A Risk-Based Methodology for Commingled Well Abandonment - Southeastern Alberta Gas Field Case Study

Commingled gas production is the production of gas from more than one geologically distinct zone in an unsegregated manner within a single borehole. Tens of thousands of commingled wells are producing natural gas across Alberta, all of which will eventually require abandonment. The abandonment of wells is strictly enforced using very specific requirements. The intent of these requirements is to prevent any unwanted impacts on the environment through gas or fluid migration, and on the ability of resource production companies to produce their fair share of hydrocarbon resources. At the time of publication, the Alberta Energy Regulator (AER) is examining if it can allow variances from the abandonment requirements for commingled zones where there is demonstrated low risk. The cost savings of such variances could be substantial to Alberta’s oil and gas industry, provided it can be shown that there is no intolerable increase in risk to the AER’s outcomes.

A risk-ranking screening tool was developed using the Alberta Table of Formations to qualitatively display the relative probability of risk from commingled well abandonments. This derivation uses knowledge of the regional geology and hydrogeology of the Alberta sedimentary basin to provide a stratigraphic zonation of groups and formations. If commingled zones or formations fall within green-coloured intervals, the risks to the AER’s outcomes are relatively low. If commingled zones or formations fall within red coloured intervals, the risks to the AER’s outcomes will be relatively high. The orange-coloured intervals represent a moderate risk to the AER’s outcomes. The risk rankings can guide a commingled well operator on how to apply for variances from abandonment requirements for certain geological units, and can be used by the regulator to guide the operator through the waiver request process. The different risk categories can be used to guide what is required to substantiate the application and satisfy the AER’s requirements in achieving its regulatory outcomes. Although green intervals might have fewer requirements in assessing risks and red intervals will likely have more, this does not necessarily mean that commingled well abandonments in red intervals will not be allowed, but rather that additional supporting materials will have to be provided to support any applications for red-interval wells. To assess what a red interval assessment might look like, the AER decided to look at the potential for the abandonment of a large number of commingled gas wells within the Southeastern Alberta Order area (Commingling Order No. MU 7490). The AER and Alberta Geological Survey undertook a detailed geological, hydrogeological, and petrophysical mapping and modelling exercise to understand the extent and properties of the gas-bearing and water-bearing units in the Milk River Formation and the Alderson and Medicine Hat members in the southeastern Alberta gas field. The geological, petrophysical mapping, and modelling results were used for numerical modelling of single-phase groundwater flow and two-phase gas and groundwater flow modelling to understand the consequences of allowing widespread commingled well abandonments.

In assessing the petroleum system and groundwater literature for the area, it was found that there are multiple plausible conceptual models of how the gas field petroleum system functions and the nature of regional groundwater flow. After compilation of the literature, five conceptual models were constructed. Each conceptual model was translated into a distinct groundwater flow model and two-phase flow model creating five equally plausible scenarios for a multimodelling exercise. In four of the five models, gas migration did not migrate into areas of potential concern during commingled well abandonment conditions, but for one of the models gas migration was observed in an area of concern. This suggests the need for further model validation achieved through additional data collection and environmental monitoring to ensure that the AER’s outcomes are satisfied.