Impact of Thermal Effect on the Fracture Plane and Formation Integrity During Cold Water Injection
Authors: Makris C. , Arogundade O. , Zhiyenkulov M. 
Published: April 2016

The extraction of petroleum and natural gas is an operation which results in the alteration of the subsurface stress distribution in geological formations. During cold water injection in these hot formations, the initial in situ stress magnitude and orientation is affected. These thermal effects have lead to a great deal, in both conventional and unconventional reservoir development.

In this study fully coupled 3D Mechanical Earth Model (MEM) simulation employing Finite Element and Finite Difference analysis was conducted to: Study the poroelastic and thermal effects on fracture propagation and formation integrity.Examine principle rock mechanics properties and its relation with thermal induced fracturing (TIF).

The analysis considers thermal flow, poroelasticity and thermoelasticity constitutive equations under isothermal and non-isothermal water flooding scenarios at different bottom hole injection pressures for a given time.

The research focusing on mechanical responses at two different reservoir rocks types for two different stress regimes based on E. M. Anderson's classification scheme. The widely used linear Mohr-Coulomb failure criterion is expressed in terms of Principal effective stresses assuming non-zero cohesion, for the determination of potential fracture plane. We conduct a detailed sensitivity analysis and the results indicate that the thermal effect becomes clearly obvious in cases of stiff reservoir rocks.

The reservoir rock was subjected initially in ideally triaxial conditions where Normal Stress regime was taking place. At late times of injection, the initial stress regime has been totally reversed (RF). Pore pressure increase causes pore extraction and tensile stress while the temperature (T) decrease create tensile load and thermal contraction to pore space. Hence, thermal tensile stress is added to tensile load during pore pressure increase causing inadvertent thermal induced fracturing.

Comparing the Sandstone and Limestone geomechanical response with respect to maximum (σ1)-minimum (σ3) effective stresses (psi) during thermoelastic loading, it is clear that in stiff reservoir rocks (limestone) where Young's modulus is large, cold water injection will lead to tensile or shear failure. This rock property combined with the thermal effect reflects to higher fracture deviation (Φxz) from Mohr-Coulomb theoretical plane.

We can conclude that thermally induced fracturing is typically observed when there is a wide differential between the reservoir temperature and the injected fluid. Thermal induced fracturing during cold water injection differs from hydraulic fracturing for well stimulation. In TIF, the propagation of the fracture is strongly dependent on the area of the cooled zone while in hydraulic fracturing the thermal effect is neglected. Moreover, the principal mechanism for TIF factures is the effect of thermally induced stress on fracture initiation pressure. Thermal induced fracture is the result of superimposed poroelastic and thermoelastic stresses in space and time.