Low-Field NMR Method for Bitumen Sands Characterization: A New Approach
- K.D. Mirotchnik (TIPM Laboratory) | K. Allsopp (TIPM Laboratory) | A. Kantzas (TIPM Laboratory/U. of Calgary) | D. Curwen (Suncor Energy) | R. Badry (Schlumberger of Canada)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- April 2001
- Document Type
- Journal Paper
- 88 - 96
- 2001. Society of Petroleum Engineers
- 5.6.1 Open hole/cased hole log analysis, 4.6 Natural Gas, 5.4.6 Thermal Methods, 5.8.7 Carbonate Reservoir, 1.6.9 Coring, Fishing, 4.1.5 Processing Equipment, 4.2 Pipelines, Flowlines and Risers, 5.1 Reservoir Characterisation, 5.2 Reservoir Fluid Dynamics, 5.5.2 Core Analysis, 4.1.2 Separation and Treating, 2.4.3 Sand/Solids Control, 5.6.2 Core Analysis, 5.8.5 Oil Sand, Oil Shale, Bitumen
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The nuclear magnetic resonance (NMR) signal obtained from conventional oil,heavy oil, and bitumen formations can consist of both hydrocarbon and watersignals. Each NMR signal can further characterize both mobile and immobilefluids in the porous media. However, as the viscosity of the hydrocarbon phaseincreases and the NMR signal shifts toward lower relaxation times, thecomposite NMR signal for the hydrocarbon-bearing formation becomes verycomplicated. As the viscosity of the bitumen exceeds 100,000 cp (at naturalconditions), the relaxation characteristics of bitumen become partiallynondetectable by both the logging and laboratory NMR tools. As a result, theconventional methods of NMR detection fail to precisely recognize thehydrocarbon components.
Laboratory NMR measurements of bitumen-bearing porous media under differenttemperatures were performed. This method delivered new information aboutbitumen reserves in situ. The results show that low-field NMR holds promise forthe characterization of recoverable heavy oil and bitumen reserves. This newapproach can be applicable for real-time monitoring of thermal extraction,including monitoring the efficiency of thermal recovery methods.
NMR logging tools are currently used for determining reservoir propertiessuch as porosity1,2 and permeability,1-4 as well as thepresence of mobile and immobile fluids.5-8 Recent developments in NMR researchoffer tools for separating water, oil, and gas from the combined NMRsignal.9-11 Very little is known about the use of NMR logging toolsfor the in-situ characterization of crude oils.1 With respect toheavy-oil and bitumen formations, NMR logging has not been very successful incharacterizing crude oil (viscosity>100,000 cp). The reason for this is thefact that the spectra from most heavy-oil and bitumen formations cannot beadequately detected by the NMR logging tools. This is because the shortestrelaxation times (t2's) of the spectra at normal temperatureconditions (T 30°C) are lost. High-field NMR technology has solved suchproblems in the past but is not currently available to be useddownhole.12,13
A fundamental objective of the research performed in our laboratory was toextend the use of NMR logging tools to heavy-oil and bitumen formations,particularly during thermal recovery projects. To this end, the NMRcharacteristics of these types of hydrocarbons in bulk volume and in porousmedia were investigated.8,14 The objective of this work was toinvestigate the NMR characteristics of these bitumens and heavy oils atelevated vs. ambient temperatures and to isolate the oil signal from thecombined NMR spectrum of the formation. The hydrocarbon and water saturationswere then determined. The possibility of increasing the quality of NMR data byincreasing the signal-to-noise ratio and by proper reconstruction of the wholet 2 spectra was also investigated.
These objectives were achieved by performing a series of experiments, whichaddressed the following issues:
Variable-temperature NMR spectra determination for bitumen-saturated coresto estimate different fluid components in porous media in situ.
NMR characterization of brines, conventional oils, heavy oils, and bitumenin bulk volume at different temperatures.
Estimation of the parameters of NMR tools and their applicability formonitoring thermal recovery processes.
It must be noted that La Torraca et al.15 providedlaboratory data that correlate NMR properties to viscosity of heavy oilsranging from <1,000 cp to >100,000 cp. Then they combined NMR log andconventional log data to predict the in-situ oil viscosity in two heavy-oilreservoirs. This work,15 although similar in nature to the workpresented here, deals with oil reservoirs having 3 to 4 orders of magnitudeless viscosity. Unfortunately, algorithms presented in the literature seem tocollapse when applied to bitumen formations.
All field measurements for bitumen sands characterization were performedwith a Schlumberger CMR-200™ logging tool. All measurements were done atnatural in-situ conditions; the maximum recorded temperature was T=14°C.One example of these results is presented in Fig. 1.
The entire NMR laboratory testing was performed with a custom-built NumarCorespec 1000™. This is a unique system with a separate temperature control forheating the magnet and the sample. The equipment was installed and tuned at theTIPM Laboratory and operates at a frequency of 1 MHz. All the samples weretested at different temperatures and at ambient pressure with a standardmethodology developed for NMR log calibration. 16 All decay datawere translated into NMR spectra with algorithms developed in-house and theNUMAR standard analysis packages 6 that are included with thespectrometer. Several sets of experiments were performed to address each of theissues mentioned earlier.
Variable-Temperature NMR Spectra of Core.
Variable-temperature NMR was used to determine the fluid components inbitumen- and water-saturated cores. The first set of experiments involved thetesting of native state cores at different thermal conditions. Native-statebitumen-saturated plug samples were cut from full-size core using aliquid-nitrogen-cooled cutter. Testing started with NMR measurements at thefollowing temperatures: 1°C, 6°C, 8°C, 12°C, 16°C, 22°C, 25°C, 30°C, 40°C,45°C, 50°C, 60°C, 65°C, 70°C, 75°C, and 80°C. For all measurements, a CPMGsequence was applied with interecho times of 0.3 and 0.6 ms. The NMR spectrawere recovered.
After completion of the temperature-cycle testing, the native-statebitumen-saturated plugs were saturated under vacuum with an aqueousparamagnetic solution (2N CuSO4, T2 1ms). The fact thatwater entered the cores under vacuum implies that some drying occurred duringcore handling. This procedure was performed to eliminate the water signal fromsubsequent NMR testing. The samples were measured at 30°C. Following the NMRdata collection with the paramagnetic solution, the sand samples were cleanedusing the Dean-Stark method (thus removing all bitumen) and resaturated with 2%NaCl brine. The brine-saturated samples were tested in the NMR again at theprevious temperatures. This determined that the NMR spectrum of the sample wasfree of any bitumen effects, measuring only structural and mineralogicaleffects.
X-ray diffraction (XRD) analyses were performed to determine clay type andconcentration. In accordance with these analyses, a set of artificial samples(sand + clay) was constructed. All samples were saturated with 2% NaCl brine.The brine-saturated artificial samples were tested to determine the NMRspectra, again at the temperatures mentioned above. Examples of the obtainedspectra set are presented in Figs. 2 through 4.
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