Laboratory Rock Physical and Geological Analyses of a SAGD Mannville Heavy Oil Reservoir, Senlac, West-Central Saskatchewan 1

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1 Laboratory Rock Physical and Geological Analyses of a SAGD Mannville Heavy Oil Reservoir, Senlac, West-Central Saskatchewan 1 D. Rokosh 2 and D. Schmitt 2 Rokosh, D. and Schmitt, D. (2004): Laboratory rock physical and geological analyses of a SAGD Mannville heavy oil reservoir, Senlac, west-central Saskatchewan; in,, Sask. Industry Resources, Misc. Rep , CD-ROM, Paper A-13, 9p. Abstract A project has been undertaken to measure geophysical and geological properties of samples from a Cummings- Dina heavy oil reservoir near Senlac (Twp 40, Rge 25W3M and 26W3M). The reservoir is undergoing Steam Assisted Gravity Drainage (SAGD) and has been the subject of a 4-D (i.e., time-lapse) single component, seismic experiment to image saturation changes in the reservoir. Velocity measurements of reservoir rocks are being taken at a suite of pressures and temperatures, including those that mimic reservoir conditions. This will allow us to model how in-situ variations in velocity over time affect seismic amplitude and travel time of the reservoir. The model can then be compared to real data, where differences in the data sets may be attributed to the factors that affect velocity variations such as reservoir pore pressure, temperature, and saturation. Keywords: Senlac, heavy oil, SAGD, rock physics, 4-D time-lapse geophysics, seismic velocity, SEM, ESEM, mineralogy, thin section, Cretaceous, Mannville, Dina. 1. Introduction The conventional development of oil and gas properties involves geological and engineering evaluations of the structure and fluid-flow properties of a reservoir based primarily on exploration seismic data, drill core, and geophysical log descriptions. These evaluations are integrated into a model of the reservoir, the temporal behaviour of which may be predicted using a reservoir simulation program. This is generally as far as conventional practice goes. Geological and engineering data derived from wells are spatially limited because of the distance between wellbores. In recent years, geophysicists have gone beyond the traditional role of interpreting geological structure and stratigraphy for the benefit of petroleum prospect generation and have begun to interpret the physical properties of rocks from seismic sections to determine rock properties between wellbores. If rock properties are to be inverted from seismic imaging data, then a fundamental pre-requisite is to have considerable confidence in the behaviour of seismic velocity and density with changes in effective stress, temperature, and saturation in the interval of interest. From 1999 to 2002, the Seismic Heavy Oil Consortium (SHOC) at the University of Alberta shot, acquired, and processed multiple data sets along lines trending east-west (seven profiles) and north-south (four profiles) over a SAGD project in the Cummings-Dina Oil Pool at Senlac in west-central Saskatchewan (Figures 1 and 2). All geophones are buried, placed at the same depth below surface, and surveyed to ensure maximum repeatability. Our purpose is to develop inexpensive, but repeatable, high spatial resolution geophysical methodologies to monitor fluid movement during production, with special focus on heavy oil reservoirs in the Western Canada Sedimentary Basin. The lines are shot and acquired using a 6000 lbs VI Minivibe (10 to 550 Hz variable sweep) with shear-wave capability, and a 240-channel state-of-the-art distributed seismic-acquisition system. All receivers and shot point locations are surveyed with our Trimble Geographical Positioning System. To gain a better understanding of the behaviour of seismic waves and changes in density in the formation during injection and production, a rock physical and geological study of the reservoir rocks and fluids accompanies the seismic acquisition and ongoing processing and interpretation stages. 1 Funding is provided to the principal researcher, Prof. D.R. Schmitt, through the Seismic Heavy Oil Consortium (SHOC), of which Husky Energy and Alberta Energy Research Institute are members. 2 Institute of Geophysics, Department of Physics, P412 Avadh Batia Physics Lab, University of Alberta, Edmonton, AB T6G 2J1; drokosh@phys.ualberta.ca and doug@phys.phys.ualberta.ca. 1

2 Well Symbols SHOC Seismic Lines Horiz. Well; symbol at well toe Drilling pad for the horizontal well pair Oil Susp Oil N Since we have only recently begun our laboratory measurements, this report will briefly summarize our rationale for this study, outline our sampling strategy, and present some of our initial results and observations of Scanning Electron Microscope (SEM) and Environmental Scanning Electron Microscope (ESEM) imaging. Abnd Oil Gas Susp Gas Abnd Gas D & A Water Disposal Water Source Observation Well T40 R26W3 1 6 Figure 1 - Location of the seismic program and sample sites near Senlac, Twp 40, Rge 25W3M and 26W3M. Sample sites are indicated by large circles. 2. Background and Rationale for the Study Seismic properties of a saturated porous rock critically depend on a variety of extrinsic factors including saturation, confining stress, pore pressure, temperature, and phase saturation state. These factors control the overall density and elastic properties of the material, and influence the seismic response of the geological structure and the characteristics of the reflections (such as their amplitudes). The strength and character of a reflected seismic wave depends on the contrast in both the density (ρ) and the seismic-wave velocity (v) of two materials (i.e., impedance Z = ρv) on either side of a contact. Hence, the seismologist is very interested in what the rock density and velocity might be as reflections are obtained only when the impedance of the different earth materials contrasts sufficiently strongly. The rock physics relationship to seismic response can be complex. Existing experiments indicate that the most important factors influencing changes in seismic velocity are stresses that confine the rock, pore pressure, saturation state, exsolution of free gas phase above the bubble point of gas-liquid mixtures, and temperature. Structural damage to grain contacts may also be important, but this has not been studied much. Temperature is often assumed to be the only factor detected in seismic experiments probably because it is the primary factor controlling viscosity and flow of heavy oils and is easily measured with in situ detectors. It considerably lowers the wave velocity in laboratory experiments (Wang and Nur, 1988; Eastwood, 1993) by as much as 20% over a range of temperatures to 200ºC. However, the seismic properties of earth materials, and unconsolidated reservoirs in particular, are highly sensitive to the other important factors mentioned above. Most of these lower the velocity to the same degree; indeed, the effect of free gas in particular on seismic reflectivity has been well known for some time and exploited as a gas exploration tool (i.e., bright spot ). Scaling problems are an additional concern in that most rock physics laboratory experiments have been carried out at 1 MHz frequencies, and therefore differ by about four orders of magnitude from field measurements, where frequencies of seismic waves are typically up to ~100 Hz. Some fundamental issues related to viscosity/frequency effects in liquids, such as frequency-dependant velocity dispersion, have not yet been resolved; this is a problematic area that we hope to address in the future in the rock physics lab. Common current industry practice is to calculate seismic velocity by inverting sonic log travel time within the formation. Although numerous problems such as wellbore fluid invasion, cycle skipping, noise, and the scale of measurement relative to seismic need to be considered, this is the method of choice and often provides the only in situ information available regarding velocity. The primary goals of our project are to measure P- and S-wave velocity on real, semi-consolidated and unconsolidated, heavy-oil saturated sands at known variations in temperature and pressure, and to apply this knowledge to improve the interpretation of seismic time-lapse images of the reservoir. Geological properties presently of interest include mineralogy and texture (especially grain size, fabric and cementation; see also Rokosh and Schmitt, 2002, 2003) R25 2

3 3. Sampling Strategy Permission was granted to extract core samples from four wells near Senlac (Figure 1, Table 1) for rock physical and geological analyses. In addition to P- and Swave velocity analyses, other analyses planned for the samples are SEM, ESEM, thin section, and mineralogy. Environmental Scanning Electron Microscope analysis will be performed before and after cleaning the sample of oil using a Soxhlet extraction system in the laboratories of the Department of Civil and Petroleum Engineering, University of Alberta. Scanning Electron Microscope and ESEM analyses should improve our knowledge of the rigidity (e.g., bulk moduli) or framework of unconsolidated sands and the influence on wave propagation and velocity. Thin section and mineralogical analyses have also begun on the samples. Particular attention was paid to the type of cements, if any, present in the sands, to the distribution of cement within the grain framework, and to the determination of heavy oil as a glue that binds the grains. The type and occurrence of cement or lack thereof has an important influence on seismic velocity (Prasad et al., 2002). Figure 2 - Four seismic profiles obtained from July 2001 to October 2002 along the north-south seismic line shown in Figure 1 (from Zhang and Schmitt, 2003). Interpretation of these profiles by correlation to changes in injection and production, and perhaps further processing, will be aided by our laboratory studies of seismic wave behaviour. Our goal is to isolate differences in seismic wave behaviour due to changes in the reservoir rather than due to differences in seismic acquisition or processing. The similarity of the present output indicates that our acquisition and processing procedures are very repeatable. The sample locations are as follows: a) the lower part of the Dina channel in CS Senlac OBS A W3 below the oilwater contact (Figure 3); b) the middle of the Dina sand from CS Senlac SWD A W3 within the oil-bearing reservoir (Figure 4); c) two samples were taken in Dome Senlac W3 (Figure 5), where a limestone core plug was taken about 60 cm below the pre- Table 1 - Location of samples taken at Senlac for the rock physics and geological analysis project. Location Depth Formation/Member Environment/Lithology W3M ~751 m Dina Member Channel, below O/W contact W3M ~ m Dina Member Channel, above O/W contact W3M ~747.7 m ~753.0 m Cummings Member Duperow, below unconformity Grey Shale Limestone W3M ~747.5 m ~ m Cummings Member Cummings Member Marine?, oil sand Marine?, oil sand 3

4 CS Senlac OBS A W3M DEN -500 mv GR api 200 AC 2000 kg/m usec/m ILD ohms Duperow CummingsDina 700 Mannville SP Sample location ~10 cm Figure 3 - CS Senlac OBS A W3 (~751 m): Sample interval (~10 cm in length) is outlined by a rectangle on the core photograph (diameter of coin is 2.8 cm); the approximate location of the sample is also indicated on geophysical logs from this well. 4

5 Murphy W3M SP -500 mv ILD 0.20 ohms 2000 Mannville GR api 750 Cummings Dina Duperow Sample location 10 cm Figure 4 - CS Senlac SWD A W3: Sample interval (~10 cm in length) at ~ m is outlined by a rectangle on the core photograph (diameter of coin is 2.8 cm); the approximate location of the sample is also indicated on geophysical logs from this well. 5

6 Dome Senlac W3M SP -500 mv 500 RHOB DT ILD Duperow Mannville Cummings - Dina GR 0 api kg/m usec/m ohms 2000 shale limestone Sample location Figure 5 - Dome Senlac W3: Geophysical well logs showing locations of shale sample (~747.7 m) and limestone sample (~753 m); samples are ~10 cm in length. Mannville unconformity within the Devonian Duperow Formation, and a 10 cm long section of half diameter core was taken in grey shale above the Dina sand; and d) two closely-spaced intervals were sampled in a clean, oilbearing sand in the Cummings above the main Dina channel in CS Senlac SWD B W3 (Figure 6). One large sample was taken at m for seismic analysis and two small (about one to two cm 3 ) loose samples were taken at m, specifically for thin section, SEM and ESEM analyses (Figures 6 to 11). 4. SEM and ESEM Images The SEM cannot view oil-stained samples because out-gassing of the sample in the vacuum chamber prevents the development of clear images. Hence, during SEM analysis, oil-stained samples are frozen using dry ice. The ESEM operates on a similar principal to the SEM, but allows for differential pressure in the vacuum chamber and gun columns, and uses a proprietary gaseous secondary electron detector that is capable of imaging in a gaseous environment. Hence, the sample can be viewed in its natural environment, even if wet. Another technique is to remove the oil from the sample using Soxhlet extraction prior to SEM and ESEM analysis (Figure 7). Shown here are four images of a sample (747.5 m) from the CS Senlac SWD B well: one SEM image before extraction (Figure 8), and four ESEM images, one before extraction (Figure 9A) and three after Soxhlet extraction (Figures 9B, 10, and 11). SEM imaging and -ray analyses after extraction have not yet begun. 5. Future Work We believe that significant progress can be made in understanding seismic velocity variations and wave propagation in heavy oil reservoirs through a more thorough understanding of rock physics, and of the framework and cementation of semi-consolidated and unconsolidated sands. We have begun to take SEM and ESEM images of the reservoir to increase our understanding of the influence of variations in grain fabric and cementation on seismic velocity and wave propagation, but have not yet started the P- and S-wave velocity analysis of the samples. We look forward to reporting our results in the future. 6

7 CS SENLAC SWD B W3M -500 SP mv 500 GR 200 DT 2000 kg/m usec/m ILD ohms 2000 Mannville api RHOB 750 Duperow Cummings Dina Sample location Top Base Figure 6 - CS Senlac SWD B W: Two small (about 1 to 2 cm3) loose samples were taken at m (interval outlined by rectangle on core photograph) and one large sample (~10 cm) was taken at m (outlined by circle on core photograph). The location of the sample interval is also indicated on geophysical logs from this well. 7

8 Figure 7 - Sample of heavy-oil saturated sand from CS Senlac SWD B W before and after Soxhlet oil extraction. The sample size is about 1 to 2 cm3. The clump of clean sand at the top centre of the right hand image was used for ESEM viewing. Images were also taken of loose sand but are not shown here. 6. References Eastwood, J. (1993): Temperature-dependent propagation of P- and S-waves in Cold Lake oil sands: Comparison of theory and experiment; Geophys., v58, p Prasad, M., Kopycinska, M., Rabe, U., and Arnold, W. (2002): Measurement of Young s modulus of clay minerals using atomic force acoustic microscopy; Geophys. Res. Lett., v.29, 8, p1-4. Rokosh, D.R. and Schmitt, D.R. (2002): Geology and production history of the Senlac Area; in Annual Report of the Seismic Heavy Oil (SHOC) Consortium at the University of Alberta. Univ. Alberta, p (2003): 1) Distribution of porosity and absolute permeability in the East Senlac Reservoir, Saskatchewan; and 2) Production and injection history of the Senlac B Pool; in Schmitt, D.R. and Rokosh, C.D. (eds.), Annual Report of the Seismic Heavy Oil (SHOC) Consortium at the University of Alberta, Univ. Alberta, p Figure 8 - CS Senlac SWD B W; oil sand, m: SEM image analysis before Soxhlet extraction; scale bar is 10 µm. The frozen oil film (smooth areas) is fractured resulting in the presence of numerous small (<10 µm across) particles resting on the surface of the grains. The frozen samples permit a good view of pore sizes, shapes and distribution. Pore shapes and sizes are irregular, although some relaxation occurs during recovery and grain rotation may occur during coring. Note the oil bridge between the two grains in the centre of the photo. The oil bridge is fractured, although whether the oil covers any cementing material is unclear. Note also the rough appearance of many of the grain surfaces. This image does not distinguish whether the surface of the grain is rough, or whether small particles adhere to the grain surface. 8

9 A B Figure 9 - CS Senlac SWD B W; oil sand, m: A) ESEM image analysis before Soxhlet extraction; scale bar is 50 µm. The photomicrograph shows two curved meniscus-like features between the grains in the centre and centre right of the photo; the central meniscus occurs at some depth below the top of the grain, hence we can be certain of no cement, at least above the meniscus. The irregular surface and stepped nature of the grains are evident. Small grains (<10 µm across) appear to adhere to the surface of the elongated grains (northwest-southeast) on the right side of the photo. B) ESEM image analysis after Soxhlet extraction; scale bar is 50 µm. The photomicrograph shows the lack of cement, although a few small (<10 µm across) particles of unknown origin adhere to smooth grain surfaces. The grains are subangular to subrounded with poor to moderate sphericity. At the upper left, note the rough grain (arrow) pictured in Figure 10. Figure 10 - CS Senlac SWD B W; oil sand, m: ESEM image analysis after Soxhlet extraction; scale bar is 50 µm. This photomicrograph shows good contrast between rough grains (black arrow, points to possible feldspar) and smooth grains (white arrow, points to possible quartz). Note the lack of cement between the smooth and rough grains, and the difference in the roundness of the edges between the two types of grains. Figure 11 - CS Senlac SWD B W; oil sand, m: ESEM image analysis after Soxhlet extraction; scale bar is 200 µm. This photomicrograph shows three or four dense areas (arrows) in the upper portion where a few grains appear to be cemented with presently unknown material. Surrounding these areas, the pore network is open and well connected. Note the smooth grains just above the letters WD in the black label and the clear lack of cement at the contacts. Wang, Z. and Nur, A. (1988): Effect of temperature on wave velocities in sands and sandstones with heavy hydrocarbons; Soc. Petrol. Eng., Resear. Eng., v3, p Zhang, Y.Z. and Schmitt, D.R. (2003): Strategies for the acquisition and processing of high-fidelity time-lapse seismic data in the Senlac area; in Schmitt, D.R. and Rokosh, C.D. (eds.), Annual Report of the Seismic Heavy Oil Consortium at the University of Alberta, Univ. Alberta, p