Petro Shine The Place for Oil and Gas Professionals.
Related Articles
Data Sources
Reservoir data relates to the type and extent of the reservoir rock, the reservoir fluids and their properties, and the reservoir’s production performance history including pressure performance. Geological information of importance to the reservoir engineer includes the reservoir aerial extent and its closure, that is, the height of the crest above the lowest contour that completely closes the reservoir, also any flow barriers such as pinch-outs, faults, and fluid contacts.
Knowing the size of the aquifer is also useful along with a sense of lithology variations and the continuity of the reservoir in the vertical and horizontal directions. Well logs provide the subsurface data needed to construct structure and contour maps to show reservoir extent and thickness, giving us an idea of the bulk volume of the reservoir. They also provide the measurements necessary to calculate porosity and fluid saturations. While well tests can provide estimates of permeability and reservoir continuity, more precise measurements of rock properties can often be obtained from analyses of cores retrieved from individual wells.
A second type of reservoir data concerns the properties of the reservoir fluids and how they react to changes in pressure and temperature. For volumetric calculation of hydrocarbon in place, a value for the oil formation volume factor is required in order to relate hydrocarbon fluid volumes in the reservoir to volumes at the surface. Material balance calculations require knowledge of oil and gas properties over a range of pressures obtained from a laboratory analysis performed on a carefully obtained representative sample of original reservoir fluid. Where sampling is impossible, empirical correlations are available to estimate oil, gas, and water properties.
Careful measurements of the volumes of oil, gas, and water produced from a reservoir over time along with carefully measured subsurface pressures are essential for an accurate material balance calculation of hydrocarbon in place. This data can also be analyzed to give an indication of the type of reservoir drive mechanism operating in the reservoir and to determine the need for pressure maintenance.
Several types of data are used in reservoir engineering calculations. The most important are
- data that pertain to the reservoir rock and its extent
- data that pertain to the properties of reservoir fluids
- production data
First we shall describe the four sources of data related to the reservoir rock and reservoir extent, which are
- geologic and seismic interpretations
- well log analyses
- well test analyses
- core analyses
Geologic and Seismic Interpretations
Reservoir geology helps the engineer to understand the external geometry of the reservoir as well as its internal architecture. Examples of the types of information it provides are
- the reservoir extent and its closure (the height of the crest above the lowest contour that completely closes the reservoir)
- flow barriers, such as faults or pinchouts
- fluid contacts, (i.e., oil-water, oil-gas, and gas-water interfaces)
- aquifer size
- lithology variations
- continuity of the reservoir in the areal as well as in the vertical direction
Calculations from Well Logs
Logging provides in-situ information about the rock and its content from the immediate vicinity of the wellbore. There are over 30 types of logs, information from which may include:
- location of the productive stratum and its boundaries
- continuity of rock strata between adjacent wells
- net pay thickness
- oil, gas, and water saturations
- porosity of the reservoir rock
- other miscellaneous information, such as the condition of the hole, the temperature gradient in the wellbore, and the condition of the cement in a cased hole
Calculations from Well Tests
Well tests measure the pressure response of the well to short-term flow periods and the subsequent pressure buildup performance after shut-in. Various mathematical models can be used to determine the reservoir characteristics responsible for a particular pressure-flow rate behavior. In particular, permeability, the presence of nearby fault boundaries, or fluid contacts may be determined from an analysis of the well test data. Keep in mind that reservoir rock characteristics as determined from well tests are averaged values over the area of the reservoir that is contacted during the test.
Core Analyses in Reservoir Engineering
Cores provide petrophysical data essential to reservoir engineering. Basic core data, such as permeability, porosity, and fluid saturations help the engineer decide whether or not to complete the well and where to complete it. Special core analyses also help in evaluating reservoir performance, estimating hydrocarbons in place and reserves, evaluating the feasibility of EOR projects, and providing input data for reservoir simulation studies.
A second type of data used in reservoir engineering concerns the properties of the reservoir fluids and how they react to changes in pressure and temperature. Expressing the original hydrocarbons in place in surface volumes requires such data. Quantitative calculation of recoverable reserves requires estimates or laboratory determinations of formation volume factor, gas-oil ratio, and oil and gas compressibility, all as a function of pressure. Determining production rates of oil or gas requires knowledge of their respective viscosities at reservoir conditions. Any assessment of the practicality of EOR methods requires an understanding of the effects of the particular method employed on the behavior of the oil in the reservoir (i.e., oil viscosity reduction in a steam flood).
Reservoir fluid data is generally determined from a laboratory analysis performed on a carefully obtained representative sample of the original reservoir fluid. Where sampling is impossible, empirical correlations are available to estimate oil, gas, and water properties.
Production Data in Reservoir Engineering
This is another important type of data used in reservoir engineering calculations. By production data, we generally mean a careful accounting of the volumes of produced oil, gas, and water, as functions of time. Pressure as a function of time is also extremely important. The decline curve analysis and the material balance equation of oil or gas reservoirs require accurate production data in order to be of any value as predictive techniques.
The accuracy of production accounting can vary from field to field, particularly in large offshore developments where isolated wells and “satellite platforms” preclude the individual measurement of well production volumes on a regular basis. In such situations, individual well production is allocated from a total field production volume based on monthly well tests. In areas with high water-production rates the accuracy of measured water cuts also becomes a factor. Some estimate of the reliability of production data should be made by the engineer using such data in his or her calculations.
