Reservoir sand

Growth faulted deltaic areas are highly prospective since they comprise of thick sections of good quality reservoir sands. Deltas usually overlay organic rich marine clays which can source the structures on maturation. Examples are the Niger, Baram or Mississippi Deltas. Clays, deposited within deltaic sequences may restrict the water expulsion during the rapid sedimentation / compaction. This can lead to the generation of overpressures. 

For example, In the following situation, two wells have penetrated the same reservoir sand. The updip well finds the sand gas bearing, with gas down to (GOT) at the base of the sands, while the downdip well finds the same sand to be fully oil bearing, with an oil up to (OUT) at the top of the sand. Pressures taken at intervals in each well may be used to predict where the possible gas-oil contact (PGOC) lies. This method is known as the gradient intercept technique. 

We have all used maps to orientate ourselves in an area on land. Likewise, a reservoir map will allow us to find our way through an oil or gas field if, for example we need to plan a well trajectory or If we want to see where the best reservoir sands are located. However, maps will only describe the surface of an area. To get the third dimension we need a section which cuts through the surface. This is the function of a cross section. Figure. 5.44 shows a reservoir map and the corresponding cross section. 

If the original field development plan was not based on a 3-D seismic survey (which would be a commonly used tool for new fields nowadays), then it would now be normal practice to shoot a 3-D survey for development purposes. The survey would help to provide definition of the reservoir structure and continuity (faulting and the extension of reservoir sands), which is used to better locate the development wells. In some cases time-lapse 3-D seismic 4D surveys carried out a number of years apart, see Section 2) is used to track the displacement of fluids in the reservoir. 

Site-specific studies will have to be done with reservoir sand, reservoir oil, and field water to determine the suitability of each design. 

Bunge, A. L. and C. J. Radke, 1982, Migration of alkaline pulses in reservoir sands. Society of Petroleum Engineers Journal 22,998-1012. 

FothergiU C. A. (1955) The cementation of oil reservoir sands and its origin. Proc. World Petrol. Cong. 59, 301 -314. 

In Eq. 10.30, the first term corresponds to accumulation in the fluid and the surfaces, the second term describes convective transport, and the third term indicates the loss by the kinetic dissolution reaction defined by Eq. 10.28. Equation 10.30 applies to any chemical transport process that includes fast and reversible ion-exchange, and slow and irreversible dissolution of the mth-order kinetics. In reservoir sands, both fine silica and clay minerals dissolve under attack by the alkali, yielding a complex distribution of soluble solution products... 

Lieu, V.T., Miller, S.G., Staphanos, S.J., 1982. Long-term consumption of caustic and silicate solutions by petroleum reservoir sands. Soluble Silicates, Am. Chem. Soc. Symposium Series 194, 215-226. 

Effect of pH. The pH of the flowing fluid is an important factor in the fines migration process (5, 12, 52, 58). Mungan (52) noted that injection of strong acids or bases could cause permeability damage. Under very high or very low pH conditions, the permeability damage is caused by dissolution of the matrix material, which produces fine particles of varied mineral composition. Somerton et al. (58) found that the water sensitivity of reservoir sands was related to the pH response exhibited by the rock after the contact with fresh water. Most sandstone cores showed an increase in the effluent pH after the switch was made... 

Solids production from these heavy oil reservoirs was first discussed in some detail by Smith (97). Smith developed an analytical model to predict production, decline, recovery, pressure, and pressure-transient behavior, together with the large solids volume production and its effect on oil rate and well productivity. Smith s model incorporated time-de-pendent properties of the oil as a result of gas evolution and treated the unconsolidated reservoir sand as a soil in which cohesion relies only on the tension of the wetting phase. This is a similar, though simpler, approach than Vaziri s (54) finite element method. Smith developed a Darcy law formulation for compressible fluid flow... 

The simulated present reservoir pressure (Fig. 5c) indicates significant decrease in effective stress, but below estimated fracture pressure (0.8-0.85 X geostatic pressure) in the Snorre field. An important factor in controlling the estimated porosities in the reservoir sands, due to the... 

Rotated fault blocks like Smorbukk and the Halten Vest high-pressure structures have a general morphology as illustrated in Figure 21 and the shales separating the reservoir sands... 

Ukhta system The access drifts and drilling chambers were completed in the tuffite caprock just above the reservoir. A dense network of straight shallow (less than SO m deep) wells, 12 cm in diameter, with well bottoms spaced at 12-20 m were drilled from the chambers into the reservoir sands below. 

Hot air is injected through the well and into the bottomhole zone of the reservoir sand. The petroleum in the sand converts to coke which then becomes the cementing agent, holding the loose sand together. At the same time, aside from the desired consolidation of the sand, a fairly high permeability is retained within the treated zone near the well bottom. 

The coking of reservoir sands in the bottomhole zone must take place under strictly controlled conditions as regards (a) the rate of air injection (b) rate at which the temperature of injected air is increased (c) the temperature maximum (d) the duration of the treatment, and (e) the energy consumption. 

Nevertheless, in case of thermal treatment, certain addidonal condidons may have to be met e.g., in shallow petroleum fields with muldple producing horizons made up of poorly cemented reservoir sands that may have tendency to run, such individual sands should not be unified unless effective measures are first taken to prevent sanding of producing wells. 

Fig. 4.15. Possible mechanism for mass transfer of silica (quartz) in moving pore fluids during diagenesis in reservoir sand units, a In fluids supplied by mechanical sediment dewatering and com-paction b in fluids by recharge from seawater c in fluids convecting within a sandstone sequence realizing SiOj of various origin including pressure soiution (from Wood and Hewett 1982)...

Long-Term Consumption of Caustic and Silicate Solutions by Petroleum Reservoir Sands... 

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