Reservoir clastic

Reservoir rocks are either of clastic or carbonate composition. The former are composed of silicates, usually sandstone, the latter of biogenetically derived detritus, such as coral or shell fragments. There are some important differences between the two rock types which affect the quality of the reservoir and its interaction with fluids which flow through them. 

With a few exceptions reservoir rocks are sediments. The two main categories are siliciclastic rocks, usually referred to as elastics or sandstones , and carbonate rocks. Most reservoirs in the Gulf of Mexico and the North Sea are contained in a clastic depositional environment many of the giant fields of the Middle East are contained in carbonate rocks. Before looking at the significance of depositional environments for the production process let us investigate some of the main characteristics of both categories. 

Shallow marine/ coastal (clastic) Sand bars, tidal channels. Generally coarsening upwards. High subsidence rate results in stacked reservoirs. Reservoir distribution dependent on wave and tide action. Prolific producers as a result of clean and continuous sand bodies. Shale layers may cause vertical barriers to fluid flow. 

It is believed that the majority of clastic reservoir rocks are water wet, but the subject of wettability is a contentious one. 

The field unit for permeability is the Darcy (D) or millidarcy (mD). For clastic oil reservoirs, a good permeability would be greater than 0.1 D (100 mD), while a poor permeability would be less than 0.01 D (10 mD). For practical purposes, the millidarcy is commonly used (1 mD = 10" m ). For gas reservoirs 1 mD would be a reasonable permeability because the viscosity of gas is much lower than that of oil, this permeability would yield an acceptable flowrate for the same pressure gradient. Typical fluid velocities in the reservoir are less than one metre per day. 

In this chapter, in an attempt to devise methods for helping to foresee such unfavorable consequences, we construct models of the chemical interactions between injected fluids and the sediments and formation waters in petroleum reservoirs. We consider two cases the effects of using seawater as a waterflood, taking oil fields of the North Sea as an example, and the potential consequences of using alkali flooding (i.e., the injection of a strong caustic solution) in order to increase oil production from a clastic reservoir. 

Fig. 30.3. Variation in pH during simulated alkali floods of a clastic petroleum reservoir at 70 °C, using 0.5 N NaOH, Na2C03, and Na2Si03 solutions. Pore fluid is displaced by unreacted flooding solution at a rate of one-half of the system s pore volume per day.

Fig. 30.4. Changes in the volumes of minerals in the reservoir rock during the simulated alkali floods (Fig. 30.3) of a clastic petroleum reservoir using NaOH, Na2CC>3, and Na2SiC>3 solutions. Minerals that react in small volumes are omitted from the plots. Abbreviations Anal = analcime, Cc = calcite, Daw = dawsonite, Dol = dolomite, Kaol = kaolinite, Muse = muscovite, Parag = paragonite, Phlog = phlogopite, Qtz= quartz, Trid = tridymite.

In the simulations, a significant fraction (about 50% to 80%) of the alkali present in solution is consumed by reactions near the wellbore with the reservoir minerals (as shown in Reaction 30.6 for the NaOH flood), mostly by the production of analcime, paragonite, and dawsonite [NaAlC03(0H)2]. In the clastic reservoir considered, therefore, alkali floods might be expected to cause formation damage (mostly due to the precipitation of zeolites) and to be less effective at increasing oil mobility than in a reservoir where they do not react extensively with the formation. 

Amy Berger helped me write Chapter 10 (Surface Complexation), and Chapter 31 (Acid Drainage) is derived in part from her work. Edward Warren and Richard Worden of British Petroleum s Sunbury lab contributed data for calculating scaling in North Sea oil fields, Richard Wendlandt first modeled the effects of alkali floods on clastic reservoirs, and Kenneth Sorbie helped write Chapter 30 (Petroleum Reservoirs). I borrowed from Elisabeth Rowan s study of the genesis of fluorite ores at the Albigeois district, Wendy Harrison s study of the Gippsland basin, and a number of other published studies, as referenced in the text. 

Clastic sediments are reservoirs of information about weathering processes, but are sufficiently complex that no study has yet to realize their potential. Despite a number of initial reports of relatively isotopically heavy samples, the majority of data for clastic sedimentary rocks have an average 8 Li 0, equivalent to the estimated average isotopic composition of the continental crust. 

Smith J. T. and Ehrenberg S. N. (1989) Correlation of carbon dioxide abundance with temperature in clastic hydrocarbon reservoirs relationship to inorganic chemical equilibrium. Mar. Petrol. Geol. 6, 129-135. 

Worden R. H. and Smalley P. C. (2001) H2S in North Sea oil fields importance of thermochemical sulfate reduction in clastic reservoirs. In Proceedings of the 10th International Symposium on the Water Rock Interaction (ed. R. Cidu). A. A. Balkema Publishers, Rotterdam, pp. 659-662. 

Aagaard P., Egeberg P. K., Saigal G. C., Morad S., and Bj0rlykke K. (1990) Diagenetic albitization of detrital K-feldspars in Jurassic, Lower Cretaceous, and Tertiary clastic reservoir rocks from offshore Norway 11. Formation water chemistry and kinetic considerations. J. Sedim. Petrol. 60, 575-581. 

Bloch S. (1994) Secondary porosity in sandstones significance, origin, relationship to subaerial unconformities, and effect on predrill reservoir quality prediction. In Reservoir Quality Assessment and Prediction in Clastic Rocks (ed. M. D. Wilson). Society for Sedimentary Geology (SEPM), vol. 30, pp. 137-159. 

Loucks R. G., Dodge M. M., and Galloway W. E. (1984) Regional controls on diagenesis and reservoir quality in Lower Tertiary sandstones. In Clastic Diagenesis (eds. D. A. McDonald and R. C. Surdam). American Association of Petroleum Geologists, Tulsa, OK, vol. 37, pp. 15-45. 

Kaiser, W.R. (1984) Predicting reservoir quality and diagenetic history in the Frio Formation (Oligocene) of Texas. In Clastic Diagenesis (Ed. McDonald, D.A. Surdam, R.C.). Mem. Am. Ass. petrol. Geol., Tulsa, 37, 195-216. 

Saigal, G.C. Bjorlykke, K. (1987) Carbonate cements in clastic reservoir rocks from offshore Norway— relationships between isotopic composition, textural development and burial depth. In Diagenesis of Sedimentaiy Sequences (Ed. Marshall, J.D.), Spec. Publ. geol. Soc. London, 36, 313-324. 

De Ros, L.F. (1990) Porosity preservation and generation in deep clastic reservoirs a review. Bol. Geoc. PETROBRAS, 4, 387-404 (in Portuguese with a summary in English). 

Johansen, S.J. (1993) Depositional and structural controls on the diagenesis of Lockhart Crossing reservoir (Wilcox), Gulf Coast of Louisiana (U.S.A.). In Marine Clastic Reservoirs Examples and Analogs (Eds Moslow, T.F. Rhodes, E.G.), pp. 117-134. Springer-Verlag, New York. 

Nagtegaal, P.J.C. (1980) Diagenetic models for predicting clastic reservoir quality. Rev. Inst. Invest. Geol., 34, 5-19. 

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