Design of a two-well field test to determine in situ residual and dissolution trapping of CO₂ applied to the Heletz CO₂ injection site

Abstract

Field testing is a critical step to improve our knowledge on in situ-trapping mechanisms of CO₂ injected in geological formations and their relative importance. In this study, we present a two-well test sequence aimed at quantifying field values of both residual and dissolution trapping of CO₂. Then, we apply it to the Heletz experimental CO₂ injection site, using numerical modelling. The sequence includes a hydraulic test to measure residual scCO₂ saturation and a novel tracer technique, together with measurements of abstracted fluid compositions for quantification of the rate of CO₂ dissolution in the reservoir. The proposed tracer technique uses a tracer with negligible aqueous solubility, which is injected with the scCO₂ and enriched in the scCO₂ phase as CO₂ dissolves. We show that this tracer can provide direct information about the dissolution of mobile scCO₂. We also show that the rate of abstracted dissolved CO₂ can be used to predict the total rate of CO₂ dissolution, provided that the amount of dissolved CO₂ in the formation stabilizes, and that this can be achieved with the proposed abstraction scheme. We conclude that the combination of these measurements is a promising tool for detailed field-scale characterization of residual and dissolution trapping processes.

Introduction

Carbon dioxide capture and storage (CCS) is a potential key technology, in the global effort of mitigating the impact of greenhouse gas emissions to the atmosphere. However, successful implementation of CO₂ geological storage projects requires that secure storage at the proposed field sites can be demonstrated. CO₂ trapping by different processes improves the storage security and are critical for the fate of the injected CO₂ as well as storage security and reservoir capacity in many geological settings under consideration for CO₂ geological storage. Highly-controlled scientific CO₂ injection experiments at the field scale are critical both for the demonstration of CCS technology and to improve current knowledge about CO₂ trapping processes at field scale, under the actual conditions of typical storage sites.

Due to the temperature field prevailing in the candidate storage layers, geologically stored CO₂ remains buoyant in brine even at depths where it has been compressed to a dense supercritical state (>800 m) and tends to migrate upwards by buoyancy unless it is trapped by other processes. The main general trapping mechanisms are (i) structural and stratigraphic trapping, (ii) residual phase trapping, (iii) dissolution trapping and (iv) mineral trapping (IPCC, 2005). A structural trap in the form of a low-permeability cap rock layer overlying the target formation for storage is generally a prerequisite for taking a site into consideration for CO₂ geological storage. While mineral trapping typically becomes significant only after long times (order of 1000s of years), residual phase and dissolution trapping can contribute to increased storage security already from the end of the CO₂ injection phase. CO₂ geological storage techniques primarily relying on these trapping mechanisms have also been proposed (e.g. Qi et al., 2009, Suekane et al., 2008), for example in open, mildly dipping formations, which lack a true structural trap (Akervoll et al., 2006).

Laboratory studies have investigated fluid migration and residual phase trapping in supercritical CO₂ (scCO₂) – brine systems at the core scale (e.g. Krevor et al., 2011) and in analogous two-phase systems at the bench scale (e.g. Fagerlund et al., 2007a, Fagerlund et al., 2007b, Polak et al., 2010). CO₂ dissolution and convective mixing has been investigated experimentally (Kneafsey and Pruess, 2010, Neufeld et al., 2010) as well as by theoretical analyses (e.g. Riaz et al., 2006). Several modelling studies have also been conducted to further study both residual phase trapping and dissolution trapping (e.g. Hesse et al., 2009, Pau et al., 2010).

Well-controlled field experiments investigating residual phase and dissolution trapping in situ, under influence of geological heterogeneity, however, remain scarce. Yet, field testing is critical to improve understanding of how the CO₂ trapping will take place in situ and to assess the relative importance of the different trapping mechanisms at field sites for geological storage. Given the challenge to measure fluid flow and trapping processes in kilometre-deep reservoirs with few boreholes and limited knowledge of the spatial distribution of geological parameters, the design of field tests that can accurately quantify the CO₂ trapping is also challenging. At the Frio site, Texas, a small-scale CO₂-injection was passively monitored at a nearby up-dip well (Doughty et al., 2008) using fluid sampling, tracers, well logs and cross-hole seismics. This study demonstrated the importance of combining different measurements for better characterization and the integration of these in a flow and transport model for the field site. At the Ketzin site, Germany, CO₂ injections have been monitored from three boreholes using geophysical, hydraulic and tracer techniques (e.g. Wuerdemann et al., 2010). Experiences from Ketzin highlight the importance of geological heterogeneity on migration and fate of the injected CO₂ (Lengler et al., 2010).

Zhang et al. (2011) have proposed a single-well test method to measure the residual gas saturation S_gr (of scCO₂) using a combination of hydraulic, thermal and push–pull tracer tests at the Otway field site, Australia. Zhang et al. (2011) demonstrate that the hydraulic test where CO₂-saturated water is injected into the formation containing residually trapped CO₂ is highly sensitive to S_gr because the relative permeability to the aqueous phase is reduced in presence of residual scCO₂. In the thermal test the formation is heated from the borehole and then allowed to cool. The temperature in the borehole depends on the thermal conductivity, which in turn depends on the fluid saturations. Thus, it can also be used to measure S_gr. This test can reach a couple of metres into the formation from the borehole depending on the time and intensity of heating as well as the reservoir properties and conditions.

The retardation of a tracer in the flowing phase (e.g. water) due to partitioning into an immobile immiscible phase (e.g. gas or oil) can also be used to infer the presence and quantity of the immobile phase. This can be done both in single-well push–pull tests and two-well interwell tests. Partitioning interwell tracer tests (PITTs) have been developed and used in the petroleum industry for measurement of residual oil saturation (Du and Guan, 2005, Tang, 2005, Sinha et al., 2004). PITTs have also been used to measure non-aqueous phase liquid (NAPL) saturations in NAPL-water two-phase systems (e.g. Nelson et al., 1999, Jin et al., 1995) and the unsaturated zone (e.g. Mariner et al., 1999). However, field methods to determine dissolution of CO₂ in situ have, to the best of our knowledge, not been presented in literature.

Within the EU-FP7 MUSTANG project, a series of field experiments are being designed for the Heletz field site, Israel (Fig. 1), where two wells to the target formation for CO₂ injection have recently been drilled and are currently being completed and instrumented. The series of experiments will include well logging, flowing-fluid electrical conductivity logging as well as hydraulic and tracer tests without CO₂ to characterize the formation prior to CO₂ injections. Then, a series of experiments involving CO₂ injections will be performed to characterize parameters and processes affecting the fate of the injected CO₂. Specifically CO₂ trapping processes will be characterized using both single-well and interwell tests.

In single-well push–pull experiments, the influence of geological heterogeneity on the fluid flow and transport parameters is somewhat reduced because the fluids are pushed out and pulled back through the same flow channels. In interwell tests the processes are affected by geological heterogeneity between the wells. The combination of single-well and interwell tests has therefore been deemed effective for studying CO₂ trapping processes as well as the effect of heterogeneity on these processes. In a dipole test with active abstraction of fluids from one well, a controlled, converging flow field is established which facilitates careful monitoring of abstracted fluid composition and tracers.

In this study, a two-well dipole test sequence for quantifying both residual and dissolution trapping of CO₂ in situ is proposed. The sequence includes: (i) a hydraulic test to measure residual scCO₂ saturation and (ii) a novel tracer technique together with measurements of abstracted fluid compositions for quantification of the effective rate of CO₂ dissolution in the formation. The hydraulic test will be repeated before and after CO₂ injection and the residual scCO₂ saturation will be inferred from differences in pressure responses. The proposed tracer technique uses a tracer with very small or, preferably, negligible aqueous solubility, which is mixed into and injected with the scCO₂. As the CO₂ dissolves into formation brine, the tracer will be enriched in the scCO₂ phase. The hypothesis therefore was that when scCO₂ arrives at the abstraction well, its tracer content carries direct information about how much dissolution this mobile scCO₂ has undergone on its way through the formation. Using numerical modelling, the proposed test sequence was applied to the Heletz field site, for which such test is under planning. The two main objectives of this study were: (i) to present and demonstrate a viable two-well test sequence for the quantification of both effective residual-phase and dissolution trapping of CO₂ in situ as the CO₂ flows through the storage formation and (ii) to test the hypothesis that an scCO₂-borne tracer of negligible aqueous solubility provides useful information for the quantification of in situ CO₂ dissolution in this test configuration. Additional tests to further improve an integrated determination of the trapping parameters, such as thermal tests, can easily be added to the proposed test sequence but are not the focus of this study.