Managed-Pressure Drilling Used Successfully for Offshore HP/HT Exploration Wells
Authors: Carpenter Chris 
Published: February 2019

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 191060, “Operational Design and Application of MPD in Offshore Ultra-HP/HT Exploration Wells,” by Qishuai Yin and Jin Yang, China University of Petroleum; Bo Zhou, CNPC; Ming Luo, Wentuo Li, and Yi Huang, CNOOC; and Ting Sun, Xinxin Hou, Xiaodong Wu, and Junxiang Wang, China University of Petroleum, prepared for the 2018 IADC/SPE Asia Pacific Drilling Technology Conference, Bangkok, 27–29 August. The paper has not been peer reviewed.

The South China YQ Basin, with its 15 trillion m3 of natural gas, is typical of ultrahigh-pressure/high-temperature (ultra-HP/HT) plays, with the highest bottomhole temperature (BHT) at 249°C, the maximum bottomhole pressure (BHP) at 142 MPa, and an extremely narrow pressure window. Predictably, drilling challenges in these plays are numerous. This paper discusses the successful application of managed-pressure drilling (MPD) in the basin with reduction in risks and well costs.

Overview of the YQ Basin

In recent years, approximately 27% of major oil and gas discoveries have come from HP/HT fields. The South China YQ Basin is one of the three major offshore HP/HT regions in the world and is located at the intersection of the Eurasian, Pacific, and Indo-Australian plates and has a complex geological structure.

The drilling in this basin, as is the case in any HP/HT area, faces various technical challenges, including the following:

The temperature and pressure gradient is very high. The highest temperature gradient is 5.51°C/100 m. The formation-pressure transition zone is short, and the formation pressure rises rapidly.

The safety drilling-fluid-density window between pore pressure and fracture pressure is extremely narrow, and the safety factor is very small.

Formation pressure is hard to predict accurately, and the associated error is greater than 20% in some complex wells.

Formation drillability is bad because the main targeted layer is over 5000 m. As a result, the rate of penetration (ROP) is very low, which leads to longer drilling cycles and more-frequent downhole accidents and issues such as casing wear.

The natural environment is harsh (i.e., frequent typhoons in summer).

Operational Design of MPD

The operational design of MPD consists of three parts: the precise calculation of drilling-fluid equivalent circulating density (ECD), the optimization of operational parameters, and well control.

Calculation of ECD. This process includes four models:

Wellbore-temperature field model

Drilling-fluid equivalent-static-density (ESD) model

Drilling-fluid rheological-property model

A model representing the effect of cuttings concentration on ECD

The process involves the following four steps:

Establish the instantaneous wellbore-temperature model on the basis of the convection and thermal conductivity theory by dividing the wellbore into five areas.

Establish the ESD model by considering the elastic compression effect of high pressure and the thermal expansion effect of high temperature.

Establish the drilling-fluid rheological property model on the basis of the Herschel-Bulkley model by considering the effect

of ultra-HP/HT on dynamic shear force, consistency coefficient, and liquidity index.

Consider the effects of cuttings concentration on ECD on the basis of the solid/liquid two-phase flow.