Above-zone pressure response to distinguish between fault and caprock leakage

Document Type

Conference Proceeding

Publication Date

1-1-2017

Abstract

Deep saline aquifers are widely considered for injection/disposal of fluids (e.g. CO2 and wastewater) in the subsurface. However, target injection aquifers may be overlain by a regionally leaky caprock, which allows for fluid migration from the injection zone. In addition, target aquifers may also be intersected by leaky faults, which can accommodate leakage by connecting the target aquifer to other permeable zones. Leakage from the injection zone to an above permeable zone will be associated with pressure rise in that zone. The above zone pressure response has recently been extensively investigated in studying leakage in the context of CO2 geological sequestration. Both leaky fault and leaky caprock are considered as potential leakage pathways. Before analyzing above-zone pressure response for leakage characterization, it is important to determine whether the feature causing leakage is a fault or the semi-pervious caprock. The focus of this study is to provide a diagnostic tool to identify the leaky fault from a leaky caprock based on the pressure response. A fault is a planar interface that can act as a conduit both normal and through the fault plane. Caprock is a very low permeability medium which is assumed to act as sealing layer to the reserved fluids. However, there may be heterogeneities associated with regional permeability variations in the caprock, which can accommodate leakage. An injection well makes a local high pressure region near the well that may result in significant leakage through the caprock or a fault intersecting the reservoir. In this work, we investigate leakage from an injection layer into an overlying layer through a leaky fault and a leaky caprock based on the pressure changes. We present analytical models for pressure response to interlayer communication via a caprock and a leaky fault. The fault leakage causes a linear flow in the overlying layer. However, leakage through the cap rock is different in nature for which most leakage occurs at the vicinity of the injection well The flow regime occurred in the above-zone due to leakage are the base of our method to identify the leaky fault from the leaky caprock. Results show that fault leakage makes linear flow and caprock leakage show late-time radial flow in the above-zone. In order to detect these flow regimes by the common pressure transient analysis methods, the observed pressure in the above-zone must be normalized based on the time variable leakage rate.

Publication Source (Journal or Book title)

SPE Western Regional Meeting Proceedings

First Page

1

Last Page

14

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