Abstract

Until recently, the drilling industry had an established range of options for dealing with lost circulation that included pre-treatment with lost circulation material (LCM) and/or remediation with LCM when losses occurred. This paper describes a fully engineered approach taken by BP and Halliburton to formulate a drilling fluid that can effectively strengthen the weak wellbore while drilling. The approach, called Stress Cage theory, is an extension of a lost circulation strategy that has been used for years in the drilling industry. However, it includes using log analysis to identify potential fracture location and determine its geometry, especially its width, and then identify a mixture of particulate materials to seal the calculated fracture width. The application of the appropriate mixture of materials can effectively increase fracture gradient (both in land and offshore), allowing the operator to drill through a weak formation pressured zone successfully with minimal to zero downhole losses. Case histories including drilling parameters, formation pressures, field and laboratory fluids testing, and some of the operational challenges are detailed. The estimated value of Stress Cage applications for BP in 2004 and 2005 has been calculated at $100MM.

Introduction

Drilling depleted zones presents many challenges. In some cases it is virtually impossible to drill through depleted zones as the mud weights required to control the adjacent zones or the main body of the reservoirformation creates a severely overbalanced state when drilling through the depleted sands. This often leads to severe mud losses that prevent drilling ahead and creates the possibility of sticking the bottom hole assembly (BHA), drillpipe, or liner / casing.

Operators can choose from among many engineered approaches to address drilling through depleted zones:

  • Under balance drilling

  • Casing while drilling

  • Additional casing or liner before and after

  • Expandable casing/liner

  • Strengthening wellbore with heat

  • Strengthening wellbore by increasing hoop stress with particles.

This paper describes development work with BP and Halliburton to produce a drilling fluid that effectively strengthens the wellbore while drilling. The fluid also can be used in a pill form for treating a drilled weak interval. This effect is achieved by changing the stress state rather than by altering the strength of the weak rock itself. Such a system will have many benefits if it can be engineered in a practical way.

Theoretical Approach

The approach1 BP has taken, called Stress Cage theory, is an extension of a lost circulation strategy that has been used for years in drilling industry. However, it includes using log analysis to identify potential fracture depth, width and length, and then identify a mixture of particulate materials to seal the calculated fracture width.

The application of the appropriate mixture of materials effectively increases fracture gradient (both in land and offshore), allowing the operator to drill through weak depleted formations successfully with minimal to zero downhole losses.

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