Importance of Thermal Consolidation of Shale During SAGD Process
- Axel-pierre Bois (CURISTEC) | Marc Mainguy (Total)
- Document ID
- Society of Petroleum Engineers
- SPE Heavy Oil Conference and Exhibition, 12-14 December, Kuwait City, Kuwait
- Publication Date
- Document Type
- Conference Paper
- 2011. Society of Petroleum Engineers
- 5.8.5 Oil Sand, Oil Shale, Bitumen, 4.1.2 Separation and Treating, 5.3.9 Steam Assisted Gravity Drainage, 2.4.3 Sand/Solids Control, 4.1.5 Processing Equipment, 5.1.10 Reservoir Geomechanics, 5.4.6 Thermal Methods, 5.5 Reservoir Simulation, 1.2.2 Geomechanics, 4.3.4 Scale, 5.5.8 History Matching, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.1 Reservoir Characterisation
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Most of the oil sand deposits contain shale barriers of various sizes and with a space distribution difficult to appraise. These shale barriers can lower the reserves and reduce the efficiency of the SAGD process by slowing down steam chamber propagation. The detrimental effect of shale barriers on the SAGD process has been demonstrated by many simulation works and research projects have been launched to develop new technologies favoring breakdown of shale barriers.
The effects of high temperatures on shale are difficult to assess. Field data from SAGD projects show that steam might either bypass shale barriers or breakthrough them, but the conditions favoring these mechanisms are unclear. Lab data show that the constitutive mechanical behavior of clayey materials is strongly sensitive to high temperatures. Of particular importance is the phenomenon of thermal consolidation of clay which was never pointed out to explain the conditions of failure of shale barriers during SAGD. Thermal consolidation denotes an irreversible contraction of clayey materials when heated under drained condition.
This paper presents the results of a project that aimed at evaluating if thermal consolidation could help increase permeability of shale barriers. A comprehensive review of published data shows that the origin of this phenomenon is still poorly understood despite some important factors can be highlighted. It also shows that stress path applied during thermal expansion tests on shale specimens strongly control the thermal consolidation phenomenon, which can lead to erroneous interpretations of these tests. It proposes a very simple model that is used to show 1) under which conditions thermal consolidation of shale barriers might happen; and 2) the consequences of thermal consolidation in terms of permeability changes of shale.
Most of the oil sand deposits contain shale barriers in the payzone of various thicknesses and with lateral continuity difficult to appraise. When produced with the Steam-Assisted Gravity Drainage (SAGD) process, the low permeability of the shale barriers slow down or even stop vertical propagation of the steam chamber in the payzone. In addition, water saturated shale layers have large specific heat such that some steam is lost in heating unproductive layers. Therefore, shale barriers reduce the efficiency of the SAGD process and are important factors contributing to the economics of a SAGD project.
As more oil sand deposits are being produced, less good-quality reservoirs are involved and the oil industry is left with more complex reservoirs that include shale barriers. Ultimately, if the oil sands deposits contain too many shale barriers, the reservoir may not be developed due to low economics. For that reason, the oil industry is looking for new technologies allowing production of the oil sand deposits without detrimental effect of intra-reservoir shale. Several technologies have been proposed: pressure cycling at high pressure to offset negative effects of shale barriers (Ipek et al., 2008), hydraulic fracturing to enhance steam chamber propagation (Chen et al. 2007), solvent co-injection or additional injector above shale barriers (Li et al., 2011). Unfortunately none of the above techniques was judged efficient and led to a pilot application.
The impact of shale barriers on the SAGD has been assessed with different approaches: in the laboratory, with reservoir simulations, and with direct observation on SAGD projects based on monitoring data.
Yang and Butler (1992) performed lab experiments on 2D reservoir models that were scaled to field conditions and allowed direct visualization of steam-chamber propagation. Several experiments were run with various sizes of shale layers including dipping barriers and injection above or below shale barriers. They concluded that short horizontal barriers located between injectors and producers do not significantly affect the general performance of SAGD process and that long horizontal barriers impact oil production not as much as expected.
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