Insights From Interval Pressure Transient Tests Derive Key Fracture Parameters
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The complete paper describes the shortcomings of traditional well testing methods and the methodology and results of applying wireline-conveyed IPTT in a light-oil reservoir offshore Norway. The study focuses on cases in which fractures are present in the near-wellbore region but do not intersect the wellbore. The study included parameters such as fracture densities and conductivities, distance between fractures and wellbore, and the vertical extension of the fractures across geological beds.
Fractures can be first-order controls on fluid flow in hydrocarbon reservoirs. Understanding fracture characteristics such as aperture, density, distribution, conductivity, and connectivity is key for reservoir engineering and production analysis.
Well testing plays a key role in fracture characterization, particularly in fractured reservoirs. New advances in pressure transient analysis (PTA) have enabled the interpretation of production data such that the resulting geological scenarios are in better agreement with fracture patterns observed in outcrop analogs.
Traditionally, drillstem-test (DST) data have been the primary source of information for well testing. However, the authors hypothesized that wireline-conveyed tools designed for interval pressure transient testing (IPTT) could yield a more-thorough description of near-wellbore heterogeneities, including fractures. To prove their hypothesis, they used a next-generation wireline testing tool to investigate the applicability of IPTT for characterizing fractured reservoirs, using detailed numerical simulations models with accurate wellbore representation to generate synthetic IPTT responses. The effect of the different fracture scenarios on the pressure transient tests was recorded as characteristic signatures on diagnostic plots, which the authors call IPTT-geotypes because they can be used to assist the interpretation of IPTT responses.
The paper includes a field example of an IPTT case that was analyzed using the concept of geological well testing. Information from petrophysical logs and the IPTT-geotypes was integrated to assist the calibration of a reservoir model developed to represent the geological setting of the tested reservoir interval. The results provided a sound interpretation of the reservoir geology and quantitative estimation of the matrix and fracture parameters.
Static characterization of fracture systems, which can be key controls on reservoir performance, rely on analog outcrop observations, seismic, image logs, and core analysis.
Direct observations in cores; image logs; a sudden increase in mud filtration during drilling; and, at later stages, substantial differences between the wells’ expected and real productivity indices are all indicators of the presence of fractures in a reservoir. Well testing provides the means for dynamic characterization and is an area that has shown significant progress in recent years.
The industry standard model for well test interpretation in naturally fractured reservoirs (NFRs) was developed in 1963. This model assumes a geometrically idealized system where flow occurs only in the fractures, which have uniform properties, while the matrix is stagnant and only recharges the fractures. Such a system normally does not represent real fracture networks. Features such as fracture geometry, density, and connectivity cannot be derived from the purely mathematical description.
Multiple interpretation models attempting to achieve more generalized and accurate representations of NFRs can be found in the literature. Many of these focus on alternative forms for quantifying matrix/fracture flow functions. However, these approaches did not address more-fundamental issues such as the fact that fractures can exist as networks or discrete features. More-recent work has dealt with more-realistic fracture scenarios and represents major contributions in the understanding of the pressure transient behavior of NFRs.
All these studies, however, were focused on DSTs. IPTTs, also known as mini-DST, possess interesting technical, financial, and environmental advantages compared with conventional DSTs. From a technical perspective, IPTTs may enable a more-detailed characterization of the near-wellbore area, in both the horizontal and vertical directions.
The complete paper investigates the application of an IPTT for fracture-system characterization. First, the authors expand the discussion on some fundamental issues related to PTA in fractured reservoirs and highlight important differences between their work and some others available in the literature. Next, they introduce the concept of geological well testing (GWT), which they used extensively in their study. This methodology is particularly useful for providing a geological framework to well-test analysis and has been used effectively in the past for PTA in NFRs. Then, they describe the detailed numerical models developed through their study and systematically investigate synthetic IPTT responses in cases in which fractures are present in the near-wellbore area but do not intersect the wellbore. They present sensitivities in the well-testing signatures for different fracture properties such as density, conductivity, distance to the well, and vertical extension, which they call IPTT-geotypes. They also present a comparison between the industry standard model developed in 1963 and these new curves. Finally, they present a field study in which the GWT work flow was applied to generate one possible interpretation of the reservoir geology and quantify key parameters—permeability, fracture conductivity, density, distance from the well, and extension—for both the matrix and fracture systems, in both horizontal and vertical directions (Fig. 1).
- This study investigated the application of IPTT for the characterization of the near-wellbore matrix/fracture system in cases in which the fractures do not intersect the wellbore. High-resolution numerical simulations with accurate representation of the wellbore were used to simulate complex geological settings beyond the reach of analytical models.
- It was demonstrated that particular features of the fracture networks will have a recognizable effect on the pressure derivative signatures (i.e., diagnostic plots) obtained from IPTT.
- Geological-type curves called geotypes were developed that are associated with different fracture conductivities, densities, distances between well and fractures, and vertical extensions. These geotypes assist the interpretation of IPTT cases run with next-generation wireline tools but can also be useful in dual- or straddle-packer tests.
- Particularly relevant from the IPTT-geotypes analysis is the effect of the vertical extension of the fractures on the pressure transient data. Well tests in formations with fractures that cross geological beds may not reflect the vertical matrix permeability as the fractures become the preferential channels for fluid flow in that direction. Bed-bound fractures, on the other hand, can be affected severely by the ratio of permeability in the vertical direction to permeability in the horizontal direction of the individual layers. In extreme cases, an additional flow period can emerge that can be confused with the formation radial flow if the test is not long enough.
- A field example was presented in which key reservoir parameters in a geologically complex formation were analyzed using IPTT data and IPTT-geotypes. The application of the GWT work flow assisted in providing a more-sound interpretation of the reservoir geology. The resulting geological concept for the reservoir includes the presence of bed-bound fractures as the first-order control for pressure transient behavior observed in the test. The geometry and conductivity of the fractures, their position with respect to the well, and their vertical extension could be estimated readily using the IPTT data and IPTT-geotypes.
Insights From Interval Pressure Transient Tests Derive Key Fracture Parameters
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