Reservoir Geochemistry: A Link Between Reservoir Geology and Engineering?
- S.R. Larter (U. of Newcastle) | A.C. Aplin (U. of Newcastle) | P.W.M. Corbett (Heriot-Watt U.) | Neil Ementon (Heriot-Watt U.) | Mei Chen (U. of Newcastle) | P.N. Taylor (U. of Newcastle)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- February 1997
- Document Type
- Journal Paper
- 12 - 17
- 1997. Society of Petroleum Engineers
- 5.5 Reservoir Simulation, 5.4.1 Waterflooding, 4.3.4 Scale, 5.1 Reservoir Characterisation, 5.1.1 Exploration, Development, Structural Geology, 5.6.1 Open hole/cased hole log analysis, 5.6.5 Tracers, 1.2.3 Rock properties, 5.8.7 Carbonate Reservoir, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 5.5.11 Formation Testing (e.g., Wireline, LWD), 5.2 Reservoir Fluid Dynamics, 4.3.3 Aspaltenes, 5.1.3 Sedimentology, 5.6.4 Drillstem/Well Testing, 1.6.9 Coring, Fishing
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Geochemistry provides a natural, but poorly exploited, link between reservoir geology and engineering. We summarize some current applications of geochemistry to reservoir description and stress that, because of their strong interactions with mineral surfaces and water, nitrogen and oxygen compounds in petroleum may exert an important influence on the pressure/volume/temperature (PVT) properties of petroleum, viscosity and wettability. The distribution of these compounds in reservoirs is heterogenous on a submeter scale and is partly controlled by variations in reservoir quality. The implied variations in petroleum properties and wettability may account for some of the errors in reservoir simulations.
Traditionally, fluid geochemistry has played little role in reservoir engineering practice, with some notable exceptions.1 Since 1985, however, the focus of geochemistry in the petroleum industry has shifted away from exploration toward reservoir appraisal and production. This new focus has been termed "reservoir geochemistry" and has been recently reviewed by Larter and Aplin.2 Because geochemists deal with both reservoir rocks and their contained fluids, reservoir geochemistry provides a natural, but underexploited, link between reservoir geologists and reservoir/petroleum engineers. The dual aims of this paper are to summarize recent developments in reservoir geochemistry and to discuss current and potential applications of reservoir geochemistry to petroleum and reservoir engineering.
The methods of reservoir geochemistry are similar to those used for many years by classic petroleum geochemists. Samples are production and well-test fluids plus materials extracted from reservoir cores or cuttings with organic solvents or deionized water. Reservoir geochemical studies differ from conventional geochemical studies only in that much larger sample sets are processed, with up to several hundred reservoir core extract samples being analyzed in addition to available fluid samples. Collection and adequate storage of fluid samples is central to effective reservoir geochemistry, and early baseline studies are desirable if changes during production are to be identified and understood.
Recent analytical developments now allow, early in field appraisal, the rapid and inexpensive production of high-resolution, three-dimensional compositional images of the petroleum column. Larter and Aplin2,3 have identified three methods that have been developed to characterize petroleum and residual salts rapidly in reservoir core. Two of these techniques plus some applications are summarized in Fig. 1.
Routine, rapid, inexpensive assessment of levels and bulk composition of petroleum extracted from reservoir cores is now possible with automated liquid chromatographic devices, such as the Iatroscan.4 This approach produces logs of both petroleum saturation (comparable with electric-log measurements5) and composition, enabling the detection of such reservoir features as oil/water contacts (OWC's) and small tar mats4 . This method has complemented reservoir screening by the Rockeval device6,7 and allows routine assessment of variation in oil quality throughout heavy- and light-oil reservoirs. Detection of small tar mats in petroleum reservoirs is critical because these can act as low-permeability zones or flow barriers (Fig. 2).
Automated thermovaporization gas chromatography and gas chromatography/mass spectrometry (GC/MS) systems capable of producing high-quality gas chromatographic and gas chromatographic/mass spectrometric data directly from reservoir cores have revolutionized the application of molecular geochemistry to the study of petroleum reservoirs.5,8 These systems allow the production of biomarker or aromatic-hydrocarbon parameter logs at near-meter resolution at a rate of one data point per hour. Detailed molecular geochemical logs produced from such systems enable the detection of compositional steps in petroleum columns, which can be sometimes interpreted as indicating the presence of a flow barrier.9,10 This interpretation is strengthened by the observation that pressure kicks sometimes occur across the same geological discontinuity across which the compositional step is seen.5 The benefit of the geochemical data is that they can be gathered before repeat-formation-test (RFT) pressure tests and can be used when deciding on the merits of making such tests. Correlations between the compositions of produced fluids with those predicted from preproduction compositional images has yet to be widely applied but may provide the ultimate test of the validity of production models based on engineering, geological, and geochemical data.
Determination of the 87Sr/86Sr isotopic composition of salts precipitated by the evaporation of formation water or residual water during core storage11,12 permits the reservoir geochemist to obtain information about the heterogeneity of waters present in the reservoir, at high spatial resolution. As for petroleum, compositional steps across geological discontinuities imply that the discontinuity is an effective barrier to fluid flow. Downcolumn trends in the ratio within the oil leg imply that the composition of water was evolving during reservoir filling and can be used to suggest detailed filling histories.11
While these new methods have had a major impact on our ability to characterize petroleum reservoirs, traditional fingerprint gas chromatography coupled with multivariate data analysis has found a new lease of life, both as a reservoir continuity tool and for detection of leaking production tubing.13
Variation in water chemistry throughout a petroleum column is increasingly recognized by reservoir geochemists, although this information is not commonly incorporated in reservoir studies. Measurements of the salinities of aqueous fluid inclusions (tiny pockets of fluid trapped during the precipitation of diagenetic minerals in the reservoir) in petroleum fields indicate that oilfield water salinities may vary substantially through time. In one North Sea field we have studied, fluid-inclusion evidence suggests that salinities varied between 4 and 25 wt% total dissolved solids during the period of field filling, implying that residual waters trapped in the oil leg also have variable salinity. Coleman14 has shown that the water trapped within the oil leg of one North Sea field has a different chemical and isotopic composition from that in the water leg. The trapped paleowaters have a salinity of around 25 000 mg/L chloride, while the present-day formation water contains 60 000 mg/L chloride. These data force us to reassess the accuracy with which we calculate oil in place (OIP) because this involves the use of resistivity logs through the use of the practical Archie equation. In this case, assuming that the residual water in the oil leg is identical to that in the water leg would result in a 10% overestimate of OIP.3
In summary, reservoir geochemical methods provide cost-effective supplements to conventional reservoir appraisal and monitoring procedures and, in some cases, provide alternatives to conventional reservoir test procedures.
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