Wellbore Stability Evaluation Guideline for Reducing Non-Productive Time
Authors: Meng Frank , Fuh Giin-Fa 
Published: March 2015

Wellbore instability is one of the most critical challenges affecting drilling and production. Borehole instability in both shale and sand formations, hole collapse, lost circulation, stuck pipe, sanding, and other related well failures can be avoided by meticulous and careful design of drilling plan, specially by determining the critical mud weights and mud fluid components which will provide sufficient support for the wellbore during drilling operation.

This paper presents a systematic program and guideline of comprehensive geomechanics investigation and evaluation that includes drilling and log data analysis for rock strength and earth stress model construction, stress regime influence, laboratory triaxial stress-strain tests on well core samples considering bedding plane inclination effects, failure criterion consideration, and optimum mud-weight (MW) window for secure and stable well drilling.

The continuous well production can also cause problems for in-field drilling as the severe pressure depletion and reservoir compaction would inevitably induce significant reduction in stresses and formation fracture gradient both inside the reservoir and significantly upward into the caprock formations depending on the reservoir/formation stiffness contrast, reservoir size, thickness and depth, etc. This paper addresses those issues with the optimum MW reexamined and recalculated based on the largely altered in-situ stresses and wellbore strengthening application if necessary. This paper will describe theories, some issues in wellbore stability work, discuss some uncertainties from authors' experience, lessons learned and guidelines for wellbore stability evaluation.

Introduction

Wellbore instability is one of the major causes of the non-productive time (NPT). Wellbore instability affects both exploration and development drilling stages. In addition to in-situ stresses and formation rock strength as the two critical contributions for the borehole instability, drilling fluid system is another principal factor. Most of the well failure happens in the shale formation, especially in the depth section where the drilling fluid interacts with shale formation.

In-situ stresses include overburden stress, two horizontal stresses and pore pressure. The stress magnitudes and direction will directly affect borehole stability. In this paper, some issues related to stresses and formation strength will be illustrated and discussed. The pore pressure deduction due to depleted reservoir is another challenge encountered in many mature field drilling operation. Fracture gradient changes with pressure depletion results in very narrow mud weight windows and potential lost circulation problem. To solve the problem and get successful drilling operation, wellbore strengthening techniques may be applied. Good results have been demonstrated and been shared in this paper. Also it is discussed that adding sufficient loading of bridging/plugging materials in the mud system can help to prevent any possible fracture changes, shale disintegration or hole cave-in resulting from the invasion of mud filtrate penetrating into the natural fractures.

In this paper, a practical guideline for wellbore stability analysis, reasonable and more accurate input data, depletion evaluation, wellbore strengthening, and real time mornitoring during drilling operation will be addressed and discussed.