Managed-Pressure Drilling Solves HP/HT Challenges Offshore Vietnam
Authors: Feder Judy 
Published: February 2019

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 191927, “Application of Managed Pressure Drilling on a Semisubmersible Tender-Assisted Rig To Address Drilling Challenges in HP/HT Gas Condensate Wells, Offshore Vietnam,” by Harpreeet Kaur Dalgit Singh, SPE, Bao Ta Quoc, and Tan Chai Yong, SPE, Weatherford; Do Van Khanh, Nguyen Xuan Cuong, and Hoang Thanh Tung, PV Drilling; and Truong Hoai Nam, Ngo Huu Hai, Dang Anh Tuan, Trinh Ngoc Bao, Tran Nam Hung, and Nguyen Pham Huy Cuong, Bien Dong Petroleum Operating Company, prepared for the 2018 SPE Asia Pacific Oil and Gas Conference and Exhibition, Brisbane, Australia, 23–25 October. The paper has not been peer reviewed.

Drilling in deeper formations with high-pressure/high-temperature (HP/HT) margins increases the tendency and frequency of well-control incidents related to anomalies of formation pressures and temperatures. This paper discusses how managed-pressure-drilling (MPD) technology led to cost savings in two wells drilled in the Hai Thach gas field offshore southern Vietnam. The wells were drilled from a semisubmersible tender-assisted rig with an automated MPD choke manifold and docking-station rotating control device (RCD), integrated on a flexible plug-and-play basis to enable easy rig up and rig down and the moving of equipment between wells.

Introduction

MPD systems have built a track record of success in mitigating drilling hazards related to uncertain formation pore pressures and narrow-pressure operating windows common to HP/HT exploration and development wells. In these wells, slow rates of penetration, wellbore-stability issues, ballooning, accuracy of influx-detection systems, and differential sticking can result in well-control problems and nonproductive time (NPT).

The Nam Con Son (NCS) basin off the southern coast of Vietnam presents such challenges. It is one of a series of Tertiary rift basins created on Vietnam’s continental shelf as a consequence of the East Sea seafloor spreading.

The formation pressure profiles of offset wells in the central NCS basin, including the Hai Thach and Moc Tinh gas fields, are characterized by over-pressure beneath the late Pliocene/Quaternary deltaic sedimentary section. The overpressure occurs with a high-pressure ramp in the thick, post-rift, deep marine, Late Miocene/Early Pliocene shale interval. Overpressure retains in the syn-rift and pre-rift sedimentary section with lower ramp.

A higher pore pressure of approximately 17.3 lbm/gal at a shallower-than-predicted depth was encountered when drilling the 12¼-×14½-in. under-reamed (UR) section of Bien Dong Well WHP-HT1. Increasing mud weight (MW) was not practical because high equivalent circulating density (ECD) exceeded the unexpected, low leakoff test result at the casing shoe. This led to the decision to call off the well total depth (TD) early and set the contingency 11¾-×135/8-in. expandable liner to cover the weak formation. A total of 46 days was required to complete the planned depth of the well section.

As a part of remedial actions based on the experience of drilling Well 1, and with added concern of potential ballooning given high pore pressures and a narrow operating window, the operator’s drilling team decided to deploy MPD technology with constant bottomhole pressure (CBHP) and early kick-loss detection capabilities on Wells WHP-HT 2 and 3.