Unitisation for CCS: Is it Coming? What will it Look Like? How to Prepare?
Authors: Peacock D. 
Published: October 2024

Abstract
The objective of this paper is to assess the issues related to subsurface cross-boundary flow of CO2. As Carbon Capture and Storage (CCS) projects expand in scale and number, it seems inevitable that CO2 injected into one licence will migrate into others, either by accident or design. Injection into saline aquifers outside of structural traps, where residual trapping may be dominant and uncertainties about plume migration are greater, may be especially prone to migration across boundaries.
If CO2 being injected into one licence migrates, or leaks, into a neighbouring licence, several issues may arise including entitlement and payments. Migration is typically considered to mean movement of CO2 within the storage complex; leakage is typically considered to mean movement of CO2 outside the storage complex. Confirmation that migration or leakage has occurred should be identified by monitoring plans, assuming such plans extend sufficiently. Liability for leakage into neighbouring blocks needs to be considered. Lack of unitisation provision may be a barrier to CCS projects going ahead. Lateral migration risk typically increases later in operational life and the early post-closure period.
Pressure management is a key issue for CCS projects. As CO2 is injected, pressure increases occur at distances much greater than where the CO2 plume is located and occur much faster. CCS projects can cause large-scale basin-wide effects. Situations may arise where projects interfere and compete within regional pressure compartments.
The issue will be assessed by considering experience and best practice from the petroleum industry and how it applies to CO2 subsurface storage. Unitisation practice has evolved over the years, especially in mature basins such as the North Sea, but regulation and practical implementation of unitisation varies widely throughout the world. CCS unitisation will likely follow a similar path, although lessons learned and good practice from hydrocarbon unitisation should ideally be followed.
Two scenarios are presented, with likely realities somewhere in-between. The first scenario envisages a situation with strong regulations, unitisation agreements or similar as a norm. Agreements would typically be simpler than many hydrocarbon agreements and may involve negotiated solutions although provision for redeterminations and expert referral may be appropriate in some cases. The second scenario is an unregulated scenario with rule of capture, or "rule of injector", being prevalent. This situation may lead to competing projects, legal action, and sub-optimal management of injection projects. An ideal scenario might be somewhere in between these extremes as over -regulation may also be detrimental.
The conclusion is that we can prepare by acknowledging the situation that CCS migration and/or leakage to adjacent licences is an issue and proactively deciding our future. Strong and appropriate regulations are critical. How the industry handles these situations depends on the reaction of governments, regulators, operators and other stakeholders. A desire by stakeholders to prevent "rule of injector" and a recognition of some the differences involved with CCS unitisation will be important as the CCS industry matures.
This issue has received little attention to date and this paper attempts to provide an overview of the subject to avoid problems and better prepare for cross-boundary flow of CO2 and related issues.