Analysis of potential leakage pathways at the Cranfield, MS, U.S.A., CO₂ sequestration site

Abstract

A 1.5-million-ton CO₂ sequestration project took place in a 3000-m-deep historical oilfield, combined with a CO₂-EOR flood. The Cranfield reservoir is found within a multikilometer domal structure related to a deep-salt diapir and consists of fluvial sediments of the Tuscaloosa Formation. An earlier analysis determined that plugged and abandoned wells provide the most likely leakage pathways to aquifers and potentially to the ground surface. Fourteen Cement Bond Logs (CBL's) were used to assess the risk. The present quality of the cement bond ranges from excellent to poor.

Geological insights, stochastic numerical modeling of the pressure field, analysis of the CBL's, and application of a wellbore flow model were used to conclude that the limited pressure increase and mostly intact wellbores result in a low CO₂- and brine-leakage risk. Statistical estimates of well properties suggest that at most two (and possibly none) could be capable of conveying a total of 1800 kg/yr CO₂ to the surface (0.0002% of annual injection rate). Given that the oilfield is an active operation, it is improbable that well leakage to the surface will go unnoticed and certain that risks will be managed through active risk mitigation and remediation if necessary.

Introduction

A risk assessment or analysis (RA) of processes and events that can derail an otherwise perfectly planned enterprise is often undertaken before the launching of any large industrial endeavor. Developing an enhanced oil recovery (EOR) operation to extract some of the remaining oil from a depleted oil reservoir that has historically produced is no different. When Denbury Onshore, LLC from Plano, TX (the operator), made the decision to invest in the Cranfield, Mississippi (MS), field in the mid-2000s, the operators performed standard technical, financial, and economic analysis of the project and brought with them their practical expertise, stemming from several similar operations across the southeast U.S. In agreement with Denbury and with its operational support, the site was also chosen by the Bureau of Economic Geology (BEG) to achieve some of the goals set forth by the DOE-sponsored SECARB partnership (Hovorka et al., 2013). Phase III of the collaborative project is geographically focused on the northeast section of Cranfield field. This limited section of the field has been called the HiVIT (high-volume injection test) and includes a smaller area, called the DAS (detailed area of study), on which BEG has performed many tests and research investigations (Fig. 1). The HiVIT area is the focus of this paper. There, the operator and BEG collaborated to inject 1.5 million tons of CO₂ over 1.5 years (Choi et al., 2013), both within the historically determined boundaries of the field and into the downdip leg of the field, where the DAS is located.

Developing a CO₂-storage operation next to a historical and active oil-production site introduces the issue of leakage through wells and prompted a risk-assessment study. However, the work presented here is not a risk assessment of the oil operation itself, which is handled by the operating company. The study followed the certification framework (CF) procedure (Oldenburg et al., 2009). CF focuses on events that are deemed the most likely to cause problems from past historical evidence, that is, wellbores and faults, as well as spill points (Fig. 2). The objective of the present analysis is to investigate the potential impact of these three categories of features on site integrity. We first describe the relevant attributes of the site and the volumes potentially impacted from the surface to the reservoir itself. We then present details of the methodology to estimate leakage potential and impacts and then apply the method to calculate risks semi-quantitatively. A discussion of the results follows that also includes likely attenuation factors limiting the negative impacts of some events.