Wellbore shielding and strengthening experiments were conducted on three outcrop sandstones with real-time CT scanning. The wellbore shielding stage was conducted by increasing confining and borehole pressures in 4-5 steps until both reached target pressures. In wellbore strengthening stage, the borehole pressure was increased until fracture was initiated, which was detected via borehole pressure trend and CT scan imaging. The effectiveness of wellbore shielding by mud compounds was evaluated based on normalised mud filtration rate. CT scan images demonstrate consistency of the filtration rates with observed CT scanned mud cakes which are dependent on the sandstone pore size and mud compound particle size distribution. In the wellbore strengthening stage, the fractures generated, particularly the dominant fractures, were observed to be plugged by mud filter solids which are visible in the CT scan images. The extent of observed fracture solid plugging varies with rock elastic properties, fracture width and mud compound particle size distribution. Based on the laboratory test data, fracture gradient enhancement concept and empirical criteria were developed for the mud compounds. In addition, the data obtained and observations from the tests were used to develop optimal empirical design criteria and guidelines to achieve dual wellbore strengthening and shielding performance of the mud compounds.

1.0 Introduction

Operators acknowledge infill drilling campaigns are more challenging due to drilling through depleted reservoirs with reduced fracture gradient and in some cases, almost impossible to drill with the equivalent circulating density of the mud exceeding the fracture gradient. Although it is known that wellbore strengthening is an enabler for such drilling campaign, it is typically a qualitative solution as there is currently no robust methodology to quantify the magnitude of the strengthening. Most of the time, it is based on information and know-how in the industry. Typically, the solution tends to be conservative and relies on the drilling experience of previous wells to estimate the wellbore strengthening. The lesson learnt from the wellbore strengthening application in these wells is then applied to the next well without knowing whether the well will be adequately strengthened and can be drilled successfully. Even if the drilling is successful, the actual increase in fracture gradient is not known. However, if the increase in fracture gradient is known in advance during the well design stage, it will impact the well and casing programme design which will maximise the benefit on operation.

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