ABSTRACT

Results of an extensive experimental and analytical investigation of wellbores strengthening parameters are presented in this study. Particle plugging tests with a wide range of particles and core fracturing tests, accompanied by novel data analysis methodologies suggested the significance of particle mechanical properties, size distribution and concentration as well as fracture surface friction coefficient. This work also sheds light on the mechanism of wellbores strengthening and provides a reliable analytical model to predict wellbore resistance with respect to downhole pressure. Outcomes of this study can aid in more realistic hydraulic and stability design leading to mitigation of circulation loss problems.

1. INTRODUCTION

Circulation loss is defined as the undesirable penetration of drilling fluids into the formation voids during drilling or cementing operations. Several mechanical and chemical parameters contribute to this issue and its consequences. Circulation loss may happen at any depth or formation type and may vary in severity based on the geometry of the “thief zone”. Lost circulation is usually encountered in high matrix permeability formations, natural and induced fractures as well as vugular and cavernous rocks. As matrix permeability reaches above 10 Darcy, a gradual drop in the mud level in pits initiates and may develop if remedial measures are not taken [1,2,3]. Although small fractures are present in almost all formations, highly conductive natural fractures which are mostly found in chalks and limestones can result in remarkable circulation losses [4]. Induced fractures are the openings resulted from tensile failure around the borehole and may happen in any formation as the pressure in the borehole exceeds the fracture gradient of the rock. Vugs and caverns are the most troublesome type of openings which are usually encountered in dolomites and limestones as a result of percolating water [1,3]. Shape of caverns varies from a few inch wide openings up to large tunnels. Lost circulation may initiate in the order of 1.59 m3/hr (10 bbl/hr) and increase over time. In some cases such as drilling through large conductive caverns, a quick and total loss of drilling fluids may be experienced. Based on the intensity of the fluid flow, circulation loss cases may be categorized as seepage, partial, severe or total circulation losses. There is not any globally accepted standard for this classification and various classifications are found in different references [4,5,6,7,8,9]. Pressure in the well is one of the most dominant factors in wellbore stability analyses which needs to be maintained within a safe window to avoid fluid inflow, circulation loss and intolerable wellbore collapse. Induced fractures open up in the formations as the tensile stress around the wellbore reaches above the toleration of the rock, as a result of elevated wellbore pressure. This phenomenon is affected by a wide range of fluid and rock properties. Impacts of drilling fluid properties have been identified and investigated the past decades through several experimental and analytical studies. Based on these observations, particles in the fluid tend to increase the pressure level at which circulation loss initiates in an induced fracture [8,9,10,11,12,13,14,15,16,17].

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