Effect of Fracture Compressibility on Gas-in-Place Calculations of Stress-Sensitive Naturally Fractured Reservoirs
- Roberto Aguilera (U. of Calgary)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- April 2008
- Document Type
- Journal Paper
- 307 - 310
- 2008. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 5.1 Reservoir Characterisation, 4.1.4 Gas Processing, 5.5 Reservoir Simulation, 1.2.2 Geomechanics, 5.8.6 Naturally Fractured Reservoir
- 4 in the last 30 days
- 1,440 since 2007
- Show more detail
- View rights & permissions
|SPE Member Price:||USD 12.00|
|SPE Non-Member Price:||USD 35.00|
A tank material-balance equation for gas reservoirs has been written, taking into account the effective compressibility of matrix and fractures.
The method has direct application to stress-sensitive naturally fractured reservoirs (NFRs). Under some conditions, ignoring the effect of fracture compressibility (Cf ) can lead to overestimating the volume of original gas in place with a crossplot of p/z vs. cumulative gas production (Gp ). The equation presented in this paper has been developed to overcome that weakness. The use of the tank material balance is illustrated with an example.
It is concluded that fracture compressibility can play an important role in the calculation of gas in place in stress-sensitive NFRs.
The subject matter is significant because, historically, formation and water compressibilities have been neglected when carrying out material-balance calculations for conventional gas reservoirs. This assumes that these compressibilities are negligible compared to that of gas. The assumption implies that the reservoir strata are static. When water influx is ignored, the assumption leads to a straight line in a crossplot of p/z vs. Gp . However, this study shows that in those instances in which fracture compressibility is large, such assumptions can lead to significant error.
Forecasting the performance of NFRs is a major challenge. Various authors have tackled the problem throughout the years with material-balance calculations. To the best of my knowledge, the effect of fracture compressibility usually has been ignored in material-balance equations for gas reservoirs. The work presented in this paper is not meant to replace a detailed reservoir simulation, which, in my opinion, is the best way to try to solve the problem, provided that the reservoir characterization and the quality of the pressure and production data is good. The idea is to have a tool that can provide a quick idea with respect to potential gas in place and recovery from stress-sensitive NFRs.
The conventional material balance for gas reservoirs leads to a straight line in a Cartesian crossplot of p/z vs. Gp , provided that (1) water influx is equal to zero, (2) the reservoir strata are static, and (3) the water and formation compressibilities are negligible compared to gas compressibility. Although these assumptions are reasonable in many instances, there are cases in which the fractures are quite compressible. In these cases, the conventional approach can lead to significant errors in the estimation of original gas in place. Similar problems have been observed in the past in geopressured reservoirs (Roach 1981; Ramagost and Farshad 1981) and stress-sensitive naturally fractured oil reservoirs. Possible solutions have been proposed by Aguilera (2006, 2007).
This paper presents a material-balance equation that takes into account the effective compressibility of matrix and fractures. Stress-sensitive properties such as fracture porosity, fracture permeability, and the portions of gas stored in matrix and fractures are taken into account.
|File Size||870 KB||Number of Pages||4|
Aguilera, R. 1999. Recovery Factors and Reserves in Naturally FracturedReservoirs. J. Cdn. Pet. Tech. 38 (7): 15-18.
Aguilera, R. 2003. Geologic and Engineering Aspects of Naturally FracturedReservoirs. CSEG Recorder 28 (2): 44-49.
Aguilera, R. 2006. Effect of Fracture Compressibility on Oil Recovery fromStress-Sensitive Naturally Fractured Reservoirs. J. Cdn. Pet. Tech.45 (12): 49-59.
Aguilera, R. 2007. Effect of Naturally Fractured Aquifers on Oil Recoveryfrom Stress-Sensitive Naturally Fractured Reservoirs. J. Cdn. Pet. Tech.46 (7): 43-48.
Hall, H.N. 1953. Compressibilityof Reservoir Rocks. Trans., AIME, 198: 309-311.
Jones, F.O. 1975. A Laboratory Study of the Effects of Confining Pressure onFracture Flow and Storage Capacity in Carbonate Rocks. J. of PetroleumEngineering 27: 21-27.
Laubach, S.E. 2003. Practical Approaches to Identifying Sealed and OpenFractures. AAPG Bull. 87 (4): 561-579.
Laubach, S.E., Olson, J.E., and Gale, J.F.W. 2004. Are Open FracturesNecessarily Aligned With Maximum Horizontal Stress? Earth and PlanetaryScience Letters 222 (1): 191-195. DOI:10.1016/j.epsl.2004.02.019.
Olson, J.E., Laubach, S.E., and Lander, R.L. 2007. Combining Diagenesis andMechanics To Quantify Fracture Aperture Distributions and Fracture PatternPermeability. In Fractured Reservoirs, ed. L. Lonergan, L., R.J. Jolley,D.J. Sanderson, and K. Rawnsley, 97-112. London: Geological Society ofLondon.
Ramagost, B.P. and Farshad, F.F. 1981. P/Z Abnormally Pressured GasReservoirs. Paper SPE 10125 presented at the SPE Annual TechnicalConference and Exhibition, San Antonio, Texas, 4-7 October. DOI:10.2118/10125-MS.
Roach, R.H. 1981. AnalyzingGeopressured Reservoirs—A Material Balance Technique. Paper SPE 9968available from SPE, Richardson, Texas.
van der Knaap, W. 1959. NonlinearBehavior of Elastic Porous Media. Trans., AIME 216:179-187.
Walsh, J.B. 1981. Effect of Pore Pressure and Confining Pressure on FracturePermeability. Intl. J. of Rock Mechanics, Minerals, Science andGeomechanics 18: 429-435.