Petrophysics: Rock Properties and Fluid Interaction

• Petrophysics studies rock properties and their interaction with fluids.

Purpose of Petrophysics

• Provides basic understanding of physical properties of porous geologic material.
• Describes the interaction of various fluids within rocks.
• Gives the distribution of pores of various sizes.

Three Dimensional Network

• The geological material must have a 3D network of interconnected pores to accumulate and store hydrocarbons, and also to allow fluid movement.

Porosity and Permeability

• Porosity and permeability are the most fundamental physical properties of rocks with respect to storage and the transmission of fluids.

5 Petroleum System Components

• Source rock
• Migration pathway
• Proper timing
• Seal rock
• Reservoir rock
• Accurate knowledge of rock properties aids in efficient development, management, and performance prediction of oilfields.

Physical and Fluid Transport

• Physical and fluid transport properties of rocks result from pore structure, grain cementation, and electrolytic properties.

Mineralogy and Geology

• Mineralogy and geology are intrinsically linked because most of the world's petroleum is found in porous sedimentary rock.

Sedimentary Rock

• Sedimentary rocks consist of fragments from other rocks that have undergone mechanical and chemical deterioration.

Particles of Erosion

• Particles of erosion are frequently transported by wind and surface streams.

Hydrocarbon Phase Behavior

• Hydrocarbon phase behavior describes the complex interaction between physically distinct, separable portions of matter.

Phases

• Phases are separable portions of matter.

Typical Phases

• Solid
• Liquid
• Vapor

Petroleum Applications of Phase Behavior

• Oil recovery
• Compositional simulation
• Geochemical behavior
• Wellbore stability
• Geothermal energy
• Environmental cleaning
• Multiphase flow in wellbores
• Pipes
• Surface facilities

Thermodynamics

• Thermodynamics, the study of energy and its transformations, is essential for understanding phase behavior.
• Thermodynamics began as the study of heat applied to stream power.

Gibbs Contribution

• Gibbs broadened thermodynamics in the mid-to-late 1880s.

Gibbs Most Significant Contribution

• Gibbs developed phase-equilibrium thermodynamics applicable to multicomponent mixtures.
• Chemical potential leads to equilibrium, ensuring each component's chemical potential is the same in all phases.

Phase-Equilibrium Thermodynamics

• Phase-equilibrium thermodynamics seeks to determine properties like temperature, pressure, and phase compositions.

Natural Occuring Hydrocarbon Systems

• A naturally occurring hydrocarbon system is a mixture of organic compounds.
• Hydrocarbon accumulation can occur in gaseous, liquid, or solid states, or in combination.

Differences in Phase Behavior

• Differences in phase behavior are coupled with the physical properties of rock that determine the relative ease with which gas and liquid are transmitted.

Petrophysical Data

• Wireline logs and logging-while-drilling
• Core analysis
• Laboratory experiment
• Seismic data
• Well test data

Hydrocarbon Phase Behavior Data

• Pressure-Volume-Temperature Relationship
• Fluid properties
• Phase equilibria and saturation
• Equation of state
• Fluid characterization

Subsurface Reservoir Rock

• A subsurface reservoir rock serves as natural storage for hydrocarbons.
• These rocks are primarily porous rocks like sandstones, limestone, or dolomites.
• The quantity of hydrocarbons depends largely on porosity and hydrocarbon saturation.

Porosity

• Porosity indicates the storage capacity of the rock based on the pore availability.

Hydrocarbon Saturation

• Hydrocarbon saturation denotes the proportion of pore spaces occupied by hydrocarbons.

Permeability

• Permeability describes the ease with which fluids migrate through the reservoir rock.

Fluid Type

• Fluid type identifies the composition of oil, gas, and water present in a reservoir.

Reservoir Extent

• Reservoir extent is the thickness and area of the reservoir, affects hydrocarbons volume.

Two Types of Migration

• Primary migration: Movement of hydrocarbons from the source rock into the reservoir rock.
• Secondary migration: Movement of hydrocarbons within the reservoir rock.

Two Techniques for Determination of Reservoir Properties

• Direct measurements on core samples: Involves extracting core samples from the subsurface during drilling.

Core Analysis

• Core analysis uses laboratory techniques to determine properties such as porosity, permeability, grain density, and fluid saturation and provides highly accurate and detailed information on the reservoir's properties.

Indirect Derivation from Well Logs

• Continuous recording of physical properties made while drilling or after the well has been completed.

Well Logging Tools

• Well logging tools measure parameters such as natural gamma radiation, electrical resistivity, acoustic travel time, and neutron porosity plus record physical properties that must be interpreted and transformed into meaningful reservoir characteristics.

Log Calibration

• Log calibration correlates log responses with core-derived properties.

Core Analysis Benefit

• Core analysis offers data that supports and validates log analysis.
• Core analysis is an expensive process, often limited to the reservoir interval to minimize costs.

Two Primary Methods for Acquiring Cores

• Conventional cores (rotary): 1½ inches (4.5 cm) to 5 ½ inches (13.5 cm) in diameter and provide a continuous record of rock formations.
• Sidewall cores: 1 inch in diameter and 3 inches in length, obtained through percussion or rotary sidewall coring methods.
• Sidewall coring is a less expensive method compared to full-diameter coring and only extracts small samples from the wellbore wall.

Two Categories of Core Analysis

• Routine core analysis (RCAL): Focuses on fundamental reservoir properties such as porosity, permeability, and fluid saturation and validating log interpretations and provides a baseline for the reservoir.
• Special core analysis (SCAL): Measures complex petrophysical properties like capillary pressure, relative permeability, wettability, and Archie's parameters to help in understanding fluid flow and reservoir behavior under various production scenarios.

Two Major Rock Classifications

• Clastic rock (sandstone): Formed from the compaction and cementation of mineral grains and other rock fragments.

Transported By

• Wind
• Water
• Ice

Environment

• Rivers
• Beaches
• Dessert

Sandstones

• Sandstones are among the most prevalent reservoir rocks due to their favorable porosity and permeability.
• Carbonate rocks (limestone, dolomite): Form primarily through the accumulation and lithification of biological materials like shells, corals, and algae and carbonate rocks are known for their complex, heterogenous matrices.

Porosity Types Often Possessed by Carbonates

• Intergranular
• Vuggy
• Fracture porosity

Clastic Rocks

• Clastic rocks are generally more predictable and consistent in their reservoir properties.

Types of Clay Distribution in Sedimentary Rocks

• The presence of clay in a reservoir rock influences its petrophysical properties and, consequently, the reservoir's capacity to store and transmit hydrocarbons.

Different Types of Clay Distribution in Sedimentary Rocks

• Laminar clay: Creates directional permeability barriers as thin clay layers alternate with sand or other granular materials that often deposit during formation, forming barriers to fluid flow and reducing overall permeability and can create anisotropy, with permeability varying by the direction of fluid flow.
• Dispersed clay: Reduces effective pore space and fluid flow, also known as authigenic clay, distributed throughout the pore spaces of the reservoir rock, often during diagenesis and reduces the effective porosity and permeability of the reservoir.
• Structural clay: Fundamentally alters the rock's framework, forming grains and acting as a fundamental rock-building component that simply occupies the pore spaces, altering the overall rock fabric.

Clastic Reservoirs

• Clastic reservoirs are primarily composed of varying proportions of clay, silt, and sand and exhibit diverse porosity.

Triangle Diagram (Ternary Diagram)

• A triangle diagram illustrates the composition of clastic sediments in terms of clay, silt, and sand fractions.

Triangle Diagram Components

• Sand occupies one apex, representing the coarse-grain fractions.
• Silt occupies another apex, representing the fine-grained, intermediate fractions.
• Clay occupies the third apex, representing the very fine-grained fraction.

Importance of Using Cores

• Cores provide direct determination of reservoir properties.
• Core analysis allows for direct measurements of key reservoir properties like porosity and permeability.
• Core data can be used to calibrate log measurements.
• Cores provide essential information on parameters like grain density and Archie's parameters.

Limitations and Challenges of Core Measurements

• Limited Representation: A core sample represents only a small section of the reservoir and may not be fully representative of entire formations.
• Alteration During Coring And Recovery: The process of coring and core recovery can alter the rock's original stress.
• Sample Preparation Artifacts: Plugging, cleaning, and drying of core samples for laboratory analysis can alter their wettability and potentially change the reservoir rock properties.
• Core analysis remains an essential component of reservoir characterization

Critical Factors

• Temperature and pressure are critical factors influencing hydrocarbon production.

Phase Behavior and Pressure Temperature Relationship

• Phase behavior is highly sensitive to changes in temperature and pressure.

Specific Phase Relationship

• Single liquid phase
• Single gas phase
• Mixture of liquid and gas

Reservoir Fluids

• Reservoir fluids can range from light, volatile oils and gas condensates.

Key Aspect of Phase Behavior

• Bubble point: the pressure at which gas begins to come out of solution from the oil when pressure decreases.
• Dew point is the pressure at which gas condensate turns into liquid as pressure increases.

Parameters That Dictate The Viscosities, Densities and Solubility of The Fluid

• Impact on Fluid Viscosities and Mutual Solubility: Higher temperature reduces the viscosity of the oil.
• Decreased Viscosity: Decreased viscosity can facilitate easier movement of oil through the reservoir rocks, improving the production
• Gas Viscosity: Gas viscosity increases with pressure but is generally lower than that of oil.
• Relatively Stable Viscosity: Relatively stable viscosity changes across the temperature and pressure ranges encountered in most reservoirs

Effect on Permeability and Fluid Flow

• As pressure drops below the bubble point in the reservoir, gas bubbles begin to form in the oil phase.
• Formation of Gas Bubbles: Gas bubbles reduce the effective permeability to oil because the gas occupies pore spaces and blocks pathways that oil would use to flow
• Blockage: Blockage reduces the relative permeability of the oil, hindering its ability to move through porous rock.
• Gas Locking: Gas locking can cause a substantial decline in oil production rates

Variable Reservoir Conditions

• The impact of temperature and pressure on phase behavior and fluid properties is highly variable and depends on the reservoir's specific conditions.
• Temperature and pressure are critical in determining hydrocarbon production
• Reservoir depth
• Geothermal gradient
• Fluid composition
• Reservoir rock

Deep Reservoirs

• Deep reservoirs with high temperature and pressure are more likely to contain hydrocarbons in a single phase.

Shallower Reservoirs

• Shallower reservoirs may exhibit multiphase behavior, with distinct oil and gas phases coexisting.