Elastic roerty changes of bitumen reservoir during steam injection Ayato Kato*, University of Houston, higenobu Onozuka, JOGMEC, and Toru Nakayama, JAPEX ummary Elastic roerty changes of bitumen reservoir during steam injection have been oorly understood. We measured and analyzed ultrasonic velocities of bitumen-saturated sediments (oil sands) and then obtained a relation of the velocities with temerature and ressure individually. We also investigated validity of the Gassmann equation for redicting velocity changes. We combined the laboratory measurement results to obtain a sequential rock hysics model that can redict the velocity changes induced by the steam injection. Introduction One of the most effective methods for roducing the bitumen in Canada is the team-assisted Gravity Drainage (AGD) method. It makes the bitumen flowable by heating it with injected steam and reducing its viscosity. The steam movement is highly influenced by comlex substructures in reservoir. Thus, the time-lase seismic survey is exected to be owerful for monitoring the three-dimensional steam movement. However, the difficulties of making quantitative interretation of the time-lase seismic data remain because there has been no model of relating seismic velocities of oil sands directly with reservoir arameters (temerature, ressure, and fluid changes). In this study, we measured ultrasonic velocities of the oil sands and roose a ractical model which can redict sequential seismic velocity changes during the steam injection. of 0 C, which corresonds to the reservoir temerature before the steam injection. Figure shows the velocities as a function of differential ressure, which is confining ressure (900 si) minus ressure. We observed that the P- and -wave velocities gradually decrease with decreasing differential ressure. Aling the natural logarithm as a fitting curve, we obtained a relationshi between the velocities and ressure. = 0.0593 P = 0.0780 P ) 0.375 + ) 0.495 + where P is ressure (si), P and are P- and - wave velocities (km/s) resectively, and and 0 are P- and -wave velocities (km/s) at the initial condition ( ressure of 300 si and temerature of 0 C) resectively. The solid lines in Figure reresent Equation. The ressure increase from 300 to 700 si induced by the steam injection causes velocity decrease; 65 m/s for and 86 m/s for s. 0 () Oil ands amles We acquired whole cores from the oil sands reservoir in the Jaan Canada Oil ands Limited (JACO) Hangingstone AGD oeration area, where the time-lase seismic data had been acquired (the baseline survey in 2002 and the reeat survey in 2006 (Nakayama et al. (2006)). The oil sands samles were high-orous clean sands with small amounts of clay. We cut the whole cores into blocks and carefully whittled four lug samles from the blocks by a lathe. Next, the lug samles were trimmed in order to be held firmly with transducers. The four lug samles were.5 inches in diameter and aroximately inch in length. We measured the P- and -wave velocities for the lug samles by the ulse-transmitted method with frequency of 0.5 MHz. Pressure Deendence The P- and -wave oil sands velocities were measured under several ressure conditions at a constant temerature Figure : P- and -wave velocities of the oil sands as a function of differential ressure at constant temerature of 0 C. The numbers (#2, #3, #7, and #0) reresent samle number of the measured oil sands. Temerature Deendence We also measured the P- and -wave oil sands velocities as a function of temerature at a constant ressure of 700 si. A sloe of the velocities to temerature significantly changes at around 30 C. In the lower temerature range the P- and -wave velocities steely decrease, where bitumen filled in the s works as a quasi-solid to stiffen the rock grain frame moduli (Han et al., 2006). We divided EG Las egas 2008 Annual Meeting 709
Elastic roerty changes of bitumen reservoir during steam injection the velocities at a given temerature by the initial velocities at 0 C to obtain normalized velocities (Figure 2). The normalized velocities were fitted with two linear lines which connect to each other at 30 C. T < 30 C P / P = 0.00550 +.06 / = 0.090 +.9 T 30 C P / P = 0.0068 + 0.940 / = 0.000640 + 0.639 where T is temerature ( C), P and are P- and -wave velocities (km/s) resectively at temerature of 0 C and ressure of 700 si. In accordance with Equation 2, the normalized P- and -wave velocities at 80 C are 0.8 and 0.59, resectively. (2) Alication of the Gassmann Equation Dry frame moduli were calculated from the ultrasonic laboratory measurement data based on the Gassmann equation in order to investigate its validity for redicting velocity changes caused by the steam injection. Porosity and water saturation were determined to be 37 % and 9.6 % resectively by etrohysical analysis of the well logs. Mineral bulk modulus was assumed to be 35.5 GPa, taking into account the small amounts of clay. The bulk modulus and density of the bitumen were calculated from the FLAG rogram develoed in Fluids/DHI consortium and its shear modulus was assumed to be zero. The calculated shear modulus raidly decreases from 0 to 30 C, and then gradually decreases from 30 to 70 C (Figure 3). The bulk modulus also obviously decreases at lower temeratures than 80 C. The significant deression of the dry frame moduli at the lower temeratures can be interreted as: () the bitumen with higher viscosity stiffens the grain contacts, (2) the Gassmann equation fails because the bitumen has substantial rigidity, or (3) there is an effect of velocity disersion between the low frequency of the Gassmann equation and high frequency of the ultrasonic measurement. On the other hand, they remain at aroximately constant values at temeratures greater than 80 C. The unvaried dry frame moduli at the higher temeratures imlies that the Gassmann equation would be alicable for redicting velocity changes. Figure 3: Dry frame moduli of the oil sands (samle #7) at ressure of 700 si. Figure 2: Normalized P- and -wave velocities of the oil sands as a function of temerature at constant ressure of 700 si. The black solid lines reresent Equation 2. In order to justify the above hyothesis, we calculated the velocity changes caused by fluid roerty changes based on the Gassmann equation. In the calculation, dry frame moduli were assumed to be constant (K and G are 0.80 GPa and 0.48 GPa resectively) even though temerature varies. The calculated velocities were fairly consistent with the EG Las egas 2008 Annual Meeting 70
Elastic roerty changes of bitumen reservoir during steam injection measurement at higher temeratures than 80 C. This result encourages the use of the Gassmann equation for redicting velocity changes caused by any fluid roerty changes (due to not only temerature changes but also water saturation and hase changes) at the higher temeratures until the rock frame undergoes direct damage from the steam injection. equential Rock Physics Model to and bottom at the well locations. The sonic interval velocities were calculated from time-deth relations of the corresonding reservoir interval. Both the interval velocities were not affected by the steam injection because the data used for the calculation had been acquired rior to it. From the comarison, the seismic interval velocity was estimated to be 7.6% lower than the one of sonic log on average. We combined the laboratory measurement results to obtain a sequential rock hysics model that can redict velocity changes induced by the steam injection:. P 300 si 700 si (at T = 0 C) = 0.0593 P ) 0.375 + = 0.0780 P ) 0.495 + 0 2. T 0 C 30 C (at P = 700 si) = ( 0.00550 +.06) P (3) = ( 0.090 +.9) 3. T 30 C 80 C (at P = 700 si) = ( 0.0068 + 0.940) P = ( 0.000640 + 0.639) 4. T 80 C (at P = 700 si) The Gassmann equation can redict velocity changes Fluid roerties are based on the FLAG rogram where P and are P- and -wave velocities (km/s) resectively at ressure of 700 si and temerature of 0 C, which can be calculated from the first relations between the velocities and ressure. Once steam is injected into the reservoir, the ressure front raidly sreads to the erihery, and the temerature front follows it. The P- and -wave velocities decrease due to the ressure increase as a natural logarithm relation with differential ressure. After the ressure change, the velocities decrease with increasing temerature as a linear relation with a change in the sloe at 30 C as shown in Figure 2. The velocities at temeratures greater than 80 C can be calculated by the Gassmann equation with the FLAG rogram. elocity Disersion We made a comarison between the surface seismic interval velocity and the sonic log interval velocity for the oil sands reservoir. The seismic interval velocities were calculated from two-way time differences between two seismic horizons which nearly corresond to the reservoir Figure 4: chematic illustration of a ractical method for calibrating the velocity disersion. We roose a ractical method for calibrating the velocity disersion (Figure 4). As stated before that the Gassmann equation is alicable at temeratures greater than 80 C for the high frequency ultrasonic laboratory measurements. When we aly the Gassmann equation to the surface seismic data, we assume that the temerature limitation of the Gassmann equation can extend to lower temeratures until the dry frame moduli change or the bitumen has a substantial rigidity. In addition, we have the average value of the interval velocity difference between the surface seismic and the sonic data at the initial condition. The seismic velocity at the initial condition is estimated from the sonic velocity with the average velocity disersion. We transform them into normalized velocities and connect the initial oint (Point-A) with the temerature limitation of the Gassmann equation (Point-B) by a linear line. However, we do not know the temerature limitation. Fortunately, the Point-B can only move in a small range from 0 to 30 C. Assuming the temerature limitation is 25 C, we modified the coefficients associated with the temerature deendence in Equation 3 and obtained new sequential rock hysics model which is adated for low frequency band of the surface seismic.. P 300 si 700 si (at T = 0 C) = 0.0593 P ) 0.375 + = 0.0780 P ) 0.495 + 0 2. T 0 C 25 C (at P = 700 si) EG Las egas 2008 Annual Meeting 7
Elastic roerty changes of bitumen reservoir during steam injection = ( 0.00433 +.04) = ( 0.0239 +.24) P 3. T 25 C (at P = 700 si) The Gassmann equation can redict velocity changes. Fluid roerties are based on the FLAG rogram. (4) of the bitumen. Finally, the water hase changes from liquid to steam at ste 2 (260 C), leading to significant P- wave velocity dro (while the -wave velocity slightly increases because of density decrease). equential Elastic Proerty Changes To understand elastic roerty changes of the oil sand reservoir during steam injection, we reresented the sequential reservoir conditions using 23 stes (Figure 5). Pore ressure change is caused from ste to 5 followed by temerature change from ste 6 to 23. In addition, adjacent to the injector well, bitumen is relaced by hot water at ste 8 and water hase changes from liquid to steam at ste 2. te 5 : ressure increases (from 300 to 700 si) te 6 23 : temerature increases (from 0 to 300 C) te 8 : bitumen is relaced by hot water at 200 C te 2 : hase changes from liquid to steam at 260 C Assuming that the sonic P- and -wave velocities at the initial condition (ste 0) are 2.4 km/s and.0 km/s resectively, the seismic velocities at ste are calculated by alying the average velocity disersion (Figure 5 and 6). The P- and -wave velocities decrease in the first ressure change from ste to 5. In the temerature increase from ste 5 to 8, the P- and -wave velocities decrease and the /s ratio significantly increases because amount of decrease of the -wave velocity is relatively larger than the P-wave velocity. After ste 8 (25 C), the P-wave velocity continues to decrease while the - wave velocity remains virtually constant (because shear modulus is constant). At ste 8 (200 C), where the movable bitumen is largely relaced by the hot water (w from 20 % to 80%), the P-wave velocity slightly increases because the hot water has a faster P-wave velocity than one Figure 6: Crosslot between and s based on the conditions of Figure 5. Conclusions We measured and analyzed the ultrasonic velocities of the oil sands cores acquired from the AGD oeration area and constructed the sequential rock hysics model. The ractical method for calibrating the velocity disersion was roosed. The elastic roerty changes induced by the steam injection were redicted. Acknowledgments We wish to thank JACO, JAPEX, and JOGMEC for ermission to ublish this aer. We also thank the cororate sonsors of the Fluids/DHI consortium. Figure 5: equential P- & - wave velocity and /s ratio changes induced by steam injection. EG Las egas 2008 Annual Meeting 72
EDITED REFERENCE Note: This reference list is a coy-edited version of the reference list submitted by the author. Reference lists for the 2008 EG Technical Program Exanded Abstracts have been coy edited so that references rovided with the online metadata for each aer will achieve a high degree of linking to cited sources that aear on the Web. REFERENCE Han, D., J. Liu, and M. Batzle, 2006, Acoustic roerty of heavy oil-measured data: 76th Annual International Meeting, EG, Exanded Abstracts, 903 907. Nakayama, T., A. Takahashi, and H. Mochinaga, 2006, Time-lase seismic survey in the oil sands area JACO AGD Oeration Area, Athabasca, Canada Model tudy and Data Acquisition: The 8th EGJ International ymosium, EGJ Proceedings, 355 358. EG Las egas 2008 Annual Meeting 73