Deepwater HP/HT Drilling-Fluid Development and Applications in the South China Sea
Authors: Carpenter Chris 
Published: October 2017

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 182429, “Deepwater HP/HT Drilling-Fluid Development and Field Applications in the Western South China Sea,” by Hexing Liu, Zhong Li, Yi Huang, Manzong Fang, and Zhiqin Liu, China National Offshore Oil Corporation, prepared for the 2016 SPE Asia Pacific Oil and Gas Conference and Exhibition, Perth, Australia, 25–27 October. The paper has not been peer reviewed.
Thirty percent of the deepwater wells in the western South China Sea have high-pressure and high-temperature (HP/HT) characteristics. After incorporating analyses of engineering and geological environments for deepwater HP/HT wells in the Lingshui Block, the authors suggest in the complete paper that the solution for drilling problems in deepwater HP/HT wells should start with drilling fluid. Researchers have developed a novel water-based-drilling-fluid system compatible with deepwater HP/HT wells in the Lingshui Block on the basis of a conventional drilling fluid and further optimization.
Introduction
Most of the reservoirs in the South China Sea are not only deeply buried with a low seabed temperature but also have HP/HT characteristics in the lower part of the well interval. The low temperature of the seawater environment causes an increase of drilling-fluid viscosity in the riser and deteriorates the rheology, leading to a buildup of equivalent circulating density (ECD) that might induce lost circulation. HP/HT reservoir conditions will also result in a narrow drilling-fluid window. The drilling fluid has to be HT-resistant and must prevent overflow and wellbore instability, as well as increase the safe drilling-fluid window.
The Lingshui deepwater gas field in the western South China Sea is a typical HP/HT reservoir. Its greatest operational water depth is 1688 m, and its highest well temperature is 167°C. The estimated reservoir temperature of the Lingshui Block is 176°C, with the maximum pressure coefficient reaching 2.01.
Challenges Encountered in Use of Drilling Fluids in the Western South China Sea
The major challenges encountered in the use of drilling fluids in deepwater HP/HT wells in the Lingshui Block can be placed into two general categories as follows:
The deep water and HP cause further narrowing of the drilling window.
The coexistence of HP and HT makes the drilling-fluid performance difficult to maintain.
The existence of a water interval of a few hundred meters or even a few kilometers in deepwater blocks usually forms a 2 to 4°C low-temperature interval from the seabed up to 500 m below sea level. The low-temperature environment gives rise to rheology variations of drilling fluid in the riser, which may cause a gelling effect with increasing viscosity and density, producing a higher friction in the wellbore flow and increasing the risk of formation leakage at the casing shoe. Conventional deepwater wells usually only focus on low-temperature resistance without considering the effect of HT. However, HT will cause degradation and cross-linking of certain drilling-fluid additives, weakening the antipollution characteristics and the water solubility of the additive.