Fractured Reservoir Petrophysics (FRP)
Module Fractured reservoir Petrophysics(FRP) helps users to interpret wireline data of fractured massive reservoir. The reservoir contains various rock types which are characterized by different mineral compositions. These mineral compositions have various influences on measured physical parameters, coupling with complex pore space structure such as Fracture-Open Vug-Isolated Vug system making conventional approach become incompatible. In a Fracture-Vug massive reservoir, the pore space model can be characterized as below:
• The macro-fracture system: mainly formed by tectonic activities. It includes faults, faults-related fracture usually enhanced by hydrothermal activity. Macro-fracture zones generate oil flows into wellbore, as well as accept injecting water. The permeability of macro-fracture zones can vary greatly from some mD to ten thousand of mD.
• The micro-fracture system is formed by tectonic activities and also magma shrinking. It occurs as either as an associated feature (parallel, beside) of macro- fracture zones or develops in among them without any preferred orientation. The micro-fracture system is characterized by capillary permeability.
• The open vug system that develops along the macro-fracture surfaces. Such system has been formed through a process of hydrothermal activities. It usually contributes significantly to accumulation ability and hydrodynamic permeability of the solid rock.
• The solid mass is the primary, non-destroyed magma/tight carbonate… without any fluid permeability and any accumulation ability.
• Isolated vugs (non-connected vugs) are formed either at the same time with magma cooling or in diagenesis. They do not contribute to storage capacity and permeability ability.
• The macro-fracture system: mainly formed by tectonic activities. It includes faults, faults-related fracture usually enhanced by hydrothermal activity. Macro-fracture zones generate oil flows into wellbore, as well as accept injecting water. The permeability of macro-fracture zones can vary greatly from some mD to ten thousand of mD.
• The micro-fracture system is formed by tectonic activities and also magma shrinking. It occurs as either as an associated feature (parallel, beside) of macro- fracture zones or develops in among them without any preferred orientation. The micro-fracture system is characterized by capillary permeability.
• The open vug system that develops along the macro-fracture surfaces. Such system has been formed through a process of hydrothermal activities. It usually contributes significantly to accumulation ability and hydrodynamic permeability of the solid rock.
• The solid mass is the primary, non-destroyed magma/tight carbonate… without any fluid permeability and any accumulation ability.
• Isolated vugs (non-connected vugs) are formed either at the same time with magma cooling or in diagenesis. They do not contribute to storage capacity and permeability ability.
In general, Petrophysical analysis of Fractured Reservoir involves the below tasks:
1. Determination of Mineral compositions and Total porosity.
2. Determination of Open Porosity.
3. Determination of Fracture porosity by correction of Vug porosity’s effect on resistivity by Maxwell formula (parallel connectivity model).
4. Identification of Macro-fracture porosity and Micro-fracture porosity.
5. Determination of permeability.
6. Swir - irreducible water saturation.
2. Determination of Open Porosity.
3. Determination of Fracture porosity by correction of Vug porosity’s effect on resistivity by Maxwell formula (parallel connectivity model).
4. Identification of Macro-fracture porosity and Micro-fracture porosity.
5. Determination of permeability.
6. Swir - irreducible water saturation.
Based on pore space model described above, total porosity ( ) which includes open and block volume that create capacitance-filtration property of reservoir rock.
Based on pore space model described above, total porosity ( ) which includes open and block volume that create capacitance-filtration property of reservoir rock.
For determination of total porosity with consideration to minimize the influence of mineral compositions on porosity evaluation, the multi-mineral solver approach with following system of equations is proposed
Where
The classification of Macro-Fracture and Micro-fracture zones are defined by data from Acoustic (DTC, DTS, DTST), Neutron (NPHI), Resistivity…..
• Macro-Fracture zones with low Resistivity and high Acoustic, Porosity (Neutron & Density) are characterized by hydrodynamic permeability.
• Micro-fracture zones have lower porosity (Neutron & Density), Acoustic and higher Resistivity than Macro-fracture zones. They are characterized by capillary permeability.
• Micro-fracture zones have lower porosity (Neutron & Density), Acoustic and higher Resistivity than Macro-fracture zones. They are characterized by capillary permeability.
Based on the conceptual model above, Basroc, a specialized algorithm for log interpretation of fractured reservoir, is developed by Dr. Quy Hoang and his group in VietsovPetro to study granitoid basement reservoir fields at CuuLong Basin in Vietnam such as White Tiger, Eastern Dragon, Sutu Den, Rang Dong…After that, this workflow is extended and applied for other fractured reservoirs such as metamorphism rock in Sudan, as well as Carbonates in Uzbekistan and so on. In all of the above reservoirs, oil is produced from Fracture-Vug system.
The first important task for petrophysical analysis of fractured granitoid reservoir is minimizing influence of mineral composition on logs data for a correct total porosity estimation.
In our approach, we use multi-mineral solver (function 1) to identify each mineral volume. Input of mineral solver requires matrix parameter of each mineral. The challenge in this step is that matrix parameter for a specific mineral have wide range, for example: density of Plagioclase ranges from 2.58 to 2.62 g/cm3. This brings uncertainties when solving set of multi-mineral equations (1), thus brings uncertainties to evaluate porosity. To help users determine the matrix parameter for each mineral, I2G has a utility “Build Mineral Parameter” which utilize petrography data (volumetric proportion of each mineral of rock) and logs of that sampled section to predict mineral matrix parameter for that rock type in interpreting well sections. Combining this utility with synthetic-input curves comparison, users can get reliable estimation of ФT, Vmi.
If petrography data from cutting is not available, i2G has a unique built in overlay lines to classified 03 main rock types: granite, granodiorite and diorite. These overlay lines are built and validated based on data from hundreds of well in granitoid basement.
For quick analysis or well without petrography data, these overlay lines in Neutron-Density, Sonic-Density can help users identify 03 main lithology groups along their well section to apply the analogue mineral model. It helps users to get starting point to analyze below mineral volume from open hole logs in fractured granitoid reservoir:
• Plagioclase
• Quartz
• K-Feldspar
• Heavy minerals and
• Secondary minerals.
• Plagioclase
• Quartz
• K-Feldspar
• Heavy minerals and
• Secondary minerals.