A design method for injection pressures to preclude formation shearing is presented. The elements of the procedure are: determine the maximum and minimum principal stresses; postulate a reasonable shear yield criterionfor the reservoir mass; ca1culate the pore pressure conditions at the point of yield; apply a small safety factor reduction to the yield pore pressure; apply corrections for losses to give an allowable injection pressure at surface; and, apply some minimum monitoring strategy to confirm the design. An example calculation is given, as well as means of estimating stresses and yield criteria in the absence of dependable data.
This article presents a design approach to calculate a maximum allowable surface injection pressure [Pmax] which will preclude formation shear yield. This value of Pmax can be used in conditions of water flood, gas injection, or as an upper limit for waste water injection wells. If shear failure of large reservoir volumes is precluded, hydro fracture is also extremely unlikely.
The term shear yield refers to the creation of a new slip plane in natural mated also It is not to be confused with hydrofracture, or with the reactivation of faults in tectonically affected reservoirs. The term yield is preferred to the term failure, as the latter implies a loss of function, and in the case of injection, a loss of function will not necessarily happen upon formation shear. Shear yield in any reservoir material is a strain weakening phenomenon and always leads to a discrete planar slip surface, the sides of which have experienced dislocations in opposite senses (Figure 1). The strain weakeningis due to the competence and density of reservoir rocks, and to the fact that injection-induced shear involves a reduction in the matrix stress, whereas strain weakening is suppressed on increases in matrix stress.
If a large volume of reservoir rock approaches a condition of shear yield simultaneously, yield will occur over a large plane, giving rise to a seismic event entirely analogous to a very small earthquake. Many reservoirs in Alberta which have been subjected to high pressure injection have exhibited induced seismicity, either due to triggering of old faults, or due to shear yield of new material.
The stress conditions at which shear yield will occur are known as the "Yield Criterion", and are expressed in matrix (effective) stresses, as these are known to govern all yield processes in porous media: Equation (1) (Available in full paper), where σ is the total stress and u is the pore pressure. Hence any computations must be in terms of matrix stress and the yield criterion is of the form:
Equation (2) (Available in full paper)
Equation (3) (Available in full paper)
It is usually assumed that strain behaviour does not affect the yield behaviour. While this is not strictly correct, it is a reasonable assumption for complex reservoir conditions. Also, it is assumed that only the minimum and maximum matrix stresses need be known. Under the conditions, and for the method suggested, this is also reasonable.
