The Role of Well Logs in Reservoir Modeling

Abstract

1980 Fall Meeting Introduction As oil and gas become scarcer and more valuable, recovery efficiency becomes a matter of increasing concern. In order co better analyze reservoir behavior and plan production strategy, the petroleum industry employs reservoir modeling, or reservoir petroleum industry employs reservoir modeling, or reservoir simulation, using a high-speed, large-memory digital computer. The technique is applicable to all phases of the life of a reservoir - from initial exploration, through primary production development, to enhanced recovery. Furthermore, the use of reservoir modeling appears certain to continue expanding as the value of oil and gas increases, and as the cost of computer power declines. Reservoir simulation applies the concepts and techniques of mathematical modeling to analysis of the behavior of petroleum reservoir systems. The goal - evaluation of the flow performance of the reservoir - is achieved through the integration of petrophysical, geological and engineering information. The petrophysical, geological and engineering information. The validity and accuracy of the results depend, of course, on how well the model duplicates the reservoir, and on the accuracy and completeness of the many required input data. Wireline well logging is one of the richer sources of these data. Measurements, computations, and inferences from well togs yield many of the petrophysical parameters required in reservoir modeling. And, for some parameters, such as porosity and water and oil or gas saturations, well logs are the only truely reliable source. Wireline data can also complement, expand, and verify data from other sources, for example, permeability, fluid densities, pressure, and capillary pressure, and they can provide insight into pressure, and capillary pressure, and they can provide insight into characteristics not otherwise readily measured or observed, such as depositional environments, structural and stratigraphic features, and orientation. The purpose of this paper is to review the role that well logs, and related wireline services, can play in reservoir modeling. Since this might require a dissertation on the entire field of well logging, and of well-log interpretation, the depth of discussion must be limited. Subjects extensively documented in the literature, such as determination of porosity and saturation from well logs, will not be reviewed in great derail. The use of the Dipmeter log to define and orient structural and stratigraphic features will also be only briefly discussed. However, less commonly recognized applications of wireline measurements to reservoir modeling, such as zonation, absolute and relative permeability, and pressure profiles, will be reviewed in greater, permeability, and pressure profiles, will be reviewed in greater, although not exhaustive, detail. SIMULATION INPUT DATA Well logs enable continuous measurement, versus depth, of various physical parameters of the rock traversed by the bore-hole, e.g. resistivity, bulk density, acoustic travel time, neutron capture, and natural radiation. From these data, important petrophysical characteristics can be ascertained, such as porosity, petrophysical characteristics can be ascertained, such as porosity, fluid nature and saturation, shaliness, permeability index, and lithology. DEPTH-CORRELATION-VERTICAL ZONING Tool depth is accurately and continuously recorded during logging. Thus, the depth and thickness of each reservoir unit are provided automatically. This can be crucial to the choice of a provided automatically. This can be crucial to the choice of a grid system for a model, particularly if significant variations in any petrophysical parameter occur within the reservoir. For example, the presence of streaks of high or low permeability can greatly influence water-oil ratios, as well as flow behavior. It will therefore be necessary to divide the reservoir into units, where a "reservoir unit" is defined as one that exhibits essentially similar or constant petrophysical characteristics (e.g. porosity, permeability, and shaliness) throughout its vertical and areal permeability, and shaliness) throughout its vertical and areal extent. The vertical detail of logging measurements and the resulting log computations provide the information necessary for delineating such units. Well logs are also useful in defining the areal extent of individual reservoir units in full-field reservoir modeling. Well-to-well log correlation studies, including Dipmeter Log analyses, allow accurate subsurface mapping of each reservoir unit. This not only facilitates identifying corresponding reservoir units but also helps in analysis of structural and stratigraphic anomalies, such as faults, unconformities, and pinch-ours. However, all available basic logs and computed logs should be used (see section on Field Studies) in order to enhance well-to-well correlation and aid in recognition of petrophysical trends, such as the gradual shaling-up of a reservoir unit or directional permeability. POROSITY POROSITY Porosity from well logs is usually determined using Sonic, Density and or Neutron logs. Although these so-called "porosity logs" are largely responsive to porosity, they are also affected by other factors, e.g., formation matrix lithology, type of fluid present in the present in the pores, type of porosity, and degree present in the present in the pores, type of porosity, and degree and type of shaliness. This has led to the use of two or more of these logs in combination for the determination of these factors and a more accurate value of porosity. P. 3