SAGD Producer Well Placement and Bottom Water

Introduction

Bottom water can be found in numerous McMurray Formation reservoirs throughout the Athabasca region of N.E. Alberta. Bottom water is defined as a zone below the base of pay with a high water saturation (Sw), direct or non-direct contact. Figure 1 shows the bottom water zone as a dark blue colour fill and low resistivity reading. 

Figure 1 Bottom Water Cross-Section (Source: AER 2016 D54 File 8591)

Established by outcrop and vertical oil sands evaluation (OSE) well control, bottom water can have an unpredictable structure, characteristics and contact with the bitumen. A key technical challenge with placing a steam assisted gravity drainage (SAGD) producer is understanding the bottom water elevation relative to the pay zone. Without an overlying thick and laterally continuous mud barrier, 3D seismic can’t resolve the bottom water because of having a similar density to the bitumen. To mitigate this challenge operators apply a stand-off height (+4m) above the bottom water. 

The opportunity associated with this technical challenge is minimizing the stand-off height while achieving the operational conditions required for the daily production of these SAGD wells. Reducing the stand-off height by 0.5m can result in improved economics for a SAGD pad, dependent on reservoir quality and bottom water structure encountered.

The data and figures used in this article are sourced from the Alberta Energy Regulator’s Directive 054 In-Situ Performance (D54) presentation site, except where noted. The opinions shared are from my first-hand experience working on six SAGD assets and over 110 SAGD well pairs. These opinions don’t necessarily represent my former employers.

Background

Throughout the history of SAGD well placement, resistivity-based logging while drilling (LWD) technology has evolved. One challenge for this technology remains, the inability to look ahead of the drill bit. The depth of investigation (DOI) for today’s ultra-deep LWD (UDLWD) has dramatically improved, and is now capable of up to 30m DOI, reservoir condition dependent. While this scale of DOI is impressive and significant to understanding the reservoir architecture, limitations remain during the drilling process, because of the tool’s relative position to the bit in the bottom hole assembly (BHA). Dependent on the service provider, and the UDLWD tool configuration being used, the UDLWD could be 30m behind the bit. The positioning in the BHA combined with the unpredictable nature of the bottom water could still result in a well intersecting the bottom water.

Figure 2 shows a bottom water (dark blue colour fill) wireline log example identified by the resistivity log’s low reading, first column from the right. This ability to identify bottom water using geophysical wireline logs aids in confirming the attributes and elevation of the bottom water at a specific location.  

Figure 2 Bottom Water Log (Source: AER 2013 D54 File 10073)

By combining the OSE well picks with a 3D seismic derived Pre-Cretaceous unconformity structure map, insight to large-scale bottom water trends can be derived, see Figure 3. The cold colours (blues) represent thick bottom water and are correlatable to Pre-Cretaceous unconformity structural lows. But, at the scale required to confidentially plan and drill a producer’s well path, the elevation of the bottom water surface between the well control can be uncertain, see Figure 4. 

Figure 3 Bottom Water Thickness Map (Source: AER 2016 D54 File 10935)

Figure 4 Bottom Water Cross-Section (Source: AER 2016 D54 File 10935)

This uncertainty can manifest as unpredictable rapid changes in the bottom water characteristics and elevation, see Figure 5.

Figure 5 Christina River Outcrop Bottom Water (Sw=100%), yellow note pad is 20 X 12 cm. (Source: Fustic, M., et al 2012)

Alternative Trajectory Design and Execution Process

To mitigate the bottom water uncertainty during the drilling process an innovative use of the UDLWD tool combined with a horizontal pilot hole is a solution that should be considered. This solution was initially tested during the drilling of the 2016 Statoil Canada Ltd. Leismer Pad L5 infill well program using the Baker Hughes Visitrak LWD tool (Source: Vetsak, A. 2017). Since this initial test in 2016, the process of using a horizontal pilot hole coupled with an UDLWD tool has evolved and a version is discussed below.

Figure 6 Conceptual Diagram for Alternative Design and Execution Process (Source: Savage, M. Feb. 2019)

Using the OSE well control both along and offsetting the producer well path and by incorporating the 3D seismic, a preliminary top of bottom water structure surface (blue dashed line) is created, see Figure 6. This preliminary surface is then used to plan the pilot hole trajectory (red line) and the preliminary trajectory for the producer well (dashed black line). The horizontal pilot hole (main hole) and producer lateral section (sidetrack) will use the same surface hole, build section and intermediate casing point. The pilot hole and producer will be drilled using the same BHA components. The pilot hole will be drilled using a simple trajectory at an elevation estimated to be between the producer and injector.

Figure 7 Mapped Top of Bottom Water Structure Surface, Wells Only (Left) and Wells with LWD Data (Right). (Source: Vetsak, A. et al 2018)

The pilot hole UDLWD resistivity and LWD gamma ray data combined with the OSE well control information can then be used to develop a revised top of bottom water surface, see Figure 7. Figure 7 is an example from Leismer Pad L5 and highlights the structural surface details gained from the UDLWD data, right-hand image. Note the structural high region (warm colours) missed by the wells only data. This revised top of bottom water surface (blue colour fill polygon) is used to finalize the producer lateral trajectory (green solid line), see Figure 6. This added information can be used to reposition the producer well to minimize stand-off height, understand and avoid reservoir complexities and ultimately better operate the producer. 

Additionally, the drilling of the producer lateral section will be more efficient as the local geological uncertainty should be minimized with the UDLWD data and the revised bottom water surface.

Depending on the degree of reservoir heterogeneity, the thickness of the reservoir and the elevation of the pilot hole, the UDLWD data could enable an understanding of the top of pay surface and the associated reservoir characteristics. This information could be used during the daily operations of the well pair and to plan future well interventions, e.g. placement of tubing deployed flow control devices, steam splitters or plugs.

In closing, this article uses the bottom water uncertainty as the inspiration for the alternative well placement process (design and execution). This same process could be used for other reservoir related well placement challenges, e.g. complex reservoir geometries resulting from heterogeneous reservoir quality at or near the base of pay or variable fluid saturation zones (lean zones) within the reservoir and near the planned well placement.

References

Fustic, M., Bennett, B., Huang, H., Larter, S. R. 2012. Differential entrapment of charged oil – New insights on McMurray Formation oil trapping mechanism. Marine and Petroleum Geology 36 (2012), pages 50 – 69.

Vetsak, A., Jablonski, B., Theunissen, I. 2017. Increased Exposure Oil Reserves by Optimizing Wellbore Placement with Extra-Deep Azimuthal Resistivity LWD Service. EAGE Horizontal Wells 2017, Kazan, Russia, May 15 – 19.

Vetsak, A., Jablonski, B. 2018. Increased Exposed Bitumen Reserves by Optimizing Wellbore Placement in Oil Sands with Extra-Deep Azimuthal Resistivity LWD Service.