document preview

Abstract
The geological storage of carbon dioxide provides the possibility of maintaining access to fossil energy, while reducing emissions of carbon dioxide (CO2) to the atmosphere. One of the essential concerns in geologic storage is the risk of CO2 leakage from the storage formations. Leakage occurs through possible pathways in the seal, which include a) transmissive faults, b) abandoned wells (penetrating the entire seal or part of it), c) active wells that partially penetrate the seal, d) and local seal weakness and fractures.

CO2 leakage to the subsurface formations can adversely affect the existing and potential energy and mineral resources and shallow ground water resources and soils. As such, detection and characterization of CO2 leakage pathways from storage formations into overlying formations is necessary. The target aquifer could be tested for the leakage pathways before CO2 storage. This will allow for the determination of proper storage aquifers and locations for the injection wells. In this work, we suggest a flow and pressure test and present an inverse methodology to detect and characterize leakage pathways based on the pressure data.

The flow test is based on the injection (or production) of water into (or from) a storage aquifer at a constant rate. The pressure is measured at a monitoring well in an aquifer overlying the storage aquifer, which is separated by an aquitard. The objective of the test is to locate and characterize any leakage through the separating aquitard. The interpretation method is based on forward and inverse solutions of a new analytical model presented in an earlier work. We present an inverse procedure to obtain the leakage pathway transmissibility and location, based on the pressure measurements in an observation well completed in the monitoring aquifer. Inversion analysis is utilized to evaluate the capability of leakage parameters’ estimation through pressure monitoring.

1. Introduction
One way to cut the carbon dioxide (CO2) emissions is their capture and storage in deep underground formations. Deep saline aquifers have the volumetric capacity to store the immense quantities of CO2; however, for the long-term entrapment of the CO2, the target aquifer must be sealed by an impermeable cap rock. The cap rock may contain natural/man-made leakage pathways, such as improperly plugged abandoned wells, leaky faults, fractures, stratigraphic heterogeneities and other local weaknesses. The detection and characterization of any leakage pathways are required before storage operation can begin. 

In this study, a leakage test is introduced, the goal of which is the detection and characterization of single leakage pathways in the cap rock overlying a target aquifer based on pressure data measured at a monitoring well perforated in an upper aquifer. We want to detect the leak prior to injection of CO2. The idea is the injection of water into the aquifer and monitoring of the pressure at a monitoring well in an upper aquifer for a specific time period, e.g. 100 days. We investigate how the location and transmissibility of the leak can be obtained based on the pressure measurement. The side and plan views of the test configurations are shown in Figure 1.

The test is similar to a vertical interference test, where two formations are tested for communication. The simplest form of an interference test is performed in one formation and involves two wells: a producer (or injector) and a monitoring well. Multiwell interference testing usually involves one producer (or injector) and several monitoring wells. The producer (or injector) is opened for production (injection) at a constant rate for a reasonable length of time. The pressure is recorded and analyzed to find reservoir continuity and detect directional permeability and other major reservoir heterogeneity.