Abstract

Effective resource management is required to assure safe And environmentally responsible disposal of fluid waste by injection wells and the development of oil and gas fields. Hydraulic fractures which break out through the upper confining stratum can carry hydrocarbon and the fluid waste up into the overlying formations, possibly contaminating water supplies

To prevent any breakdown of the upper confining stratum during water flooding and waste disposal operation, an understanding of stress state in the confining layer and pore pressure distribution in the injection zone is required. The stress gradient in the underlying confining layer is usually, higher than in the injection zone, therefore it is less likely to break and has not been considered in the study. The pore pressure distribution in the injection zone was considered for steady state, transient, and pseudo steady-state flow systems. The radial and tangential stress profile In the confining layer were developed under complete wellbore stress relief and under the conditions of applied wellbore stress. Analytical equations for one dimensional models have been developed for the radius of potential fracture region in the confining layer considering an anisotropic stress field around the wellbore. Subsequently, the effect of pure pressure on the horizontal stresses In the confining layer has been studied to determine the potential fracture zones.

Introduction

Attempts are made to confine injection or disposal fluids in the desired injection zone for hydrocarbon conservation and ground water protection. For the confinement of disposal fluid the classical approach emphasizes on thewellbore Integrity and avoids interface with offset wells. Any fluid movement behind pipe is usually attributed to bad cementing operations and in attempt to mitigate this problem secondary cementing is usually recommended. The fluid movement behind the pipe could, however, be the result of local and regional stresses in the overlying formations.

In the ground prior to injection, one of two possible conditions exists:

  1. there IS no pre-existing crack.

  2. a fracture starts from a pre-existing opening.

To initiate a fracture in condition (I). the injection pressure should be higher than the tensile strength of the rock. However, in case the of condition (2). an injection pressure slightly higher than the least principal stresses would propagate the fracture if the crack is normal to the least principal stress 1.

When a fluid is injected into a permeable formation with no pre-existing crack. the pore pressure is increased near the wellbore and it will approach the far field pressure away from the well bore. If the injection pressure is lower than that required to reduce the compressive stresses across a plane ill the permeable formation to zero, then there can be no fracture in that formation. However high pore pressure can be transmitted into the impermeable confining stratum, if there exists a small defect in the form of a crack or notch at the interface between the confining layer and the permeable injection zone.

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