Pemex controls lost circulation in South Mexico

Aug. 2, 1999
A reactive, water-based, lost-circulation material (LCM), used by Pemex Exploration-Production Co. in South Mexico, can effectively control lost circulation in a single treatment.

A reactive, water-based, lost-circulation material (LCM), used by Pemex Exploration-Production Co. in South Mexico, can effectively control lost circulation in a single treatment.

In addition to eliminating lost circulation problems in the Rio Nuevo, Sen, and Fortuna Nacional fields, this system helped Pemex Exploration-Production Oil Co. to rescue a Rio Nuevo well from abandonment, saving about $750,000 in well costs.

The LCM treatment, consisting of a blend of clays, polymers, buffers, crosslink, and dispersants, can be used to stop losses of drilling and completion fluids. In addition, it can help increase and restore the integrity of weak formations, extend casing shoe points, and eliminate intermediate casing strings.

How it works
The LCM treatment reacts with drilling muds to create a barrier at the face of the lost-circulation zone. After reacting with the mud, the LCM develops into a moldable consistency typically within 30 sec.

This moldable consistency allows the material to form an adjustable, ductile, nonbrittle bridge against the opening of the fluid-loss zone, allowing it to conform to well bore or near well-bore fractures as it changes shape. Because this treatment only minimally penetrates the formation matrix or fracture system, the reactive, water-based LCM causes little formation damage.

In laboratory tests, the material penetrated samples less than 1/64 in. LCM treatments are available for use with water, oil, and synthetic-based muds.

Pumping procedure
Because the LCM reacts with the drilling fluid, a compatible spacer must be pumped ahead of and behind the material. The general pumping procedure for treatments, developed by Halliburton Energy Services Inc., is as follows.

The lead spacer, LCM, and tail spacer are pumped through the drillstring. The blow-out preventers (BOPs) are then closed and an oil-based mud is pumped though the drillstring until the leading edge of the spacer has cleared the drillpipe.

To initiate reaction, oil-based drilling mud is simultaneously pumped through the work string and annulus at the same rate. When the drilling mud and water-based LCM intermix, they react to form a moldable plugging material that can effectively seal off the lost-circulation zones.

After placement, the LCM can be removed from the well bore almost immediately, and drilling can continue with higher equivalent circulating densities (ECDs).

Benefits
In addition to stopping circulation in natural and induced fractures, vugs, channels, weak zones, and underground cross-flow zones, the LCM treatment has the following benefits:

  • It can be pumped through most bottom-hole assemblies (bit, motor, measurement, and logging-while-drilling tools).
  • It usually requires less treatment volume (10-60 bbl) than other circulation-loss treatments. A treatment volume is the LCM slurry volume plus an equal volume of mud/well bore fluid.
  • A single treatment is usually effective, helping reduce rig time.
  • Treatment does not require removing pipe from the hole, which helps to reduce or eliminate trip time.
  • The quick reaction time can reduce or eliminate time spent waiting on strength development of conventional cements or lost-circulation treatments.
  • Treatments are not dependent on mud chemistry.

Case 1
In April 1998, Pemex drilled a well in Rio Nuevo, Reforma, to 10,496 ft, whereupon an asphalt zone prevented further drilling. To avoid this drilling problem, the crew set a 500-ft cement plug to conform to the hole wall.

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When the operator began to drill through the plug, the formation began to lose fluid at a rate of 25 bbl/hr, eventually increasing to 106 bbl/hr. The crews tried to control the lost circulation with other treatments, failing in these attempts.

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To stop the lost circulation, the reactive, water-based LCM treatment, consisting of 25 bbl of lead spacer, 20 bbl of LCM, and 19 bbl of tail spacer, was applied to the zone (Figs. 1-3). To help ensure success of the LCM treatment, the bottom of the well was circulated to condition the 13.30 ppg oil-based drilling mud.

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Next, the crew ran a tri-cone roller bit to a depth of 9,361 ft and performed an injection test to help ensure an effective plug design. This prejob test consisted of closing the BOP and pumping through the drill stem and annulus at a rate of 4 bbl/min until the surface pressure reached 1,700 psi (Table 1).

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The LCM treatment procedure consisted of five steps:

  1. Shut the well down and wait for the pressure to decrease. It reached 950 psi in 1 min and 0 psi in 10 min.
  2. Conduct a surface-reaction test to verify that the mud and LCM would react properly.
  3. Pump the lead spacer, LCM, and tail spacer.
  4. Displace the lead spacer until the leading edge of the spacer reaches the bottom of the bit.
  5. Close the BOP and simultaneously pump oil-based drilling mud down the drillpipe and annulus at the same rate.

When a surface pressure of 2,200 psi (17.40-ppg ECD) was obtained, pumping was discontinued, followed by monitoring the surface pressure for 15 min. When no loss of surface pressure was observed, the crew slowly released the surface pressure, opened the BOPs, and lifted the drill stem.

The LCM treatment was successful and allowed Pemex to continue drilling. The savings achieved by using this method are shown in Table 2.

Case 2
Concerning the second lost circulation problem in this well, encountered in May 1998, a third-party service company set a cement plug, and Pemex decided to continue drilling. While drilling, however, Pemex exceeded the well`s ECD of 14.7 ppg by 1.75 ppg and the well began to lose mud at a rate of 106 bbl/hr until the asphalt zone prevented further drilling.

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To save the well, a cement plug was set in front of the asphalt zone, then cement was squeezed into the formation, and the reactive, water-based LCM treatment, consisting of 25 bbl of lead spacer, 20 bbl of LCM, and 19 bbl of tail spacer, was again pumped down (Fig. 4).

To help ensure success, the bottom of the well was conditioned with a 13.30 ppg oil-based drilling mud. Next, the crew ran a tri-cone bit to a depth of 9,361 ft and performed an injection test to help ensure effective plug design. This prejob test consisted of closing the BOP and pumping through the drill stem and annulus at 4 bbl/min until the surface pressure reached 1,200 psi.

The LCM treatment consisted of 6 steps (Table 1):

  1. Stop pumping and monitor the surface pressure. It decreased to 0 psi in 5 min.
  2. Mix the LCM treatment.
  3. Conduct a surface reaction test to verify that the mud and LCM would react properly.
  4. Pump the lead spacer, LCM, and tail spacer.
  5. Displace the lead spacer until the leading edge of the spacer reaches the bottom of the bit.
  6. Close the BOP and simultaneously pump oil-based drilling mud down the drillstring and annulus at the same rate.

After the crew stopped pumping, the pressure equalized in 5 min to 1,300 psi. When they obtained a surface pressure of 1,700 psi, the crew shut the well in and monitored the pressure for 10 min. When no loss of surface pressure was observed, the crew released the pressure, opened the BOP, and lifted the stem without problems.

The LCM treatment successfully saved the well, resulting in the costs savings shown in Table 2, and allowed Pemex to begin well production.

The Authors
Rafael Rodriguez Monroy is an engineer for the well services department at Pemex Exploration-Production Oil Co. He graduated from Universidad Aut?noma de Mexico in 1982 with a degree in petroleum engineering.

Hector Mandujano is an engineer for the drilling engineering department at Pemex. He graduated from Instituto Polit?cnico Nacional in 1974 with a degree in petroleum engineering.

Jabal Tejeda is an engineer for the technology development department at Pemex. He graduated from Instituto Polit?cnico Nacional in 1962 with a degree in chemical engineering.

Epitacio Solis is in charge of Pemex's drilling department in the southern region of Mexico. He graduated from Instituto Polit?cnico Nacional in 1985 with a degree in petroleum engineering.

Francisco Rueda is a technical advisor for production enhancement and zonal isolation. For the past 6 years, he has worked for Halliburton Energy Services Inc. in the southeast Mexico area. Rueda graduated from La Salle University in 1990 with a degree in mechanical engineering.