High-Temperature Blowdown Experiments in a Vuggy Carbonate Core
- Kaveh Dehghani (Chevron Petroleum Technology Co.) | Jairam Kamath (Chevron Petroleum Technology Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- September 2001
- Document Type
- Journal Paper
- 283 - 287
- 2001. Society of Petroleum Engineers
- 5.8.6 Naturally Fractured Reservoir, 5.4.6 Thermal Methods, 5.2.1 Phase Behavior and PVT Measurements, 5.4.9 Miscible Methods, 4.6 Natural Gas, 5.3.4 Reduction of Residual Oil Saturation, 5.8.7 Carbonate Reservoir, 4.1.9 Tanks and storage systems, 1.6.9 Coring, Fishing, 5.5.2 Core Analysis, 5.4.1 Waterflooding
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We conducted a series of tests on a vuggy carbonate core sample to quantify recovery mechanisms during a high-temperature blowdown process. We heated the core to 300°F, keeping the pressure above the bubblepoint, and then reduced the pore pressure at a constant rate from one end of the sample. Tests were conducted with the core at initial oil saturation, Soi, and at residual oil saturation, Sor, to water, with pressure reduction from top or bottom of the sample. The blowdown experiments show that 50 to 68% of the oil is recovered, with 8 to 20% owing to thermal expansion, 20 to 24% owing to thermally enhanced solution gas drive, 12 to 16% owing to distillation, and 8 to 10% owing to in-situ steamdrive.
Steamflooding of light oil in vuggy and fractured reservoirs has been examined in several field trials.1-3 The existence of vugs and fractures is often detrimental for water or miscible gas. However, they can be utilized in a steamflood for effective heating of the matrix rock. A steam-injection process can then be designed to take advantage of the transferred thermal energy to recover oil from parts of the reservoir that were not accessible to water and miscible gasfloods.
Steamflooding, followed by pressure drawdown using both injectors and producers, has been suggested to recover oil from the waterflood-bypassed regions of a vuggy carbonate reservoir.1 Steamflooding heats the waterflood-bypassed oil and raises the vapor pressure. This pressure increase is employed as the primary source of energy for an oil displacement process during the drawdown period. The hot, volatile oil and water are flashed during the drawdown period, resulting in oil displacement from the waterflood-bypassed matrix into the high-permeability vuggy channel.
Recovery mechanisms in conventional steamflooding have been identified and studied in the laboratory.4,5 There is very little laboratory data2,6,7 on the blowdown process and no systematic data quantifying the contributions of the various mechanisms.
The objectives of this work are to investigate the recovery mechanisms during the high-temperature blowdown process. To properly interpret the results, we first characterize the connectivity of vugs and matrix in the core. We then conduct three blowdown experiments, varying the initial oil saturation state and the location of the pressure sink. We analyze the data to quantify the relative contributions of thermal expansion, thermally enhanced solution gas drive, distillation, and in-situ steamdrive to oil recovery.
Characterization of Core Sample
The experiments were conducted on a whole core sample (15 cm long, 10 cm diameter) from the vuggy interval of a carbonate reservoir. We solvent-cleaned and air-dried the sample, and then measured the porosity, using a helium porosimeter, to be 22.7%.
The dry core was computer tomography (CT)-scanned, saturated with 0.5 mol% KCl, and scanned again for calculating the porosity distribution. Fig. 1, a typical cross-sectional image, shows the presence of vugs, high-porosity matrix, and low-porosity matrix. Fig. 2 is the histogram of porosity pixel* values. It shows that there is a significant amount of high-porosity pixels, and some pixels have unit porosity. These are likely caused by vugs. Fig. 3 displays pixels with porosity values greater than 42% that are connected to a "seed" pixel at the bottom end of the core sample. This figure clearly shows a good static connectivity for nonvuggy high-porosity values. Thin sections and scanning electron microscopy photomicrographs from the core samples in the same area show that porosity types are both moldic and intercrystalline.
We next measured the brine permeability for this core sample to be 1000 md. To map the dynamic fluid connectivity, we CT-imaged a unit-mobility, equal-density, KBr-displacing KCl miscible flood. The normalized KBr concentration profiles for different pore volumes injected (PVI's) are shown in Fig. 4. The sample is heterogeneous, and breakthrough occurs at 0.2 PVI. Large areas of the core are unswept, and the KBr concentration is less than unity even after 2.7 PVI. Fig. 5 compares the average KBr concentration in the core to analytical calculations for different values of core dispersivity, a. In addition to its heterogeneity, this core sample has a large dispersivity, a>15 cm.
Fig. 6 contains CT-derived concentration images during the miscible flood. As the flood proceeds, the fluid in the vugs connects up through the high-permeability matrix rock and finally with the low-permeability matrix rock. The fluid interaction resulting from connectivity between vuggy and low-permeability zones is important for a successful application of the steamflood/blowdown process. The effective heating of the low-permeability reservoir by vuggy and high-permeability regions during steamflooding, and the subsequent oil displacement during the blowdown period, rely on this interaction.1
We conducted three blowdown experiments using different initial oil saturation states and locations of the pressure drawdown sink. Table 1 summarizes the test conditions.
Dead reservoir crude (molecular weight=237) was combined with synthetic-solution gas to create a live oil sample with bubblepoint pressure of 200 psia at reservoir temperature (88°F). Compositions of the dead crude, the synthetic-solution gas, and the live oil are shown in Fig. 7.
In all the experiments, we first established oil saturation at ambient temperature and using a backpressure greater than the live-oil bubblepoint. Once the sample was ready for the experiment, one end of the core sample was closed. The backpressure on the open end was then increased to 600 psia and the oven temperature increased to 300°F. The backpressure of 600 psia prevented any free gas from being generated in the core during heating, as the live-oil bubblepoint at 300°F is 570 psia. After the temperature stabilized at 300°F, we decreased the backpressure at a constant rate of 18 psi/hr. We continuously monitored the upstream and downstream core pressures, core temperature, and effluent volumes of oil, water, and gas. Oil and gas samples were collected periodically for compositional analysis.
We displaced the brine-saturated core with live oil. The high-temperature blowdown experiment was conducted with the pressure drawdown from the top of the core.
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