Abstract

A laminated and fissured formation around a horizontal borehole is modeled as a combination of intact rock blocks and permeable fractures with a DEM code. The model is used for understanding the mechanism of rock failure problems observed in a shale formation of an offshore field. Different mud weights and stress conditions are applied to the model to simulate the rock failure around deviated holes in laminated and fissured shale.

The numerical simulation shows that shear failure along the bedding planes is the predominant mode of collapse of the hole when mud under pressures penetrate into bedding planes on which shear stress is applied. The severity of wellbore instability varies with stress states and wall conditions. The effect of a plugging agent in mud is verified by comparison between permeable and impermeable wellbore models.

Introduction

In recent decades, geomechanical and rock mechanics studies for drilling optimization and reservoir management have been eagerly conducted by many researchers and engineers. As a new result, fundamental understanding of failure mechanism of rocks and geomechanical environment is shared by academic and industrial societies.

For the purpose of reducing drilling troubles and optimizing drilling operations, rock failure analysis for wellbore instability has become popular and common after its remarkably successful application in the Cusiana field in Colombia1. Intensive studies have been conducted to improve mechanical behavior models of various rock types, numerical simulation codes and computer aided engineering tools, and engineering procedures to apply such techniques to field operations. However, currently available techniques and numerical tools in which elastic, homogeneous, continuous, and isotropic rock models are adopted could give misleading results due to the complex nature of real formation rocks.

In fissured or highly laminated formations, anisotropy and discontinuity of the rock mass have predominant effects on rock failures, as well as poroelastic effects caused by pore pressure change in the fractures and rock matrix.

In this paper, a two-dimensional, fluid-solid coupled model of Discrete Element Method (DEM) simulator is applied to wellbore failure modeling of a laminated rock to analyze the effect of pore-pressure penetration through the fractures. Key factors of the stability and anticipated failure mechanism of the hole troubles observed in a real field are discussed.

Field experiences of rock failure in a heavily laminated formation

Hole instability phenomena observed in real fields.

The authors investigated the records of drilling troubles in Japanese oil fields2 and found many phenomena which can not be explained by conventional approaches and understanding of borehole failures, such as:

  1. Severe caving is observed simultaneously with lost circulation.

  2. Increasing mud density to maintain wellbore stability does not work well, and increased mud weight may ever worsen the hole condition.

  3. Break-out directions determined by a caliper log vary with depth, and suddenly change discontinuously.

These experiences suggest the limitation of conventional mechanical models. Many factors complicate of the situation, such as complex constitutive laws of each type of rock, fluid motion and poroelastic effect, fluid exchange between borehole and formation by pressure and chemical potential effects, anisotropy and heterogeneity of the formation, and so on. The real situation must be a combination of such effects.

Hole instability phenomena observed in real fields.

The authors investigated the records of drilling troubles in Japanese oil fields2 and found many phenomena which can not be explained by conventional approaches and understanding of borehole failures, such as:

  1. Severe caving is observed simultaneously with lost circulation.

  2. Increasing mud density to maintain wellbore stability does not work well, and increased mud weight may ever worsen the hole condition.

  3. Break-out directions determined by a caliper log vary with depth, and suddenly change discontinuously.

These experiences suggest the limitation of conventional mechanical models. Many factors complicate of the situation, such as complex constitutive laws of each type of rock, fluid motion and poroelastic effect, fluid exchange between borehole and formation by pressure and chemical potential effects, anisotropy and heterogeneity of the formation, and so on. The real situation must be a combination of such effects.

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