Home > Ground Freezing > Prediction of Ground Thaw Formations around an Oil Well

Prediction of Ground Thaw Formations Around an Oil Well

Oil well operations in permafrost areas cause the formation of thaw bulbs around wellbores, that may result in borehole and pipeline buckling failure. Consequently, well design requires the simulation of the permafrost thermal regime and thaw bulbs around the well cluster.

 

Today, Frost 3D Universal is the most convenient tool for performing such simulations. In order to create a computer model of the borehole thermal influence on the permafrost, the following information is needed:

 

1. Meteorological data: air temperature variation, wind speed, change in snow cover thickness.

2. Geological soil structure and thermophysical properties around simulated boreholes: thermal conductivity and volumetric heat capacity in thawed and frozen state, initial temperature of the water-ice phase transition, density of dry soil, total gravimetric soil moisture (over all types of soil water), dependence of unfrozen water content on temperature.

3. Temperature and velocity of pumped oil

4. Well structure and thermophysical properties of used material (cement, thermal insulation, etc.)

 

Based on this data, Frost 3D Universal creates a three-dimensional simulation model for the thermal influence of the boreholes on permafrost.

 

Computer model of borehole thermal influence on permafrost

Computer model of boreholes’ thermal influence on permafrost

 

Geometric dimensions of the computational domain (4 production wells and 5 different soil types) are: length – 60 m, width – 40 m, height – 200 m.

 

Appropriate thermophysical properties are specified for each geological layer.

Parameters material short text
Layer1 Layer2 Layer3 Layer4 Layer5
Volumetric heat capacity of thawed ground,
 J/(м3оС)
2.92×106 3.20×106 2.89×106 2.86×106 2.79×106
Volumetric heat capacity of frozen ground
J/(м3оС)
1.83×106 2.20×106 2.01×106 1.92×106 2.19×106
Heat conductivity of thawed ground, W/(m•оС) 1.86 1.8 1.78 1.8 1.27
Heat conductivity of frozen ground,W/(m•оС) 2.1 1.88 1.91 1.93 1.85
Total gravimetric soil moisture, % 0,2 0.3 0.25 0.15 0.25
Density of dry soil,Kg/m3 1400 1600 1400 1400 1600
Phase transition temperature, оС 0 -0.2 -0.1 -0.6 -0.2

 

The dependence of unfrozen water content on temperature is also given for each geological layer.

 

Unfrozen water content on temperature dependence

Dependence of unfrozen water content on temperature

 

Initial time is specified for vertical temperature distribution over the soil depth. To consider the influence of air and ground surface on heat exchange, the changes in snow cover thickness are specified.

 

Initial temperature distribution over the soil depth

Initial temperature distribution over the soil depth

Snow cover thickness and temperature changes

Snow cover thickness and temperature changes

 

The heat exchange coefficient and changes in air temperature over the time(based on wind speed) are specified on the boundaries of the computational domain and atmosphere by means of boundary conditions.

 

Changes of air temperature over time

Changes of air temperature over time

Ground surface and air heat exchange coefficient changes

Ground surface and air heat exchange coefficient changes

 

On the side surface of the computational domain, heat flow is equal to zero because the left and the right boundaries lie on the plane of symmetry, and the front and back boundaries of the computational domain are at a sufficient distance from the simulated wells (the heat flow from the well does not reach these boundaries).

The heat flow is equal to zero because of identical soil layer and borehole extension below the lower boundary of the computational domain (heat flow through the lower bound is equal to zero). Thermal interaction between the wells and the ground around them are simulated by the third type boundary conditions. Thermal properties, velocity of pumped oil, and borehole heat insulation thickness are taken into account when calculating the heat exchange coefficient between the ground and the borehole wall.

 

Specification of boundary conditions for borehole thermal analysis

Specification of boundary conditions for borehole thermal analysis

 

Simulation of bulb thawing and thermal field changes was performed for a 20-year period. From the simulation results, thermal field distribution around boreholes was analyzed at specific moments in time in different sections of the 3D simulation area.

 

Soil thermal field distribution over 5 years in the XZ plane

Soil thermal field distribution over 5 years in the XZ plane

 

Frost 3D Universal ground analysis

Soil thermal field distribution over 5 years in the YZ plane

 

Thermal borehole simulation on permafrost

Soil thermal field distribution over 20 years in the XZ plane

 

Borehole permafrost thawing with Frost 3D Universal

Soil thermal field distribution over 20 years in the YZ plane

 

Thermal field simulation can also be represented with isolines in the cross section of the simulation area.

Thaw front around the borehole

Soil thermal field distribution over 20 years in the 2D plane in the form of temperature isolines

 

Similarly, we can analyze thaw bulbs around wellbores. The relative distribution of unfrozen water amount in the ground is shown below. The red color corresponds to regions of the soil were all ice is melted; the blue color corresponds to regions in which all the moisture is frozen.

 

Thaw bulbs around well cluster

Relative unfrozen water content distribution after 5 years in the XZ plane

 

Ground thawing around borehole

Relative unfrozen water content distribution after 5 years in the XZ plane
 

Thawing of the soil along the oil well

Relative unfrozen water content distribution after 20 years in the XZ plane

 

Borehole in permafrost

Relative unfrozen water content distribution after 20 years in the YZ plane

 

Thus, visualizing the relative distribution of unfrozen water content in the cross section of the boreholes, we can determine the size of thaw bulbs around wellbore at specified points in time, and draw conclusions regarding the effectiveness of borehole insulation and the selected distances between them.

 

Thaw bulbs around borehole

Relative unfrozen water content distribution after 20 years in the 2D plane

 

FROST 3D UNIVERSAL

Software package for simulation of heat processes in grounds with the account of:

 

  • Phase transitions
  • Filtration
  • Snow cover thickness
  • Operation of cooling devices
  • Heat impact of buildings and constructions
  • Complex ground structure

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Christopher Demez
CEO
Spectral Pixel. Belgium

 

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