A Coupled Model To Predict Interformation Flow Through an Abandoned Well
- Elise A. Striz (U. of Oklahoma) | Michael L. Wiggins (U. of Oklahoma)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- February 2002
- Document Type
- Journal Paper
- 11 - 22
- 2002. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 4.1.5 Processing Equipment, 2.2.2 Perforating, 3 Production and Well Operations, 5.1.1 Exploration, Development, Structural Geology, 1.14 Casing and Cementing, 2.4.3 Sand/Solids Control, 6.5.3 Waste Management, 1.2.3 Rock properties
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This paper presents a novel approach for predicting flow between two formations coupled by an abandoned well. A steady-state, abandoned-well model is derived analytically and applied with a "series of steady states" to create the transient flow response. The model is verified by comparison to the results of a transient, analytical, interformation well model for a single injector given by Avci. This model is the first to include true pressure losses in the abandoned wellbore calculated from established equations for turbulent or laminar pipe flow, plugs, and a casing perforation. The model can also predict flow behind pipe through an open or plugged annulus or a fracture in the annular plug. Finally, the model is the first to incorporate the effect of pressure variations on well flow caused by production or injection in the overlying formation.
Application of the model in a single-phase simulation demonstrates the impact of location and condition, rock and fluid properties, initial pressures, and well operations in the coupled formations on the abandoned-well flow. Evolving pressure distributions in the coupled formations may move flow up or down the hole. Pressure losses in the wellbore or behind pipe (through plugs) are shown to preclude flow. The model is also applied to a field case to demonstrate its use as a prediction tool for abandoned-well flow. As opposed to the limited analytical solutions or complex numerical- simulation methods currently available, the new abandoned-well model provides a simple means to determine flow for realistic field conditions.
Abandoned wells may pose an environmental concern because of their potential to act as conduits for flow between otherwise hydrogeologically isolated formations, including underground sources of drinking water (USDW). Of greatest concern are unplugged abandoned wells, those with compromised casing, or those lacking cement behind pipe or with fractures in the cement behind pipe. The transport of fluids through abandoned wells is of particular interest to the petroleum industry, as the pressure buildup from Class II injection wells in target reservoirs can act as a driving force for interformation flow. Class II injection wells are defined by the Environmental Protection Agency (EPA) as wells that are strictly associated with the production of oil and gas, including saltwater disposal wells and secondary recovery wells.
To address the problem, the EPA developed the underground injection control (UIC) program to protect USDW from potential abandoned-well flow near Class II injection wells. Under UIC, Class II injection wells are subject to area-of-review (AOR) regulation, which requires all abandoned wells within a "radius of endangering influence" to be located and evaluated for plugging, with corrective action to be taken if adequate plugging cannot be demonstrated.1 Variances from the AOR may be obtained if the risk of migration into a USDW is minimal based upon2:
The absence of USDW.
Pressure relationships between the injection well and overlying USDW.
Presence of local geologic conditions that preclude fluid movement.
Other compelling scientific or engineering data supporting the issuance of a variance.
A methodology has been developed and applied to obtain variances for Class II injection projects based on the first three mitigating factors.3,4 Two other studies have used single-phase numerical simulation to show that transport to a USDW would not occur for a proposed Class II injection well based on fluid diversion to other formations and assumed plugging in the abandoned wellbore to obtain a variance under Risk 4, but the numerical simulations were very labor-intensive.5,6
Currently, there is no simple method to realistically quantify fluid transport to USDW through an abandoned well in the area of a Class II injection operation to determine if a variance from the AOR regulations may be obtained. The literature contains analytical models that are for simple physical systems or complex numerical simulations that lack the versatility to incorporate many of the factors truly influencing transport. None of the available models directly address the influence of wellbore resistance conditions, such as pipe friction, plugs, and casing perforations, on abandoned- well flow. In addition, the models do not account for behind- pipe flow and fracture flow in the annulus. Finally, they do not consider the impact of evolving pressure distributions from more than one active well in the coupled formations on flow rate and the direction of flow. Therefore, they fail to incorporate the critical factors shown to control abandoned-well flow rates.
This work was undertaken to develop a realistic and practical method to predict the transport of fluids between two formations coupled by an abandoned well. A steady-state abandoned-well model is derived analytically and applied with a "series of steady states" to create the transient flow response. The model is verified by comparison with the results from the one available, transient, analytical, interformation well model for a single injector given by Avci.7 The new model is the first to include true pressure losses in the abandoned wellbore, calculated from established equations for turbulent or laminar pipe flow, cement plugs, and a casing perforation. The model can also predict flow behind pipe through an open annulus, a plugged annulus, or a fracture in the annular plug. Finally, the model is the first to incorporate the effect of pressure variations caused by production or injection in both the coupled formations on the abandoned-well flow.
The model is applied in a single-phase simulation to predict interformation abandoned-well flow. Hypothetical cases are investigated to demonstrate the many factors that influence abandonedwell flow and assess their impact on enhancing or mitigating flow. The model is also applied to one field case to demonstrate its use as a prediction tool.
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