Importance of Reservoir Geology - Essay Example

Reservoir Geology Reservoir Geology Introduction Reservoir geology refers to the studies of the nature of rocks in which the subsurface hydrocarbons such as petroleum, oil are trapped in. In oil and gas engineering, reservoirs generally refer to porous or permeable lithological set of units that trap hydrocarbon reserves. An analysis of both the external and internal geology of reservoirs is critically important for their development, management and production. For example, external geology of the reservoir is primarily concerned with the geological forces behind the creation of hydrocarbon trap while internal geology of the reservoir is closely linked to the nature of the rocks under which the hydrocarbons are found (Dake, 2011). According to many experts, permeable and porous reservoir rocks are normally the primary basis for the presence of petroleum gas or oil as well as the extent to which these important resources can be effectively produced. In its simplest form, analysis of reservoirs normally involves a critical assessment of their porosity to help calculate the potential volume of hydrocarbons as well as the permeability to help calculate how easily the hydrocarbons are likely to flow out of the rocks (Rousel-Houston, 2008). This paper critically describes the reservoir geology with particular focus to its significance in the mining of naturally occurring hydrocarbons such as natural gas and crude oil that are usually trapped below the overlying rock formations with lower permeability or porosity. Importance of Reservoir Geology There are a diverse number of reasons why the understanding of reservoir geology is critically important particularly for oil and gads engineers. For example, engineers require developing a 3- D conceptual model of the reservoirs to facilitate efficient extraction of the hydrocarbons. Additionally, the conceptual model helps in the decision-making process in selecting perforations and forecasting production. On the other hand, many of the engineering measurements currently being used on reservoirs have little spatial information. For instance, core measurements do not usually have any dimensional information and continuous core measurements and wireline logs are only 1 dimensional. However, geologic information contains critically valuable spatial data that helps in visualizing the reservoir in 3-D. In this regard, knowledge of reservoir geology is critically important because to oil and gas engineers because it enables them to understand important geological data that not only improves their conceptualization of the reservoir but also their engineering decision making processes. Lastly, one of the most important sources of geological information is the external geometry of the particular reservoir as defined by the flow barriers and seals that normally inhibit the migration of hydrocarbon during tectonic movements thereby forming a hydrocarbon trap. On the other hand, the other important geologic information is the architecture of the internal reservoir. Generally, engineers can acquire significant information regarding the potential flow of gas or oil through the reservoir by analyzing the stratigraphy, sedimentology as well as the geometry of the different constituent geological formations of a given area. External Geology of the Reservoir External geology of the reservoir is largely concerned with the geological forces behind the creation of hydrocarbon trap. The information from the External geometry of a reservoir is critically important during the exploration as well as the initial development of a reservoir. According to many experts, external geology of the reservoir is defined by the flow barriers and seals that normally inhibit the migration of hydrocarbon during tectonic movements thereby forming a hydrocarbon trap. In most case, the migration is particularly driven by the buoyancy force generated by the difference in intensity between the hydrocarbons and water. However, when the hydrocarbon undergoing migration encounters a trap, the migration is stopped and a hydrocarbon reservoir is then formed. Seals A Seal is a low permeable to impermeable rock .it has a big capillary entry pressure capable of trapping hydrocarbons (Sneider et al,. 1997).Traps originate from structural, sedimentary or digenetic geometries. Seals prevent the migration of hydrocarbons, forming the hydrocarbon trap. They define the external geometry of the reservoir. Density difference between water and hydrocarbons creates abuoyancy that powers migration of hydrocarbons. The hydrocarbon trap stops the migration forming the reservoir. Generally, traps are normally comprised of different types of seals such as: Bottom seals Lateral seals Top seals Traps A simple trap is convex and has a layer that that form a dome or double dipping anticline. Complex structural traps are formed when rounded structures are cut by faults (Sneider, 1997). Stratigraphic traps may form when deposition creates topographic high encased by impermeable lithology such as shale or salt. Stratigraphic traps are formed by deposition activity that results in paleotopographic highs encased in impermeable materials. The traps are closed when the seal materials and the underlying sediment come into contact. The volume of oil and gas that accumulates in a reservoir is defined by the height of the trap because additional hydrocarbons spill out to the bottom (Bowen, 2004, p.12). The base of the trap is referred to as the spill point. They trap may not fill up because height of the oil column is controlled by the ability of the seal to impeded flow. Subsurface fluids are usually not static and hence oil and water contacts need not be horizontal. The geometry of traps normally have three common origin types namely: Structural Sedimentary Diagenetic Fig 1: An example of a structural Strap Internal Reservoir Architecture The internal reservoir architecture contributes important geological information that is critically important during the development of a reservoir particularly in making the predictions regarding the distribution of the reservoir quality in order to allow for the planning of both primary and secondary reservoir development programs. Some of the key information that are usually obtained from internal reservoir architecture include: Seal capacity Trap configuration The base of the reservoir A reservoir is made up of rocks of varying reservoir quality systematically stacked according to stratigraphic and digenetic principles. Depositional environment and vertical stacking are related to lateral distribution of depositional textures. The changes that happen after deposition are referred to as digenesis. They control the lateral continuity and vertical stacking of reservoir rock types. This commonly happens in carbonate reservoirs where conversion of limestone to dolostone and dissolution of carbonate have a significant effect on internal reservoir architecture. Petrochemical engineers must be able to equate log and core measurements with geologic models, as the measurements do not contain spatial data. The measurements are important when performed at the rock fabric level. The rock fabric controls pore size distribution. Information describing the external reservoir geometry is very important during exploration and initial development of a reservoir. The trap configuration, seal capacity and the base of the reservoir give key details about a reservoir. Generally, reservoir architecture helps engineers to predict the reservoir quality so that primary and secondary development programs can be planned. Additionally, the reservoir architecture provides the basis for distributing petro physical properties in 3-D space. This is usually done by relating lithofacies to petro physical properties because lithofacies can be linked directly to depositional processes for prediction. Formation of hydrocarbons Anaerobic decomposition of organic matter deposited deep in the ocean basins forms hydrocarbons. High temperatures in the buried organic matter produce oil and gas. Oil forms first followed by wet gas and finally dry gas. Once made oil and gas travel vertically and laterally through overlying sediments because of density differences between hydrocarbons and water. Their movement along permeable formations is stopped by reservoir trap in which they accumulate. As the oil is first produced, it fills the trap. High temperatures result from continued burial causes gas to be produced. Migrating gas displace oil from the traps because it has a lower density. The displaced oil migrates and fills any traps encountered. A hydrocarbon trap is defined by the geometry of its seals that are formations with very low permeability and very small pores, which impede the flow of hydrocarbons. Seals may occur in the body of impermeable lithology. Sequence stratigraphy The chronostratigraphic method of correlation is called sequence stratigraphy. It groups lithofacies into time stratigraphic units between chronostratigraphic surfaces which can be defined by unconformities and faces shifts. These surfaces are principally thought to be created due to eustatic sea level changes of various scales and periodicity. Eustatic seas level changes can be linked to climatic changes and eccentricities in the earth’s orbit. Sequence stratigraphy is crucial to reservoir modeling because chronostratigraphic surface is found in every well in the reservoir. Geologists use this to correlate packages of lithofacies between wells. This allows a realistic image of the reservoir to be constructed by distributing lithofacies and petro physical properties within a detailed sequence stratigraphic framework. Conclusion In conclusion, knowledge of reservoir geology is critically important because to oil and gas engineers because it enables them to understand important geological data that not only improves their conceptualization of the reservoir but also their engineering decision making processes. The external geometry of the reservoir provides information on the process that created the hydrocarbon trap while the internal geology of reservoirs is more concerned with the nature of the rocks in which the hydrocarbons exist. References Bowen DW. 2004. Reservoir Geology of the Morrow Formation, Eastern Colorado. Development of V7 Valley-Fill. Retrieved March 15, 2014 from www.netl.doe.gov/.../05/.../Poster%2039.pdf‎ Dake L.P. 2011. The Practice of Reservoir Engineering. NEW York: Routlege. Retrieved on March 15, 2014 from www.inf-aspirantura.ipter.ru/Biblio/02.pdf‎ Guerriero V.2011. Improved statistical multi-scale analysis of fractures in carbonate reservoir analogues. Tectonophysics 504: 14–24.  Rousel-Houston, J. YouTube Video: The Role of Geology in Reservoir Development. Retrieved March 15, 2014 from www.youtube.com/watch?v=30Uh7ThgsKU Sneider, R.M. 1997. Comparison of Seal Capacity Determinations: Conventional Cores Versus Cuttings. Seals, Traps, and the Petroleum System, R.C. Surdam ed., 12. AAPG Memoir 67. http://archives.datapages.com/data/specpubs/mem67/ch01/ch01.htm. Read More