Sedimentary Rocks in the Production of Hydrocarbons

 

There are five types of sedimentary rocks that are important in the production of hydrocarbons:

Sandstones 

Sandstones are clastic sedimentary rocks composed of mainly sand size particles or grains set in a matrix of silt or clay and more or less firmly united by a cementing material (commonly silica, iron oxide, or calcium carbonate).

The sand particles usually consist of quartz, and the term “sandstone”, when used without qualification, indicates a rock containing about 85-90% quartz.

Carbonates, broken into two categories, limestones and dolomites.

Carbonates are sediments formed by a mineral compound characterized by a fundamental anionic structure of CO3^-2. 

-      Calcite and aragonite CaCO3, are examples of carbonates.

-  Limestones are sedimentary rocks consisting chiefly of the mineral calcite (calcium carbonate, CaCO3), with or without magnesium carbonate. Limestones are the most important and widely distributed of the carbonate rocks.

-    Dolomite is a common rock forming mineral with the formula CaMg(CO3)2. A sedimentary rock will be named dolomite if that rock is composed of more than 90% mineral dolomite and less than 10% mineral calcite.

Shales

Shale is a type of detrital sedimentary rock formed by the consolidation of fine-grained material including clay, mud, and silt and have a layered or stratified structure parallel to bedding. Shales are typically porous and contain hydrocarbons but generally exhibit no permeability. Therefore, they typically do not form reservoirs but do make excellent cap rocks. If a shale is fractured, it would have the potential to be a reservoir.

Evaporites

Evaporites do not form reservoirs like limestone and sandstone, but are very important to petroleum exploration because they make excellent cap rocks and generate traps. The term “evaporite” is used for all deposits, such as salt deposits, that are composed of minerals that precipitated from saline solutions concentrated by evaporation. On evaporation the general sequence of precipitation is: calcite, gypsum or anhydrite, halite, and finally bittern salts.

Evaporites make excellent cap rocks because they are impermeable and, unlike lithified shales, they deform plastically, not by fracturing.

The formation of salt structures can produce several different types of traps. One type is created by the folding and faulting associated with the lateral and upward movement of salt through overlying sediments. Salt overhangs create another type of trapping mechanism.

 

 

Defining Flow Units and Container

 

Introduction

To understand reservoir rock–fluid interaction and to predict performance, reservoir systems can be subdivided into flow units and containers. Wellbore hydrocarbon inflow rate is a function of the pore throat size, pore geometry, number, and location of the various flow units exposed to the wellbore , the fluid properties, and the pressure differential between the flow units and the wellbore. 

Reservoir performance is a function of the number, quality, geometry, and location of containers within a reservoir system, drive mechanism, and fluid properties. When performance does not match predictions, many variables could be responsible , however, the number, quality, and location of containers is often incorrect.

What is a flow unit?

A flow unites a reservoir subdivision defined on the basis of similar pore type. Petrophysical characteristics, such as distinctive log character and/or porosity–permeability relationships, define individual flow units. Inflow performance for a flow unit can be predicted from its inferred pore system properties, such as pore type and geometry. They help us correlate and map containers and ultimately help predict reservoir performance.  

What is a container?

A container is a reservoir system subdivision consisting of a pore system, made up of one or more flow units, that responds as a unit when fluid is withdrawn. Containers are defined by correlating flow units between wells. Boundaries between containers are where flow diverges within a flow unit shared by two containers . They define and map reservoir geology to help us predict reservoir performance.

Defining flow units

To delineate reservoir flow units, subdivide the wellbore into intervals of uniform petrophysical characteristics using one or more of the following:

• Well log curve character

• Water saturation (SW–depth plots)

• Capillary pressure data (type curves)

 • Porosity–permeability cross plots

 

The diagram below shows how flow units are differentiated on the basis of the parameters listed above.

Procedure: Defining containers:

Defining containers within a reservoir system is relative to the flow quality of the rock. Flow units with the largest connected pore throats dominate flow within a reservoir system. Follow the steps listed in the table below as a method for defining containers.
·         Correlate flow units between wells in strike and dip-oriented structural and stratigraphic cross sections.
·         Identify the high-quality flow units from rock and log data.
·         Draw boundaries between containers by identifying flow barriers or by interpreting where flow lines diverge within flow units common to both containers.

 

 

Common open-hole tools and their uses

Tool Physical Measurement Use Comments Logging Conditions Temperature (BHT)     Temperature Borehole temperature for resistivity calculations . Corrected with Horner plot  Pressure (PRESS) Fluid Pressure   Fluid pressure for formation volume factor calculations . Incorporated in RFT Borehole diameter   Borehole diameter Data quality, in situ stress tensor, lithology and permeability indicator. Available in 2.4. or multi-arm versions Lithology Gamma Ray (GR) Natural radioactivity  of the formation. Shale indicator and depth matching Can read through casing.   Spontaneous Potential (SP) Sand/Shale interface potential . Permeable beds Resistivity of formation water Does not work in conductive muds, or offshore. Porosity Sonic (BHC, LSS ) Velocity of an elastic wave in the formation. Effective (connected ) porosity . Compaction gas and vugs, calibration of seismic data. Density (FDC , LDT ) Bulk density of the formation. Total porosity . Used to calculate synthetic seismograms . Neutron (SNP, CNL ) Hydrogen concentration in the formation. Total porosity (shale increases measured porosity , gas reduces measured porosity ) Can read through casing Resistivity Simple electric log (SN, LN ,Lat ) Resistivity of flushed , shallow and deep zones respectively . Used in water saturation calculations. Now obsolete, not focused, can’t be used in oil based muds, prone to invasion. Induction Logs (IES , ISF, DIL, DISF, ILm, ILd ) Conductivity of the formation. Conductivity and resistivity in oil based muds, and hence calculation of water saturation. Focused devices. Use in oil based and fresh water muds. Range of depths of investigation. (Vertical resolution 5-10 ft.) Laterolog (LL3, LL7, DLL, LLs, LLd) Resistivity of the formation. Resistivity in water based muds, and hence calculation of water saturation Focused devices. Use in salt water based muds. Range of depths of investigation. (Vertical resolution 2-4 ft.) Microlog (ML) Resistivity of mudcake and flushed zone . Indicator of permeability. Detector of thin beds. (vertical resolution about 1 ft.) Micro-laterolog (MLL) Resistivity of flushed zone. Measures RXO Not good with thick mudcakes. Proximity Log (PL) Resistivity of flushed zone . Measures RXO Not good if invasion is small. Micro-spherically focused log (MSFL) Resistivity of flushed zone . Measures RXO Part of DLL-RXO tool.

Geological model vs. Reservoir model

What is model ?

• A model is a representation of some aspect of reality.
• The Purpose of creting a model is to help understand, describe , or predict how things work in the real world by exploring  a simplified representation of a particular object or setting .

 

Cellular  models : 

A cellular model is a schematic description of a reservoir that represents its properties. 

·         Geological model =Geomodel =Static model

·         Reservoir model =Dynamic model 

   A Cellular Model is required:

·         To understand the complexity of reality

·         To quantify reality

 

®     Modeling objectives: Simplify to Quantify

Geological Model vs. Reservoir Model : 

Reservoir model definition: 

Reservoir model construction is the main objective of an integrated reservoir study. It is a grid of cells which allows manage.

Geomodel represents one of the most important phases in an integrated reservoir study workflow, because : 

• It integrates reservoir geometry, Lithofacies and petrophysical properties distribution consistency.
• It takes into account dynamic information.
• It provides key heterogeneity modeling (main flow units ).

Upscaling : a challenge for information integration.  

A Reservoir model : what for ? 

It is used to simulate the evolution of a field throughout time.

• Well production
• Fluid movements within the reservoir
• Pressure evolution 

A good Reservoir Model is strongly constrained by the Geological Model:

·         Fluid flow simulation is more realistic and more reliable.

A reliable Geological Model takes into account dynamic data:

·         Identification of main faults which impact fluid flow (either permeability barrier or conductive faults).

·         Stratigraphic barriers or multiple reservoirs.

 

®     Need strong integration between geo-disciplines (Geophysicist, Geologist, Reservoir engineers) to ensure final model consistency  (Volume , pressure , rates )