Casing Integrity in Hydrate Bearing Sediments Reem Freij-Ayoub, Principal Research Engineer CESRE Wealth from Oceans

Well integrity in hydrate bearing sediments (HBS) JIP sponsors Shell Global Solutions Heriot Watt University Institute of Petroleum Engineering

Outline What are gas hydrates Possible drilling and well completion problems The model The dissociation algorithm Cases studied Results Conclusions/Future work

Ice-like structures composed of water and natural gas molecules Under conditions of high pressure and low temperature, water molecules form cages which encapsulate gas molecules inside a hydrogen-bonded solid lattice Large gas storage capacity: 1 volume of gas hydrate contains up to 180 volumes of gas at stp Gas hydrates

Drilling and well completion problems Gas hydrate-related drilling problems (Adapted from Maurer Engineering, Inc.) Gas hydrate-related casing problems (Adapted from Maurer Engineering, Inc.) Hole enlargement Casing collapse Hydrate dissociation  Gas release (gasified mud)  Loss of cohesion

Model Casing heating During drilling of lower sections of the wellbore or During production

Dissociation algorithm Φ: current porosity Φ o : initial porosity V c : volume of the Structure I crystal Porosity Pore Pressure

Strength degradation with hydrate dissociation Friction angle UCS, cohesion, tensile strength C is cohesion in MPa UCS is unconfined compressive strength in MPa Φ is angle of internal friction in degrees ( o ) is tensile strength in MPa porosity in percent (%). Tan et al. (2005)

Well integrity in HBS Geomechanical strength- petrophysical correlations Heat transfer into the formation Hydrate dissociation & strength degradation algorithm In situ stress & PP formation, cement & casing strength

Model and cases studied Cases studied-SymbolCement Strength Hydrates Case 1: strong cement and no hydrates in sediments (S-NH). strongno Case 2: weak cement and no hydrates in sediments (W-NH). weakno Case 3: strong cement with hydrate bearing sediments (S-H). strongyes Case 4: weak cement with hydrate bearing sediments (W- H). weakyes

Formation properties ParameterValue Biot’s coefficient1.0 Sediment porosity0.25 Sediment porosity of the middle layer after hydrate dissociation 0.4 Thermal conductivity1.4 Wm -1 K -1 Specific heat capacity1.9 x10 3 JK -1 kg -1 Linear thermal expansion coefficient of hydrates 7.7x10 -5 K -1 Linear thermal expansion coefficient of pore fluid 30x10 -5◦ K -1 sediment solid dry density2.800 kgm -3 Water density1030 kgm -3 Modulus of Elasticity of sediments MPa Poisson’s ratio0.35 Cohesion1.7 MPA Angle of internal friction 33.17º Tensile strength0.53 MPa Water depth800 m Top level of hydrate layer below seabed 20 m Bottom level of hydrate layer below seabed 60 m In-situ temperature15 ºC (288 K) Gas constant JK -1 Mol -1 Hydrates crystal volume 1.728x m 3 Avogadro number x10 23 In situ stress ratio1

Cement and casing properties ParameterValue Casing properties Linear thermal expansion coefficient of steel 37x10 -7◦ K -1 Thermal conductivity1.4 Wm -1 K -1 Casing thickness0.635in Casing Poisson’s ratio0.3 Young’s modulus of steel210 GPa Casing yield stress379 MPa Casing external diameter20 in Cement-casing bond properties Coupling spring tensile strength limit1x10 20 MPa Coupling spring cohesion limit1x10 20 MPa Coupling spring friction angle0º Initial temperature3 ºC (288 K) Casing raised temperature33 ºC (298 K) Casing density7.85 x10 3 kg m -3 Casing top axial load1.37 million Pound Force Cement thermal properties Thermal conductivity0.66 Wm -1 K -1 Specific heat1.9x10 3 JK -1 kg -1 Thermal expansion7.7x10 -5 K -1 Strong cement Modulus of elasticity55.16 GPa Poisson’s ratio0.4 cohesion11.4 MPa Friction angle10º Tensile strength2.6 MPa Weak cement Modulus of elasticity807.6 MPa Poisson’s ratio0.35 Cohesion1.7 MPa. Friction angle10º Tensile strength0.53 MPa

Von Mises stress at t=0

Maximum von Mises stress

Ellipse of plasticity

Thrust The profile of the thrust (normal force per linear meter of casing length) along the casing for the case of strong cement, and hydrates in the sediments, tension positive compression negative. The combined effect of heating the casing and the dissociation of hydrates creates compressive normal forces in the casing at the interval where hydrates exist.

Maximum thrust The absolute maximum thrust (hoop stress) in the casing decreases with heating. This maximum thrust occurs at the top of the casing for all the cases studied and is detected close to the base of the hydrate layer in case of its presence after 4 days of heating. No risk of hydrostatic buckling is found.

Seabed subsidence or heave After heating Before heating

Formation yield Tensile failure Hydrate bearing layer Casing After heating Before heating Maximum yield radius Location of maximum yield radius

Conclusion In the scenario considered the casing remains safe. The main impact of heating the casing and dissociating the hydrates was on the formation integrity. It is necessary to consider fluid flow (one or two phase flow) and its impact on reducing the pressure on the casing. This requires certain assumptions about the permeability of the cement and whether it will serve as a flow channel or not. Such a model should also allow for the reformation of hydrates. The accurate consideration of fluid flow requires modelling crack growth which can be done in a discrete modelling code. It is important to examine the effects of the depth-proximity to seabed- and thickness of the hydrate layer.

Model Fault reactivation and flow through faults using ABAQUS Future work