Pressure Calculations and Kick Detection with Synthetic-Based Muds in a Riserless Drilling Configuration
Authors: Lima H.R. , Barrufet M.A. , Juvkam-Wold H.C. 
Published: October 2008

This paper presents an analysis of the temperature and dynamic pressure behavior occuring in a Riserless Drilling (RD) configuration, deepwater well when circulating a ynthetic-based mud (SBM). Also discussed are some aspects of kick detection in this new configuration related to the gas solubility of SBM's.

A numerical model has been developed to simulate the mud flow through the Open-Seawater-Drillstring-and-Return-Line configuration system. The simulator is based on the numerical solution for the system of flow equations using finite-difference approximations. The model accounts for the pressure and temperature dependence of mud viscosity and mud density. Through a sensitivity analysis, the effects of pertinent parameters - specially the circulation rate - on the pressures generated in the system are examined.

Also, an EOS model was calibrated with PVT data to investigate the swelling of the synthetic-based mud containing dissolved gas. An equation for the solubility of methane in synthetic oil and a procedure for calculating the pit-gain with increasing time are presented.

Introduction

The petroleum industry is contemplating drilling in waterdepths of 10,000 ft in Brazil and West Africa and possibly 13,000 ft at the turn of the century in the U.S. Gulf of Mexico1. The current offshore technology for deepwater drilling operations requiring the use of a 21-in marine riser seems to have reached its limit at water depths about 7,500 ft.

Riserless Drilling appears to be the solution for the challenge that faces the industry today. The RD concept involves utilizing a bare drillstring and a separated nonconcentric return line. A rotating Blowout Preventer (RBOP) caps the return drilling fluid and forces it to circulate through the return line to the surface. A subsea pump is designed to maintain a constant inlet pressure equivalent to the hydrostatic pressure of seawater at the sea floor (Fig. 1).

Use of SBM has increased over the last decade in offshore drilling operations because of their lower environmental impact. However, there is a particular problem experienced when designing hydraulic programs with SBM's, i.e., the strong pressure and temperature dependence of SBM density and viscosity. In a RD system the problem tends to be worse following the combination of high pressures with low temperatures at the sea floor. Lost circulation can follow unreliable predictions for equivalent-circulating-density (ECD) made without accounting for the effects of pressure and temperature on SBM density and viscosity. Also, gas is highly soluble in the oil phase of this type of mud. Therefore, the dissolution of gas after a gas kick is taken in a SBM may mask the surface responses and the kick shall travel undetected up to near the surface. Dangerous amounts of gas may be released over a short period of time catching the drilling crew virtually by surprise.

For this reason the primary objective of this study is to present a mathematical model to calculate the dynamic pressures during routine drilling operations in a RD system based on the circulating temperature distribution and changes in downhole mud properties with pressure and temperature. The secondary objective is to describe the surface responses associated with a gas kick taken when using SBM in a RD configuration.