occurrence of well kick, wellbore collapse and lost circulation in drilling. According to the basic principles of rock mechanics and geomechanics, the analysis of wellbore stability of complex pressure strata can predict drilling risk, prevent complex accidents and improve drilling efficiency. In the South China Sea, due to the special stratigraphic depositional characteristics and development process, the W oilfield is confronted with the problems of high collapse pressure, high pore pressure and depleted pressure, low leakage pressure in adjustment wells drilling. The main difficulties are collapse problems of shale with bedding and cracks in upper formation of the W-2 section, and coexistence of abnormally high pressure and low pressure formations in the W-4 section where lost circulation is easy to occur in abnormally low pressure formation. By the analysis of wellbore stability mechanics, it is recognized that the collapse of W-2 shale is mainly controlled by weak bedding planes, and borehole is more stable when drilling with smaller inclination angle, and both collapse pressure and fracture pressure decrease with pore pressure drawing down in the W-4 reservoir. Based on the above analysis, a targeted safety drilling design was carried out, which is in the shale of W-2 section the well track design should choose lowest deviation angle and drill separately, and in the W-4 reservoir high pressure differential shielded drilling fluid and low fluid density should be used to reduce the risk of borehole instability.

1. Introduction

In oil and gas drilling, safe density window is determined by formation pore pressure, collapse pressure and leak-off pressure. In order to ensure drilling safety, drilling fluid density is required within the safe window. When drilling abnormal high or low pressure formation, and sloughing or leaking formation, the pressure system is complex, which causes safe density window is too narrow, and brings great difficulty to drilling design and operation.

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