Well-Test Analysis for Naturally Fractured Reservoirs
- Heber Cinco-Ley (CSIPSA and UNAM)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- January 1996
- Document Type
- Journal Paper
- 51 - 54
- 1996. Society of Petroleum Engineers
- 5.6.3 Pressure Transient Testing, 5.8.6 Naturally Fractured Reservoir, 1.6.9 Coring, Fishing, 5.6.1 Open hole/cased hole log analysis, 5.5 Reservoir Simulation, 5.6.4 Drillstem/Well Testing, 5.1.2 Faults and Fracture Characterisation, 5.1.1 Exploration, Development, Structural Geology
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Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area,these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.
Well testing provides a powerful tool to detect and to evaluateheterogeneities in naturally fractured reservoirs (NFR's). Experience has shownthat this type of reservoir may display behavior that consists of a variety offlow models. This paper presents a discussion of the applications andlimitations of pressure-transient tests in the evaluation of NFR's.
Optimizing the exploitation of a reservoir requires a complete descriptionof the formation. A combination of information from various sources allows areliable characterization of the system so that data from seismology, geology,well logging, well tests, core and fluid analysis, and well flow rates can beused to estimate reservoir geometry, oil and gas in place, and flowcharacteristics of the porous medium among other factors. Welltesting1-10 provides an ideal tool to find reservoir-flow parametersand to detect and evaluate heterogeneities that affect the flow process in theformation.
NFR's contain a considerable amount of the world hydrocarbon reserves. AsFig. 1 shows, the rock in these types of systems may include severalelements (i.e., vugs, fractures, and matrix). Hydrocarbons are contained inboth fractures and rock matrix usually, fractures act as channels to yield highwell-flow rates. Reservoir studies must consider these heterogeneities becausethey can affect oil and gas recovery significantly. Because of the importanceof NFR's, many publications haye appeared that provide an understanding of thebehavior of these types of reservoirs.11,12 Currently, advances inwell-test analysis allow a more reliable characterization of these systemsbased on new flow models that properly account for heterogeneities of fracturedreservoirs. Owing to space limitations, discussing every publication thatcontributed to the technology used is impossible.
Experience has shown that NFR's may behave according to a variety ofreservoir-flow models: (1) homogeneous reservoir, (2) multiple region orcomposite reservoir, (3) anisotropic medium, (4) single fracture system, and(5) double-porosity medium. Fig. 2 shows the main elements of thesemodels, and Table 1 shows the different sets of parameters that have tobe found to describe the flow behavior in the reservoir for each case. Next, wediscuss the application and limitations of these models in well-testanalysis.
Homogeneous Reservoir Model.
This model considers that reservoir properties are constant and do not varythroughout the reservoir. Fractures and rock matrix act as a single medium sothat fluid production is caused by the simultaneous expansion of both elements,and fluid transfer between them, if any, occurs instantaneously withoutresistance. This behavior is exhibited by either a heavily fractured reservoirwith small matrix blocks (Fig. 2a) or by an NFR where fluids are containedmainly in the fracture system (Fig. 2b). The presence of fractures can bedetected by the analysis of well logs and cores.
In general, well-test-analysis methods have been developed for homogeneousreservoirs. The pressure behavior in these systems is controlled by theformation flow capacity, kh; porosity f, fluid viscosity µ, and totalcompressibility, ct. An essential part of well-test-analysismethods is a flow-regime diagnosis achieved through the application of alog-log graph of both pressure and pressure derivative.13 Thisprocess allows the detection of flow geometries and the presence ofheterogeneities in the reservoir. The parameters of the system are estimated byuse of the specialized graphs of pressure, p, vs. time, t (e.g.,p vs. log t, p vs. t1/2, p vs.t1/4, p vs. t-1/2,p vs.1/t, and p vs. :t, correspending to radial,14linear,15 bilinear,16 spherical,17 constantpressure boundary,18 and pseudosteady-state2 flows,respectively.
Fig. 3 illustrates the behavior of a single-well test (drawdown orbuildup) for radial flow in homogeneous systems. The first part of thepressure-derivative graph (unit-slope straight line) shows the presence ofwellbore-storage effects followed (after a transition zone) by a horizontalportion representing a radial-flow behavior. The kh ofthe formation andthe skin factor can be determined from the straight line on the semilog graph.The estimated kh for an NFR represents an equivalent value for thefracture/matrix system. As a complement to single-well tests, interferencetests are used to estimate the storativity of the formation (fcth); this value is usually high for NFR's because it includes thestorativity of both fractures and matrix.
Sometimes a high-productivity well produces from a reservoir with smallhydrocarbon reserves; here, the fluids are contained mainly in the fracturesand the system behaves as a homogeneous medium. Extended drawdown tests allowthe early detection of this case during the appraisal phase of thereservoir.
Multiple Region or Composite Reservoir Model.
Some NFR's are fractured regionally. (Fig. 2c) and can therefore beconsidered to be composed of two regions: a high- and a low-transmissibilityregion. In this case, the reservoir behaves as a composite radialsystem.19 Wells producing from the fractured region exhibit higherproductivity than those in the unfractured region. The system is characterizedby the flow capacity of both regions (kh)1 and(kh)2.
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