The New, Generalized Material Balance Equation for Naturally Fractured Reservoirs
- Publio Alejandro Sandoval Merchan (Natfrac Corporation) | Zuly Himelda Calderon Carrillo (U. Industrial de Santander) | Anibal Ordonez (Ecopetrol)
- Document ID
- Society of Petroleum Engineers
- Latin American and Caribbean Petroleum Engineering Conference, 31 May-3 June, Cartagena de Indias, Colombia
- Publication Date
- Document Type
- Conference Paper
- 2009. Society of Petroleum Engineers
- 4.6 Natural Gas, 5.8.6 Naturally Fractured Reservoir, 5.1.1 Exploration, Development, Structural Geology, 5.2 Reservoir Fluid Dynamics, 4.1.9 Tanks and storage systems, 5.8.2 Shale Gas, 5.8.3 Coal Seam Gas, 5.2.1 Phase Behavior and PVT Measurements, 5.2.2 Fluid Modeling, Equations of State
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The complexity associated to naturally fractured formations constrain reservoir engineers to use simplified versions of the material balance equation for determining the initial hydrocarbon in place and predicting reservoir performance. This work presents a new material balance equation for naturally fractured reservoirs, which is presented by complementing a mathematical model introduced by Peñuela et al 2001, that considers a porous medium composed of interdependent matrix and fracture system. By including both dissolved gas and volatilized oil was able to overcome the long standing limitations of the material balance in naturally fractured reservoirs resulting a unique material balance equation which was applicable to full range of reservoir fluids -including volatile oils and gas condensates.
The proposed equation extend the equations presented by Peñuela et al 2001 and Niz et al 2004, in handling compositional effects of fluid composition in naturally fractured reservoirs. The new equation can be shown to degenerate to preexisting methods under appropriate reservoir conditions. Synthetic examples are given to validate the approach and illustrate how to apply this method in compositional and no compositional reservoirs. All previously published works become subsets of the present method.
During the Iast few years, research has been done on material balance equations for Naturally fractured Reservoirs (NFR) in order to improve the reservoir performance analysis. However, all past efforts are applicable to only restricted ranges of reservoir fluids.
In 1994, Walsh developed a generalized straight line method to estimate petroleum reserves in conventional reservoirs applicable to full range of reservoir fluids. In 2001 Peñuela et al. introduce an original mathematical model that considers an initially-undersaturated black-oil fluid in a porous medium composed of interdependent matrix and fracture systems. The proposed equation improve the method of modeling naturally fractured reservoirs by considering the compressibility difference between fractured and matrix systems. Finally in 2003 Niz Proposed a extended straight line formulation to NFR presented by Peñuela to initial gas cap reservoirs. This work completes the search for a general, straight line method to estimate the original oil and gas in -place in NFR without restrictions on fluid composition. This was possible by including both volatilized oil -Rv- and dissolved gas -Rs- in the individual component balances
The model is based on the following assumptions:
- The reservoir is an isothermal system.
- The reservoir is composed of four components: stock-tank oil, surface-gas, production water and naturally fractured rock.
- The reservoir is composed of four phases: oil, gas, water and naturally fractured rock.
- The stock-tank oil component exists in the oil-phase and the gas-phase. Does not partition into the water or rock phases.
- In the reservoir, the surface gas component exists free in the gas-phase and dissolved in the oil-phase.
- The water component exists in an immobile water-phase that, for material balance purpose, only reduces the pore space available for hydrocarbon accumulation and flow.
- The rock component exists only in the rock-phase.
- The rock-phase is composed of two porous media in hydraulic communication: the fractured system and the (primary porosity) rock-matrix system.
- The fracture and porous matrix systems are compressible.
- There is no water influx and water production is negligible.
- There is no fluid injection (water and/or gas) into the reservoir.
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