Design of Caprock Integrity in Thermal
Stimulation of Shallow Oil-Sands Reservoirs
Yanguang Yuan, SPE, and Bin Xu, SPE, BitCan G&E Incorporated; and Claes Palmgren, SPE, Alberta Oilsands Incorporated
Stakeholders in in-situ oil-sands development take caprock-integrity issues seriously. The industry is faced with the challenge of determining an optimal operating pressure in the reservoir where, in general, the pressure should stay significantly low to ensure the caprock integrity while being significantly high for enhanced oil production and economics. This paper presents a comprehensive work program on the subject for a shallow oil-sands play.Caprock
Caprock integrity considers the induced stress and deformation in a caprock during the thermal stimulation of an oil-sands reservoir. A minifrac-test program is undertaken to define the original in-situ stress state. Laboratory tests are carried out to measure the deformation and strength properties. Simulations are run to calculate the induced stresses and evaluate them against the mechanical strength. This paper describes some important quality-control issues for these activities. For the minifrac tests, multiple cycles and use of flowback are promoted for enhanced efficiency and accuracy. Laboratory tests are recommended on whole cores in a drained condition at a slow strain rate. Numerical simulations should use site-specific and laboratory-measured material properties. On the basis of the limited sensitivity analyses, the thermalexpansion coefficient of the reservoir and Young’s modulus of the caprock are found to significantly affect the caprock deformation and/or induced stresses.
Collectively, caprock refers to a certain interval in the overburden rock formations above a petroleum reservoir containing the reservoir fluids within the reservoir. It is normally shaly, with high clay content and low permeability. Sometimes, it immediately overlies the pay zone. In other cases, there is a buffer zone between the caprock and pay zone. During petroleum exploitation, the caprock plays an important role in safeguarding against the hydrocarbon fluid, stimulating materials, and/or their mixture invading zones above the caprock. Often, these zones contain groundwater aquifers.
Ultimately, caprock integrity considers hydraulic integrity—no reservoir fluids should escape through the caprock into the groundwater aquifers or to the surface. In general, the hydraulic integrity is already maintained naturally, as in the geological history of the caprock preventing further upward hydrocarbon migration. It is the process-induced mechanical deformation and potential failure of the caprock during thermal operations that may introduce new hydraulic conduits and thus compromise the hydraulic integrity. Therefore, hydraulic integrity becomes a mechanical integrity issue.
Caprock integrity also considers caprock mechanical integrity (i.e., deformation and failure in the caprock strata). For example, surface heave, which is rock deformation reflected on the ground, can alter the environment by changing the landscape or the surface or shallow subsurface hydrogeological conditions. Such surface heave could damage surface installations and infrastructures, and have other unintended impacts. Furthermore, rock deformation and failure (e.g., reactivation of pre-existing weak planes) can damage the well casing, breaking its hydraulic-sealing capacity.
Whether hydraulic or mechanical, caprock integrity becomes a geomechanical issue related to caprock deformation and potential failure. Caprock-integrity analysis compares the prevailing stress conditions against the material strength. The ongoing stress condition is the induced stresses superimposed on the virgin in-situ stresses. Therefore, there are three major components to any geomechanical work program: determination of the original in-situ
stresses, evaluation of the induced stresses, and measurement of the mechanical properties. Minifrac tests are the most reliable method to measure the in-situ minimum stress; the induced stresses are normally inferred from geomechanical simulations; and the mechanical properties should be measured ideally from cores obtained from the project sites. This general geomechanical work program is followed in the oil-sands industry now. However, special considerations are warranted for oil-sands development
where the formations are relatively shallow. This paper will present some relevant examples of how to ensure quality control in these special cases.
Both the industry and government regulatory agencies take the caprock-integrity issue seriously. For the in-situ development of the Alberta oil-sands reservoirs, systematic efforts in this regard were first initiated for cyclic-steam-stimulation (CSS) operations (Smith et al. 2004). More recently, steam-assisted gravity drainage (SAGD) has become another mainstream commercial in-situ oil-sands recovery process. SAGD is normally thought to be gentler on the caprock because it operates at a lower pressure than
CSS. However, as the operational experience with SAGD grows, evidence indicates that proactive precautions are still necessary to safeguard the caprock integrity. Yuan (2008) presented theoretical arguments about why attention also should be paid to integrity issues in SAGD. The current paper will apply these theoretical principles in the context of shallow oil-sands reservoirs.
In the following sections, case histories are given to illustrate important quality-control issues in various caprock-integrity studies. First, complexities encountered in the oil-sands and the corresponding best practices during minifrac tests are described. Next, geomechanical laboratory tests are explored. The subsequent section is dedicated to a study of the geomechanical simulations which combine both the field-obtained data and laboratory-measured material properties to derive the safe SAGD operating pressure. The effect of reservoir depth on caprock integrity is also specifically discussed.
The target reservoir discussed in this paper belongs to the McMurray formation sandstones of the Lower Mannville in the Fort McMurray area. Fig. 1 offers a schematic about the general stratigraphic column. The reservoir is generally less than 100 m deep, but its thickness is approximately 40 m, making the asset attractive. The Clearwater formation constitutes the regionally continuous caprock, which is mostly shale with interbedded siltyto-sandy mudstone. The caprock thickness ranges from 46 to 61
m. A Wabiskaw member is sandwiched between the Clearwater
caprock and McMurray pay zone. It is a marine shore face system
conformably overlying the McMurray formation. It may also contain
sand facies that can be bitumen-saturated. Therefore, the Wabiskaw member constitutes a transitional buffer zone from the reservoir pay zone upward to the caprock. A solvent-assisted lowpressure SAGD is proposed to develop the reservoirs (Palmgren et al. 2011).
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