Reservoir quality

Shallow water carbonate (reefs carbonate muds) Reservoir quality governed by diagenetic processes and structural history (fracturing). Prolific production from karstified carbonates. High and early water production possible. Dual porosity systems in fractured carbonates. Dolomites may produce H S. 

Shelf (elastics) Sheet-like sandbodies resulting from storms or transgression. Usually thin but very continuous sands, well sorted and coarse between marine clays. Very high productivity but high quality sands may act as thief zones during water or gas injection. Action of sediment burrowing organisms may impact on reservoir quality. 

The maps most frequently consulted in field development are structural maps and reservoir quality maps. Commonly a set of maps will be constructed for each drainage unit. 

Reservoir quality maps are used to illustrate the lateral distribution of reservoir parameters such as net sand, porosity or reservoir thickness. It is important to know whether thickness values are isochore or isopach (see Figure 5.46). Isochore maps are useful if properties related to a fluid column are contoured, e.g. net oil sand. Isopach maps are used for sedimentological studies, e.g. to show the lateral thinning out of a sand body. In cases of low structural dip (<12°) isochore and isopach thickness are virtually the same. 

The macroscopic sweep efficiency s the fraction of the total reservoir which is swept by water (or by gas in the case of gas cap drive). This will depend upon the reservoir quality and continuity, and the rate at which the displacement takes place. At higher rates, displacement will take place even more preferentially in the high permeability layers, and the macroscopic displacement efficiency will be reduced. 

The number of injectors required may be estimated in a similar manner, but it is unlikely that the exploration and appraisal activities will have included injectivity tests, of say water injection into the water column of the reservoir. In this case, an estimate must be made of the injection potential, based on an assessment of reservoir quality in the water column, which may be reduced by the effects of compaction and diagenesis. Development plans based on water injection or natural aquifer drive often suffer from lack of data from the water bearing part of the reservoir, since appraisal activity to establish the reservoir properties in the water column is frequently overlooked. In the absence of any data, a range of assumptions of injectivity should be generated, to yield a range of number of wells required. If this range introduces large uncertainties into the development plan, then appraisal effort to reduce this uncertainty may be justified. 

The geometry and reservoir quality have a very important influence on whether horizontal wells will realise a benefit compared to a vertical well, as demonstrated by the following example. 

Horizontal wells have a large potential to connect laterally discontinuous features in heterogeneous or discontinuous reservoirs. If the reservoir quality is locally poor, the subsequent section of the reservoir may be of better quality, providing a healthy productivity for the well. If the reservoir is faulted or fractured a horizontal well may connect a series of fault blocks or natural fractures In a manner which would require many vertical wells. The ultimate recovery of a horizontal well is likely to be significantly greater than for a single vertical well. 

The data gathered from the logs and cores of the development wells are used to refine the correlation, and better understand areal and vertical changes in the reservoir quality. Core material may also be used to support log data in determining the residual hydrocarbon saturation left behind in a swept zone (e.g. the residual oil saturation to water flooding). 

FIGURE 3.7 Diagram showing effects of sedimentary structures and textures on the flow of fluids in a point-bar sandstone reservoir. The cross-bedded unit is coarser grained and is inferred to have better reservoir properties (i.e., permeability) than the overlying rippled unit (A). Uneven advance of injected fluids illustrating permeability variations results from differences in reservoir quality. (Modified after Ebanks, 1987.)... 

Johnson (Personal Communication, March 6, 2006) suggests that hydrate indications by BSRs have been superseded by other, more reliable geological factors, The key elements of commercial gas hydrate prospects are (1) reservoir quality sands, (2) sufficient gas flux, and (3) the GHSZ. Where all three are together commercial accumulations are likely. If any one is missing, there will be no prospect. ABSR will help identify the phase boundary, but has little other value. ... 

Aase N. E., BjprkumP. A., and Nadeau P. H. (1996) The effect of grain-coating microquartz on preservation of reservoir quality. Am. Assoc. Petrol. Geologists 80, 1654-1673. 

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. 

Chuhan F. A., Kjeldstad A., Bjprlykke K., and Hpeg K. (2000b) Porosity loss in sand by grain crushing—experimental evidence and relevance to reservoir quality. Mar. Petrol. Geol. 19, 39-53. 

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. 

RammM. andRyseth A. E. (1996) Reservoir quality and burial diagenesis in the Statfjord Formation, North Sea. Petrol. Geosci. 2, 313-324. 

Dewers, T. and Ortoleva, P.J. 1990. Interaction of reaction, mass transport, and rock deformation during diagenesis mathematical modelling of integranular pressure solution, stylolites, and differential compaction/cementation. In l.D. Meshri and P.J. Ortoleva (Editors), Prediction of Reservoir Quality through Chemical Modelling, Memoir, 49. Am. Assoc. Pet. Geol. Tulsa, OK. 

Heather 1 Lower shoreface to offshore transition zone Strongly layered, partly calcite cemented, poor reservoir quality Very fine to fine sand... 

Middle Ness Upper delta plain (abandoned lobe, lacustrine, swamp) Reservoir quality poor to absent Predominantly fine grained components... 

Ehrenberg, S.N. 1991. Relationship between diagenesis and reservoir quality in sandstones of the Gam Formation, Haltenbanken, mid-Norwegian continental shelf. Am. Assoc. Pet. Geol. Bull., 74 1538-1558. 

There might, however, be pressure communication within the He Formation between hydraulic compartments I and II either due to cross-fault Ile/Ile communication, or due to communication downflank in the water zone at around 3900 m MSL around the tip of the fault separating these two compartments. For the Tilje Formation, however, there is no indication of pressure communication between hydraulic compartments I and II, i.e., no cross-fault communication and no downflank communication (possibly due to sub-seismic reservoir discontinuities and deteriorated reservoir quality). 

The scales of carbonate redistribution, and thus reservoir quality enhancement, are difficult to constrain. Several workers have argued that the reservoir properties of sandstones are greatly enhanced due to large-scale carbonate dissolution (Lonoy et al., 1986 Schmidt McDonald, 1979). As the undersaturated waters have to circulate through large volumes of permeable sediment to cause economically important carbonate cement dissolution, it is expected that such secondary porosity develops in partially rather than pervasively cemented sand-... 

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. 

Moraes, M.A.S. SuRDAM, R.C. (1993) Diagenetic heterogeneity and reservoir quality fluvial, deltaic, and turbiditic sandstone reservoirs, potiguar and reconcavo rift basins, Brazil. Bull. Am. Ass. petrol. Geol., 77, 1142-1158. 

Carbonate cements are often among the dominant components of diagenesis and hence are of decisive importance in determining the reservoir quality of sandstone sequences. Despite this, the timing, the geochemical conditions of precipitation and dissolution, as well as the source and fate of these cements are not fully understood. In continental and near-shore sediments, cements commonly precipitate as calcretes and dolocretes in the vadose and phreatic zones, and attain a variety of mineral-ogical, textural and distribution patterns as well as elemental and isotopic compositions. These cements form lenses and layers of densely cemented... 

The best reservoir quality potential expected for the Serraria Formation is in structural blocks in the distal and middle domains affected by porosity enhancement through extensive feldspar and carbonate cement dissolution in connection with the post-rift exposure and telogenetic infl ux of meteoric waters. 

Core analysis data are displayed as continuous logs in Fig. 2. Porosity varies from > 0 to 19%. Permeability varies from <0.1 mD to > 5000 mD. Porosity and permeability are highest where the rocks are most coarse grained. However, again the correlation is not perfect the tops of the sandbodies tend to have low porosity and permeability values relative to the middle and lower portions of sandbodies (Fig. 2). Consequently, grain size and facies variations cannot be used in isolation to understand or predict variations in reservoir quality. 

Fig. 8. Combination diagram of grain size data (derived from core description, Fig. 2) and mineral proportions, porosity and permeability (derived from wireline log analysis). There is excellent correlation between quartz proportion and reservoir quality. The correlation of these with grain size is complex. The tops of some sandbodies have a high dolomite content and correspondingly poor reservoir quality (e.g. 2470-2471 m). Sandbodies are numbered for reference to Fig. 9. Core analysis porosity and permeability data (dashed and faint) have been added to the diagram for comparison with the wireline-derived data.

Cement distribution could be influenced by reservoir quality at the time of cementation. High-permeability streaks or gradational permeability may have focused the flow and input of CO2 into specific portions of the rock. Fluvial sandstones usually fine upwards, resulting in diminishing permeability towards sandbody tops. This would lead to the most extensive dolomite cementation at the bases of sandbodies. However, note that the Chaunoy sandstones do not fine upwards (Fig. 2) and do not have dolomite preferentially at sandbody bases. 

Reservoir quality in the Chaunoy Formation is a function not just of depositional facies but also of localized cement distribution. Building a reservoir model using primary sandbody architecture alone is insufficient to correctly describe reservoir quality. 

Galloway, W.E. (1979) Diagenetic Control of Reservoir (Quality in Arc-Derived Sandstones Implications for Petroleum Exploration (Eds Scholle, P.A. Schluger, P.R.). Spec. Publ. Soc. Econ. Paleont. Miner., Tulsa, 36, 251-262. 

The Namorado Sandstone is one of the main oil reservoirs of the Campos basin, offshore Brazil, and consists of a thick turbiditic sequence deposited during the Albian-Cenomanian. Calcite cement is volu metrically the most important diagenetic parameter controlling reservoir quality in the sandstones. 

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

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