Method and apparatus for controlling pressure and detecting well control problems during drilling of an offshore well using a gas-lifted riser

A method for controlling the pressure at the base of a gas-lifted riser during drilling of an offshore well, said method comprising the steps of:

• determining riser base pressure (prb) and the mass flow rate into the base of said riser ({dot over (m)}i);
• using a throttling device located at or near the top of said riser to adjust the mass flow rate out of the top of said riser ({dot over (m)}o) and the riser surface pressure (prs) to compensate for changes in the riser base pressure (prb);
• and
• adjusting one or both of the boost mud flow rate (qb) and the lift gas flow rate (qg) to substantially minimize variations in the mass flow rate into the base of said riser ({dot over (m)}i).

A method for controlling the pressure at the base of a gas-lifted riser during drilling of an offshore well, said method comprising the steps of:

• determining boost mud density (ρb), boost mud flow rate (qb), lift gas density (ρg), lift gas absolute temperature (Tg), lift gas flow rate (qg), riser mix density (ρmix), riser mix absolute temperature (Tmix), well return density (ρw) and riser base pressure (prb);
• calculating the well return flow rate (qw), where
• qw=Aqg−Bqb,
• A=([Tmix/Tg]ρmix−ρg)/(ρw−ρmix),
• B=(ρb−ρmix)/(ρw−ρmix);
• calculating the mass flow rate into the base of said riser ({dot over (m)}i), where
• {dot over (m)}i=ρwqw+ρbqb+ρgqg;
• adjusting the mass flow rate out of the top of said riser ({dot over (m)}o) and the riser surface pressure (prs) to compensate for changes In the riser base pressure (prb);
• and
• adjusting one or both of the boost mud flow rate (qb) and the lift gas flow rate (qg) to minimize variations in the mass flow rate into the base of said riser ({dot over (m)}i).

A method for controlling the pressure at the base of a gas-lifted riser during drilling of an offshore well, said method comprising the steps of:

• a) determining a setpoint value for riser mix density (ρmix);
• b) determining the actual value of riser mix density (ρmix);
• c) adjusting one or both of the boost mud flow rate (qb) and the lift gas flow rate (qg), to substantially minimize the difference between said setpoint value and said actual value;
• d) determining well return flow rate (qw) and drill string flow rate (qc);
• e) comparing the drill string flow rate (qc) to the well return flow rate (qw) to detect well control problems such as kicks or lost circulation;
• f) determining boost mud density (ρb), boost mud flow rate (qb), lift gas density (ρg), lift gas flow rate (qg), lift gas absolute temperature (Tg), well return density (ρw), riser mix density (ρmix), and riser mix absolute temperature (Tmix);
• and
• g) calculating the well return flow rate (qw), where
• qw=Aqg−Bqb,
• A=([Tmix/Tg]ρmix−ρg)/(ρw−ρmix),
• B=(ρb−ρmix)/(ρw−ρmix).

Apparatus for controlling the pressure at the base of a gas-lifted riser during drilling of an offshore well, said apparatus comprising:

• means for determining riser base pressure (prb) and the mass flow rate into the base of said riser ({dot over (m)}i);
• a throttling device for adjusting the mass flow rate out of the top of said riser ({dot over (m)}o) and the riser surface pressure (prs) to compensate for changes in the riser base pressure (prb), said throttling device being located at or near the top of said riser;
• and
• means for adjusting one or both of the boost mud flow rate (qb) and the lift gas flow rate (qg) to substantially minimize variations in the mass flow rate into the base of said riser ({dot over (m)}i).

Apparatus for controlling the pressure at the base of a gas-lifted riser during drilling of an offshore well, said apparatus comprising:

• means for determining boost mud density (ρb) boost mud flow rate (qb), lift gas density (ρg), lift gas absolute temperature (Tg), lift gas flow rate (qg), riser mix density (ρmix), riser mix absolute temperature (Tmix), well return density (ρw) and riser base pressure (prb);
• means for calculating the well return flow rate (qw), where
• qw=Aqg−Bqb,
• A=([Tmix/Tg]ρmix−ρg)/(ρw−ρmix),
• B=(ρb−ρmix)/(ρw−ρmix);
• means for calculating the mass flow rate into the base of said riser ({dot over (m)}i),
• where
• {dot over (m)}i=ρwqw+ρbqb+ρgqg;
• means for adjusting the mass flow rate out of the top of said riser ({dot over (m)}o) and the riser surface pressure (prs) to compensate for changes in the riser base pressure (prb);
• and
• means for adjusting one or both of the boost mud flow rate (qb) and the lift gas flow rate (qg) to minimize variations in the mass flow rate into the base of said riser ({dot over (m)}i).

Apparatus for controlling the pressure at the base of a gas-lifted riser during drilling of an offshore well, said apparatus comprising:

• means for determining the actual value of riser mix density (ρmix);
• means for adjusting one or both of the boost mud flow rate (qb) and the lift gas flow rate (qg) to substantially minimize differences between said actual value of riser mix density (ρmix) and a predetermined setpoint value of riser mix density (ρmix);
• means for determining the drill string flow rate (qc);
• means for determining he well return flow rate (qw), wherein said means for determining comprises means for determining boost mud density (ρb), boost mud flow rate (qb), lift gas density (ρg), lift gas flow rate (qg), lift gas absolute temperature (Tg), well return density (ρw), riser mix density (ρmix), and riser mix absolute temperature (Tmix);
• means for calculating the well return flow rate (qw), where
• qw=Aqg−Bqb,
• A=([Tmix/Tg]ρmix−ρg)/(ρw−ρmix),
• B=(ρb−ρmix)/(ρw−ρmix);
• and
• means for comparing the drill string flow rate (qc) to the well return flow rate (qw) to detect well control problems such as kicks or lost circulation.