Abstract

Wellbore drilling in highly fractured and unstable formations has always been one of the greatest difficulties in drilling operations. Failure and collapse of wellbore wall frequently occur in the fractured zone and may ultimately obstruct any further drilling of the wellbore. A number of techniques have been proposed and tested to improve drilling conditions in the fractured and unstable formations, but a highly effective method is still lacking. For many years chemical consolidation techniques have been studied and applied for ground support in underground mine openings. Very successful results have been achieved employing the techniques to stabilize fractured zones surrounding underground openings. For these a number of highly effective chemical materials for consolidation have been developed.

In order to examine the feasibility of this technology for enhancing wellbore stability, a series of laboratory experiments have been carried out in this study. These experiments included uniaxial compressive strength tests, triaxial compressive strength tests, shear bond strength tests, indirect tensile strength tests and thick wall cylinder tests. Chemical grouts Terraset B1000, Webac 4110 and others were used in these tests and their mechanical properties were assessed. The results of the experiments show that chemical grouting is likely to improve the stability of wellbore in fractured and unstable rock formations. Among the grouting materials tested, the Webac 4110 had the best performance. Its use increased the shear strength by 100–150%.

Introduction

Borehole instability is one of the major problems frequently encountered in oil well drilling. The formation instability may cause hole size reduction by plastic rock squeezed into the hole, hole enlargement by caving shales or hard rock spilling, or formation fracture by a high drilling fluid pressure. In particular wellbore drilling in highly fractured and unstable formations has always been the greatest challenge in drilling operations. Failure and collapse of wellbore wall frequently occur in the fractured zone and may ultimately obstruct any further drilling of the wellbore. Techniques have been proposed and tested to improve drilling conditions in the fractured and unstable formations, but a highly effective method is still lacking.

For many years chemical consolidation techniques have been studied and applied for ground support in underground mine openings. Very successful results have been achieved employing the techniques to stabilize fractured zones surrounding underground openings and a number of effective chemical materials for consolidation have been developed. This technique is likely to be effective for wellbore stabilization in oil field drilling. A preliminary investigation to evaluate some commercial chemical grouts for wellbore stabilization was conducted in this study.

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