Giin-Fa Fuh

Wellbore Stability Evaluation Guideline For Reducing Non-Productive Time

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.

Borehole-Stability Analysis for the Design of the First Horizontal Well Drilled in the U.K. Southern V Fields

This paper presents the method and result of a borehole-stability analysis conducted on the first horizontal well successfully drilled the North Valiant oil field in the Southern North Sea. Previous drilling data obtained from the same area were analyzed by numerical simulations of potential mechanical behavior of the wellbore. The magnitude of the in-situ horizontal stresses was determined by the analysis of fracture-stimulation data from wells in the fields. The directions of the stresses were derived from anelastic-strain-relaxation (ASR) measurements of fresh core. Effects of hole azimuth on well bore stability were evaluated. The analysis resulted in a design of optimum mud weights for maintaining borehole stability during drilling. This paper also discussed field results and well completions.