Evaluating Fissured Formation Geometry from Well Test Data: a Field Example

Abstract

The paper illustrates, using a field example, how well test data can be used to evaluate matrix block size and fissure volume in fissured reservoirs. The method is based on type-curve analysis with a recently published type-curve for double porosity reservoirs, published type-curve for double porosity reservoirs, and is applied to two tests on the same well, performed at a few years interval, with reservoir pressure above and below the bubble point, respectively. Detailed analysis of the two tests is presented, that yields consistent values for fissuration parameters, w and X, although pressure behavior during the tests is very different. Interpretation results were used as inputs to a numerical reservoir simulator, which was then calibrated against actual performance history to yield the size of the matrix blocks. The result was in excellent agreement with that obtained from X, and with available geological information. Introduction Naturally fissured reservoirs are recognized as major contributors to the world oil reserves. As a result, they have been the subject of many studies over the last twenty years. Although several fissured reservoir models have been presented in the literature, field examples to support presented in the literature, field examples to support these models are scarce. In fact, there still is some debate on fissured reservoir behavior, and consequently, on the ability of well tests to provide specific information on fissure parameters, such as spacing and porosity. The general feeling appears to be that fissured reservoir behavior is too complex and too averse to be analyzed in a systematic and unique way. In this paper, field data from two different tests on the same well in a fissured reservoir are analyzed with the help of a recently published type-curve. It is shown that, although the well behavior was very different in the two tests, the same model could be used for analysis, and provided values of the parameters that were consistent with reservoir parameters that were consistent with reservoir information obtained from other sources. INTERPRETATION MODELS FOR FISSURED RESERVOIRS DIRECT PROBLEM The direct problem (i.e., predicting the pressure behavior of a fissured reservoir from the knowledge of pertinent reservoir parameters) has been investigated by many different authors. Most of them, however, used the same basic model to describe a fissured reservoir, and their solutions only differed by the nature of fissure-matrix interaction, the boundary conditions included in the model, and the computation techniques employed. Fissured formations are commonly represented by a double porosity reservoir (i.e. a reservoir with two porous regions of distinctly different porosities and permeabilities). One region (the porosities and permeabilities). One region (the fissures) has a high conductivity, and carries the reservoir fluid to the well, whereas the other region (the blocks) has a low conductivity and feeds the fluid only to the fissures. Parameters necessary to describe a fissured reservoir are the same as those used for homogeneous reservoirs, plus two specific ones, namely. (1) and (2) In Eqs 1 and 2, subscript f refers to the fissure system, m to the matrix and (f+m) to the total reservoir.

Uncertainties in Reservoir Limit Test Results - Effect of Input Parameters

“…In the late 70's and beginning of 80's, type curve matching is developed by A. Gringarten, Burgess, Viturat, Pelissier, and Aubry (1981) to identify more complex well behaviors. Thus make well test analysis more useful as a reservoir description tool.…”

Semi-analytical solution of flow to a well in an unconfined-fractured aquifer system separated by an aquitard

“…When applied to data from the glacial out-wash aquifer in Cape Cod reported by Moench et al (2000), the models of Neuman (1972Neuman ( , 1974, Moench (1995Moench ( , 1996Moench ( , 1997 and Tartakovsky and Neuman (2007) failed to achieve reasonable estimates of the specific yield (Malama, 2011 Modeling of fractured aquifers and reservoirs is a long-standing research topic. The dual porosity concept presented by Barenblatt et al (1960) is often used to describe fractured aquifer/reservoir hydraulics/dynamics (Bourdet and Gringarten, 1980;Gringarten et al, 1981;Deruyck et al, 1982;Park and Zhan, 2003;Sedghi and Samani, 2010;Sedghi and Samani, 2015). For example, Warren and Root (1963) used steady state inter-porosity flow to characterize the behavior of naturally fractured reservoirs.…”

“…An unconfined fractured aquifer was considered using the double-porosity concept by Boulton and Streltsova (1977). Bourdet and Gringarten (1980) and Gringarten et al (1981) presented a method of determining block size and fissure volumes using type-curve matching. Deruyck et al (1982) obtained Laplace-domain inter-porosity flow equations for both spherical and slab-shaped matrix blocks.…”

From Straight Lines to Deconvolution: The Evolution of the State of the Art in Well Test Analysis

“…This period marked the beginning of the end of manual analysis, because the full application of the new, integrated methodology required the use of computers. With these and new numerical techniques such as the Stehfest's algorithm for Laplace inversion (Stehfest 1970), new interpretation models were developed that made it possible to identify more complex well behaviors such as double porosity (Gringarten et al 1979;Bourdet and Gringarten 1980;Gringarten et al 1981;Gringarten 1984). As a result, well test analysis started becoming more useful as a reservoir description tool, both during exploration and for reservoir simulation.…”