Reservoirs hydrocarbon

A further assumption is that the pattern and intensity of this leakage also provides information on preferential pathways that the leakage follows, and as such can be combined with additional geologic information to predict broad subsurface hydrocarbon fairways. In fact, in some instances it has been claimed that such data can identify areas of reservoired hydrocarbons. This last claim is often the subject of heated debate, however, commonly depending in which camp (for or against geochemistry) the explorationist resides. 

Fig. 5-28. Fluorograms of reservoir hydrocarbons and corresponding hydrocarbons extracted from sediments in the Gulf of Mexico.

Scott, L.F. and McCoy, R.M., 1993. Near surface soil the interface between deep reservoir hydrocarbons and metals in sagebrush, Lodgepole oil field, Utah. Proc. Ninth Thematic Conference on Geologic Remote Sensing, Pasadena, pp. 861-869. 

This trend basically follows a maturity map based upon the depth to base Cretaceous and it is hence inferred that lateral migration distances for the presently reservoired hydrocarbons is short, i.e. shorter than 15-20 km, with notable exceptions concerning migration of the hydrocarbons found in Midgard and Draugen. 

The fundamental question asked when evaluating an exploration prospect is how much hydrocarbon resource is in place To answer this question, the conventional reservoir engineering approach is to compute the probable reservoir hydrocarbon pore volume, i.e. the size and extent of the hydrocarbon container , from estimates of reservoir rock and fluid properties. Uncertainties in the geological model, source rock characteristics, trapping mechanisms and seal rock effectiveness, and the presence of sufficient reservoir rock for commercial hydrocarbon production, are often represented as a singlevalued probability of success. 

THE ROLE OF ASPHALTENE AGGREGATION IN VISCOSITY VARIATION OF RESERVOIR HYDROCARBONS AND IN MISCIBLE PROCESSES... 

The aggregation and segregation of asphaltene primarily Influence the viscosity of heavy oils and bitumens and as such can be used to explain variations of viscosity of reservoir hydrocarbons. 

The Role of Asphaltene Aggregation in Viscosity Variation of Reservoir Hydrocarbons and... 

The cubic plus association equation has been applied to oil reservoir hydrocarbons with the ubiquitous water using the combining rule given by eq 5.200. The phase equilibria of (water- -CO2) is important for carbon sequestration in geological formations and requires methods to predict the phase boundaries and solubility. Pappa et al estimated the phase boundaries... 

OIL RECOVERY VIA DIRECT OXIDATION OF RESERVOIR HYDROCARBONS BY AIR OXYGEN... 

The H2S formed can react with the sulfates or rock to form sulfur i (Equation 8.2) that remains in suspension as in the case of crude from Goldsmith, Texas, USA, or that, under the conditions of pressure, temperature I and period of formation of the reservoir, can react with the hydrocarbons to give sulfur compounds ... 

H2S is found with the reservoir gas and dissolved in the crude (< 50 ppm by weight), but it is formed during refining operations such as catalytic cracking, hydrodesulfurization, and thermal cracking or by thermal decomposition of sulfur[Pg.322]

Migration describes the process which has transported the generated hydrocarbons into a porous type of sediment, the reservoir rock. Only if the reservoir is deformed in a favourable shape or if it is laterally grading into an impermeable formation does a trap for the migrating hydrocarbons exist. 

The pores between the rock components, e.g. the sand grains in a sandstone reservoir, will initially be filled with the pore water. The migrating hydrocarbons will displace the water and thus gradually fill the reservoir. For a reservoir to be effective, the pores need to be in communication to allow migration, and also need to allow flow towards the borehole once a well is drilled into the structure. The pore space is referred to as porosity in oil field terms. Permeability measures the ability of a rock to allow fluid flow through its pore system. A reservoir rock which has some porosity but too low a permeability to allow fluid flow is termed tight . 

Even if all of the elements described so far have been present within a sedimentary basin an accumulation will not necessarily be encountered. One of the crucial questions in prospect evaluation is about the timing of events. The deformation of strata into a suitable trap has to precede the maturation and migration of petroleum. The reservoir seal must have been intact throughout geologic time. If a leak occurred sometime in the past, the exploration well will only encounter small amounts of residual hydrocarbons. Conversely, a seal such as a fault may have developed early on in the field s history and prevented the migration of hydrocarbons into the structure. 

The objective of any exploration venture is to find new volumes of hydrocarbons at a low cost and in a short period of time. Exploration budgets are in direct competition with acquisition opportunities. If a company spends more money finding oil than it would have had to spend buying the equivalent amount in the market place there is little Incentive to continue exploration. Conversely, a company which manages to find new reserves at low cost has a significant competitive edge since it can afford more exploration, find and develop reservoirs more profitably, and can target and develop smaller prospects. 

An intermediate casing is usually set above the reservoir in order to protect the water bearing, hydrostatically pressured zones from influx of possibly overpressured hydrocarbons and to guarantee the integrity of the well bore above the objective zone. In mature fields where production has been ongoing for many years, the reservoir may show depletion pressures considerably lower than the hydrostatically pressured zones above. Casing and cementing operations are covered in section 3.6. 

Laminae of clay and clay drapes act as vertical or horizontal baffles or barriers to fluid flow and pressure communication. Dispersed days occupy pore space-which in a clean sand would be available for hydrocarbons. They may also obstruct pore throats, thus impeding fluid flow. Reservoir evaluation, is often complicated by the presence of clays. This is particularly true for the estimation of hydrocarbon saturation. 

For example, the many deepwater fields located in the Gulf of Mexico are of Tertiary age and are comprised of complex sand bodies which were deposited in a deepwater turbidite sequence. The BP Prudhoe Bay sandstone reservoir in Alaska is of Triassic/ Cretaceous age and was deposited by a large shallow water fluvial-alluvial fan delta system. The Saudi Arabian Ghawar limestone reservoir is of Jurassic age and was deposited in a warm, shallow marine sea. Although these reservoirs were deposited in very different depositional environments they all contain producible accumulations of hydrocarbons, though the fraction of recoverable oil varies. In fact, these three fields are some of the largest in the world, containing over 12 billion barrels of oil each ... 

Fault seals are known to have been ruptured by excessive differential pressures created by production operations, e.g. if the hydrocarbons of one block are produced while the next block is kept at original pressure. Uncontrolled cross flow and inter-reservoir communication may be the result. 

Carbonate rocks are more frequently fractured than sandstones. In many cases open fractures in carbonate reservoirs provide high porosity / high permeability path ways for hydrocarbon production. The fractures will be continuously re-charged from the tight (low permeable) rock matrix. During field development, wells need to be planned to intersect as many natural fractures as possible, e.g. by drilling horizontal wells. 

In rare cases compaction may be artificially Initiated by the withdrawal of oil, gas or water from the reservoir. The pressure exerted by the overburden may actually help production by squeezing out the hydrocarbons. This process is known as compaction drive and some shallow accumulations in Venezuela are produced In this manner in combination with EOR schemes like steam injection. 

If compaction occurs as a result of production careful monitoring is required. The Ekofisk Field in the Norwegian North Sea made headlines when, as a result of hydrocarbon production, the pores of the fine-grained carbonate reservoir collapsed and the platforms on the seabed started to sink. The situation was later remedied by inserting steel sections into the platform legs. Compaction effects are also an issue in the Groningen gas field in Holland where subsidence in the order of one meter is expected at the surface. 

While the long chain hydrocarbons (above 18 carbon atoms) may exist in solution at reservoir temperature and pressure, they can solidify at the lower temperatures and pressures experienced in surface facilities, or even in the tubing. The fraction of the longer chain hydrocarbons in the crude oil are therefore of particular interest to process engineers, who will typically require a detailed laboratory analysis of the crude oil oomposition, extending to the measurement of the fraction of molecules as long as C3Q. 

As the conditions of pressure and temperature vary, the phases in which hydrocarbons exist, and the composition of the phases may change. It is necessary to understand the initial condition of fluids to be able to calculate surface volumes represented by subsurface hydrocarbons. It is also necessary to be able to predict phase changes as the temperature and pressure vary both in the reservoir and as the fluids pass through the surface facilities, so that the appropriate subsurface and surface development plans can be made. 

In the production of hydrocarbon reservoirs, the process of isothermal depletion is normally assumed, that is reducing the pressure of the system while maintaining a constant temperature. Hence, a more realistic movement on the pressure-temperature plot is from point A to A . 

The example of a binary mixture is used to demonstrate the increased complexity of the phase diagram through the introduction of a second component in the system. Typical reservoir fluids contain hundreds of components, which makes the laboratory measurement or mathematical prediction of the phase behaviour more complex still. However, the principles established above will be useful in understanding the differences in phase behaviour for the main types of hydrocarbon identified. 

The above equation is valid at low pressures where the assumptions hold. However, at typical reservoir temperatures and pressures, the assumptions are no longer valid, and the behaviour of hydrocarbon reservoir gases deviate from the ideal gas law. In practice, it is convenient to represent the behaviour of these real gases by introducing a correction factor known as the gas deviation factor, (also called the dimensionless compressibility factor, or z-factor) into the ideal gas law ... 

In Section 5.2.8 we shall look at pressure-depth relationships, and will see that the relationship is a linear function of the density of the fluid. Since water is the one fluid which is always associated with a petroleum reservoir, an understanding of what controls formation water density is required. Additionally, reservoir engineers need to know the fluid properties of the formation water to predict its expansion and movement, which can contribute significantly to the drive mechanism in a reservoir, especially if the volume of water surrounding the hydrocarbon accumulation is large. 

In order to contain normal or abnormal pressures, a pressure seal must be present. In hydrocarbon reservoirs, there is by definition a seal at the crest of the accumulation, and the potential for abnormal pressure regimes therefore exists. 

A vast variety of logging tools are In existence and Section 5.4 will cover only those which enable the evaluation of essential reservoir parameters, specifically net reservoir thickness, lithology, porosity and hydrocarbon saturation. 

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