Borehole Stability Management Using the New Mudweight Window Concept; A Case Study of Well KTY 02, KTY 03 and KTY 04

Abstract

Borehole instability is one of the major factors that contribute significantly to additional unplanned cost in drilling operations irrespective of the wellbore inclination. Problems generally build up in time, starting with the tensile or compressive failure of the borehole wall, followed by transfer of fragments to the annulus and finally-if hole cleaning is insufficient-culminating in such difficulties as tight hole, breakouts, caving, pack off, borehole collapse and stuck pipe. Horizontal and highly deviated wells in normal faults stress regimes present more difficult challenges than low inclined wells due to compressive or shear failure of the wellbore. Wellbore stability issues are more pronounced as the wellbore stress difference reaches at maximum with increase in inclination. Proper planning of the well trajectory and mud weights are crucial to avoid such complexity which causes huge rig downtime, NPT and cost. With the aid of in-situ stress, pore pressure and rock strength analysis the wellbore stability can be assured with suggested optimum deviation profile and mud weights window for different inclinations and azimuths. An attempt has been made to perform the wellbore stability analysis for three high angle and three horizontal wells (with drain-extension of about 500ftahd) development wells in Niger Delta which is planned for production in near future. In our workflow, the seismic data and offset well information have been incorporated to generate pore pressure, optimum mudweight, shear failure (minimum envelope) and fracture pressure gradient. Rock physical parameters have been calculated from the offset well's logs and calibrated with laboratory tested dataset to use in the stability analysis for well KTY 02, KTY 03 and KTY 04. In the study area, wellbore stability analysis was carried out in both the pilot and drain hole sections of the horizontal wells. However, because of horizontal drilling plan in drain holes, differences in principal stresses in the wellbore and their physical implication on stability was plotted and interpreted through stress concentration plot and safe MW window analyzer extensively. The stresses and the rock strength datasets input were used to derive the collapse failure gradient curve (CFG) using the Mogi failure criteria. The wellbore circumferential and radial stress distribution analysis has been done for different depths with the inputs from stresses and the corresponding cohesive strengths and the Frictional Angles. With these analytical results, we have recommended the wellbore trajectory along Shmin and SHmax direction and along the maximum good reservoir facies with corresponding mud weight (window) profile required to drill these wells with NPT as a result of instability consequences such as stuck pipes and jeopardizing wellbore integrity during logging, casing running and completions.