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Abstract

A number of parameters influence the development of gas kicks in oil based drilling muds. The dynamic effects of operating conditions (drilling rate, pulp flowrate, well depth and casing depth), reservoir characteristics (permeability and formation pressure) and control methods (drillers and wait pressure) and control methods (drillers and wait and weight methods) have been studied with a dynamic simulator for gas kicks in oil based muds. The model accounts for the effect of pressure, temperature and gas solubility on the mud density, and the mass and momentum balance equations are solved to give the detailed pressure and flowrate distribution in the well as a function of time.

The control phase is studied in two kick cases; a distributed kick and a concentrated kick. A perfect control is performed using wait and weight and drillers methods, and the corresponding surface and casing shoe pressures are analyzed.

The study also identifies the most sensitive parameters for kick development, and gives guidelines on how to optimize the drilling operations related to well control.

The simulations are also analyzed with relation to early surface detection of kicks, and the most sensitive detection parameters are identified.

Introduction

The various aspects of gas kicks in oil based muds (OBM) have been in focus the last years. This applies to detection problems, special characteristics of such kicks, and well control methods. To be able to study the development and control of such kicks, advanced computer models have been developed. O'Bryan and Bourgoyne have developed an approximate method to estimate the swelling of OBM due to gas dissolution. Swanson et. al have performed computer simulations of kick development in deep, hot wells; showing that the higher the gas concentration in the ODM is, the easier it is to detect the kick. Rommetvelt and Blyberg have performed computer simulations revealing some of the performed computer simulations revealing some of the differences between kicks in OBM and water based mud (WBM) in the control phase of the kick.

This paper focus strictly on gas kicks in OBM. The effects of reservoir and operational conditions are studied in the kick development phase. Operational factors such as rate of penetration, pump flowrate, casing shoe depth, well depth and oil/water ratio of the mud are considered. In the control phase the drillers and wait and weight control methods are studied for two different gas distributions.

The simulations have been performed with a dynamic computer model for gas kicks in both water and oil based muds. This paper also briefly describes the governing equations and submodels of the simulator.

The model has been implemented as a well planning tool, and is under continuous development. Data from recent full scale gas kick experiments will be used to verify the simulator. Further plans include development of a PC-version for well design and planning, and a training version with interactive graphics, simulating real life control actions.

3 MATHEMATICAL MODEL

We assume that all variables depend on only one spatial coordinate (length along flowline) and neglect effects from cross sectionally non-uniform velocity profiles and mass distribution profiles. The temperature profile is assumed to be known at each point along the flowline. The fluid system is treated as a black oil system. Gas can exist either as free gas or as dissolved gas.

The basic equations are:

Mass conservation of mud:

(1)

Mass conservation of free gas:

(2)

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