ABSTRACT:

Frequent wellbore instability and lost circulation problems were encountered while drilling the first batch of exploration wells in a low-permeability, high pressure and high temperature (HPHT) gas field in the South China Sea, due to very narrow drilling mud weight windows resulting from HPHT reservoir conditions. This study focuses on combining and leveraging geophysical data, rock mechanics testing data, and well-logging data on predicting wellbore instability under HPHT conditions. Full-scale core samples were acquired from HPHT reservoirs of the field, and uniaxial and triaxial rock mechanics tests were conducted to determine the mechanical properties of the rock. The influences of heat transfer between the wellbore and the formation, mechanical properties, in-situ stresses, collapse pressure, and fracture pressure are investigated. Wellbore temperature change induced thermal stress result in an aggravated risk of wellbore instability. Furthermore, the safe mud weight window becomes narrower under the influence of temperature change while it was narrow originally because of the HPHT conditions. A dramatic pore pressure elevation in the reservoir formation leads to an extremely narrow mud weight window less than 0.1 g/cm3 in this oilfield.

1. Introduction

In the petroleum industry, wellbore instability is one of the most common problems in the process of drilling in exploration areas. This is due to the lack of available data and drilling experience. The costs associated with wellbore instability problems account for about 5% to 10% of drilling costs in the exploration and production phase, which means that the global annual costs can reach billions of dollars (Fjaer et al. 2008). Therefore, accurate estimation of safe mud density windows can provide guidance for safe drilling and reduce the cost (Baouchea,2020).

In this work, we studied the wellbore instability problem in a low-permeability, high pressure and high temperature (HPHT) gas field in the South China Sea. Frequent wellbore instability and lost circulation incidents were encountered while drilling the first batch of exploration wells in this gas field. These incidents are primarily due to the very narrow drilling mud weight windows resulting from HPHT reservoir conditions. Therefore, this work leverages multiple data, including geophysical data, rock mechanics testing data, and well logging data, to predict wellbore instability under HPHT conditions. The pore pressure, horizontal stresses, collapse pressure, and fracture pressure are estimated, respectively. The prediction results agree with the drilling performances quite well. Furthermore, the safe mud weight window of a planned horizontal production well is designed based on this integrated study.

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