Flow Assurance

What is the effect of field age in connection with flow assurance?

  1. Water-cut increases with time and thus the fromation of emultions
  2. Pressure reduces and deposition is easier. However, hydrate fromation is more difficult because of the lower pressure. 
  3. Boosting may be needed

What flow assurance problems are anticipated during planned or unplanned field shut-downs?

  1. The fluid in the SPS starts to cool down
  2. If the temperature is reduced below the hydrate formation temperature
  3. Wax formation

How to prevent/mitigate flow assurance problems during planned or unplanned field shut-downs?

  1. Cool down time can be desinged to be sufficient for the operator to take remedial actions such as:
    - blow-down to reduve the flowline pressure below the critical hydrate formation pressure
    - if hydrostatic head of liquids in the riser is high flowlines and risers can be pigged and displaced 
  2. Thermal insulation or heating of line and riser to reduce cool-down time.

Sands get in the produced fluids as the pass through the formation in to the well.

Sand can cause:

  1. Erosion of the SPS components
  2. Block of SPS
  3. If enough sand is produced a big void may be formed at the bottom of the well which can lead to a collapse and total destruction of the well casing and tubing

Eliminate or mitigate sand related flow assurance problems:

1. Reduce production rate. By reducing the production rate the hydraulic gradient of the flow in the well is also reduced. As as result the sand production will reduced because the flow through the reservoir pores will not be strong enough to pull sand from the reservoir. If the production of the field need to be kept then drilling another production well should be considered

2. Pigging and flushing flowlines if sand is deposited in the flowlines

 

4. Emulsions

Oil and water form emulsions that result in high pressure losses and reduction of production

Examples

Flow assurance Definition
Flow assurance is contined ability of subsea field to produce hydrocarbons efficiently thought the whole field life. the most critical task during deep water energy production because of the high pressures and low temperature (~4 degree Celsius) involved. The financial loss from production interruption or asset damage due to flow assurance mishap can be astronomical.

As flow moves from the reservoir to the topside process facility the temperature decreases. At the reservoir, the temperature is more than 100 C. At the seabed the ambient temperature is 4C and the fluid will start to cool down. Cooling down causes flow assurance problems such as:

Hydrate Fromation
Deposition of waxes

Mitigation of flow assurance problems is othen accomplissed by:

Thermal Insulation
Chemical injection
Heating

 

As the flow (mixture of oil, gas and water) moves from the reservoir to the process facilities the pressure decreases since:

a) the well fluids move upwards in the production tubing of the well. The larger the distance from the reservoir to the sea bed the larger the pressure drop (distance a, image 1). The distance a is in the order of magnitude of 2000-6000 m.

b) the well fluid passes through the wellhead, X-mas tree, the manifold and through the flowlines

c) from the upward flow in the riser and into the process facilities.

In the design of the system the pressure loss is taken into account during the hydraulic design. The input are the fluid properties, reservoir pressure and the flowline parameters. With input we can calculate the pressure through the system and check wheather the proposed production system will allow the oil to flow to the process plant and consider what should be planted for the pressure maintenance requirements,

Possible ways to boost the pressure are:

  1. water or gas injection
  2. well gas injection
  3. downhole pumping
  4. seabed pressure boosting (multiphase pumps, separation and then boosting, subsea separation and injection)
  5. riser lift

Problems associated with pressure loss

The dissolved gas in the flow of oil will bubble out of the oil. Thus, the flow becomes multiphase (liquide and gas). Mulitphase is undisirable because it us unstable and pressure boosting can only be applied with expensive and unreliable new technology such as mulriphase pumps or subsea seperation.

How can we maintain the pressure of the reservoir?

Reservoir pressure can be maintaned over the life of the field by water or gas injection. The injection well location and depth is selected such that the optimum result is accomplished