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Mature and abandoned fields offer major but often overlooked opportunities for development of new oil and gas reserves. To reverse the decline rate of mature reservoirs an integrated approach that includes geochemistry is needed to characterize these reservoirs before undertaking revitalization. Geochemical soil gas surveys are an important part of the reservoir characterization process because it establishes the amount and location of potential reserves, which are important in deciding whether a revitalization scheme would be worthwhile. These untapped resources can exist in between production wells, at the edges of reservoirs and at deeper previously unexplored horizons as well as in attic areas higher than existing production. To reverse the decline rate the traditional approach is to conduct secondary and tertiary recovery schemes followed by horizontal drilling and multistage fracture stimulations. While such schemes do reverse the decline rate, they do not necessarily tap all the potential resources.  geochemical expression of bypassed plays

How do you revitalize mature/abandoned oilfields that have been extensively explored with a mosaic of drilling and seismic and that have undergone secondary and tertiary recovery? Before embarking on a revitalization scheme for mature or abandoned fields, the explorationist would want to have a clear picture of remaining resources. Cost-effective geochemical methods are employed for finding bypassed plays and lost compartments in mature/abandoned oil fields. They are also employed in finding reserves beneath existing fields and attic areas higher than existing production. It would be important to confirm before infill drilling whether seismic structures are charged with hydrocarbons and whether such drilling is in the optimum locations. Soil gas surveys can find previously undiscovered reserves in statigraphic pools.

Depicted in the accompanying picture is a geochemical survey of a mature field showing different concentration patterns. Notwithstanding pattern drilling it is obvious that the field has not been uniformly exploited. Some wells have been highly productive with concomitant depletion of reserves as shown by blue and green areas. On the other hand, the reservoir remains intact around some wells. Obviously there has been compartmentalization by faults or facies changes indicating lower productivity. A number of vertical and horizontal drilling opportunities present themselves, particularly in the NE corner of the map. The same approach can be applied to increase the effectiveness of coal bed methane (CBM), water flooding and CO2EOR projects. The sweeping action of injected fluids and gases are often impeded by faults and facies changes.  c2+ contours showing lost compartment incised valley

Illustrated in the accompanying picture is a C2+ contour map of an incised fluvial-river valley. This meandering valley was found by 2D seismic in the Lower Cretaceous Mannville formation. Meandering-river deposits tend to be fine-grained, lenticular, and partially or completely encased in floodplain shales. Interpretation of the contours indicates two large channel deposits and one point bar. Seismic located the two main channel deposits but missed the point bar, a lost compartment.

Soil gas surveys may be the only method to assess complex faulted reservoirs that have been split into a number of compartments. Each compartment will require a dedicated production well to drain the hydrocarbons within it.  oil is entrapped in four types of reservoirs

With beginning of production, apical anomalies will change depending on the type of drive. Four types of drives are depicted in the accompanying picture. Before drilling, light hydrocarbons migrate vertically in the direction of lower pressure to the surface where they form anomalous patterns. However, once a well begins to produce, the light hydrocarbons are short-circuited horizontally to the well bore or area of lower pressure, thus modifying the original apical anomaly. Dissolved gas reservoirs such as heavy oil deplete very quickly forming doughnut or halo types of anomalies around well heads. Oil fields with water and gas cap drives also show this effect at the surface but because of the uniformity of pressures, the depletion of apical anomalies will be more uniform over a longer period of time.  varying oil-water contacts and lowest known oil depths define compartments

The accompanying picture illustrates intricate fault compartments. Varying oil-water contacts and lowest known oil depths have been used to define compartmentalization. Production from each compartment can be monitored with soil gas surveys.

Shown in the accompanying picture are areal compartments that have been picked out by looking for domains with wells showing similar-looking production profiles. Soil gas surveys can probably accomplish the same results or at least confirm the existence of separate compartments. The production profiles should correlate with decreasing anomalous values in repetitive surveys over time.  compartments defined by known oil-water contacts and oil depths

"The world average oil recovery factor is estimated to be 35% ...... Oil fields after a certain production period are called mature fields. A more specific definition of mature fields is the fields reaching the peak of their production or producing fields in declining mode. A third definition could be the fields reaching their economic limit after primary and secondary recovery efforts....Mature field development...can be divided into well development and reservoir development."(Source:Development of Mature Oil Fields; Journal of Petroleum Science 57 (2007), 221-246; Tayfun Babadagli). Determination of the amount and location of the remaining oil in a mature field are the keys to well and reservoir development.  range oil recovery efficiencies for clastic reservoirs

"Fluvial-deltaic sandstone reservoirs represent significant opportunites for re-development in mature fields throughout the world....The stratigraphic complexity inherent in these deposits is responsible for low recovery efficiencies in large part because of the isolation of significant volumes of mobile oil in undeveloped reservoir compartments....Many fields throughout the world have been abandoned leaving behind significant unrecovered resources in untapped and incompletely drained reservoirs....The unproduced zones can be identified by integrated geological and engineering reservoir characterization and targeted by incremental recovery by recompletion and infill drilling".(Source:Revitalizing an Oil Play: Strategies for Finding and Producing Unrecovered Oil in Frio Fluvial-Deltaic Sandstone Reservoirs of South Texas; Prepared by University of Texas under Contract with DOE; July 1995). Geochemical surveys provide a more cost-effective method in finding bypassed oil and lost compartments in both mature and abandoned fields.


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