Type-Curve Analysis: What It Can and Cannot Do
- Alain C. Gringarten (Scientific Software-Intercomp)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- January 1987
- Document Type
- Journal Paper
- 11 - 13
- 1987. Society of Petroleum Engineers
- 5.6.4 Drillstem/Well Testing
- 6 in the last 30 days
- 843 since 2007
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Summary. Type-curve analysis has been used for more than 20 years in the oil industry and for more than 40 years in hydrogeology. Yet, what it involves and how useful it is in practice is still being debated within the oil industry. This paper attempts to answer some of the questions that are most commonly raised about type curves and type-curve analysis.
What Is a Type Curve?
A type curve is a graphic representation of the theoretical response during a test of an interpretation model that represents the well and the reservoir being tested. For a constant-pressure test, the response is the change in production rate: for a constant-rate test, the response is the change in pressure at the bottom of the well. Other types of response are also used, such as the time derivative of the bottomhole pressure.
Type curves are derived from solutions to the flow equations under specific initial and boundary conditions. For the sake of generality, type curves are usually presented in dimensionless terms, such as a dimensionless pressure vs. a dimensionless time. A given interpretation model may yield a single type curve or one or more families of type curves, depending on the complexity of the model.
What Is Type-Curve Analysis?
Type-curve analysis consists of finding a type curve that "matches" the actual response of the well and the reservoir during the test. The reservoir and well parameters, such as permeability and skin. can then be calculated from the dimensionless parameters defining that type curve.
The match can be found graphically, by physically, superposing a graph of the actual test data with a similar graph of the type curve(s) and searching for the type curve that provides the best fit. Alternatively, an automatic fitting technique involving a linear or nonlinear regression can be used.
Fig. 1 gives an example of a graphic type-curve match. The graph of the data is positioned over the graph of the type curves, with the axes kept parallel, so that the test data match one of the type curves. Reservoir parameters are calculated from the value of the dimensionless parameter defining the type curve being matched and from the x and y axis shifts.
How Can One Select a Type Curve To Match Test Data?
First, one must find the interpretation model that best represents the dynamic behavior of the well and reservoir during the test. This interpretation model must be identified from the test data because it is usually difficult to predict from static information.
The most efficient way to identify the interpretation model is to use the derivative of the pressure with respect to the natural log of some function of elapsed time. A log-log plot of the pressure derivative vs. elapsed time yields a limited number of characteristic features for the various components of the interpretation model that are easy to recognize. These features are illustrated in Fig. 2. The possibilities are (1) a maximum. (2) a minimum, (3) a stabilization, and (4) an upward or a downward trend. The maximum is found at early times and indicates wellbore storage and skin: the higher the maximum, the more damaged the well. No maximum indicates a nondamaged or a stimulated well. The stabilization indicates semilog radial flow and corresponds to the semilog straight line on a Horner plot. A minimum indicates heterogeneous behavior. An upward or downward trend at the end of the data indicates boundary effects. The complete interpretation model is then obtained by combining these various components. Examples of interpretation models are shown in Fig. 3.
Once the interpretation model has been identified, one must select the type curves corresponding to that model that are the most appropriate for the range of available test data.
What Is the Difference Between the Various Published Type Curves?
For a given interpretation model, the mathematical solution to the flow equations is unique, and type curves derived from that solution should all be identical. In practice, however, type curves may differ by their presentation-e.g., if they use different dimensionless or dimensional parameters-or by their range of application. As a result, some published type curves may not be usable with the available test data, or may be more or less convenient to use. But even if they look different, type curves corresponding to the same interpretation model will give the same analysis results if they all cover the range of available test data.
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