Rajagopal Raghavan

New Solutions for Well-Test-Analysis Problems: Part 1-Analytical Considerations(includes associated papers 28666 and 29213 )

In this paper point-source solutions are derived in the Laplace-transform domain and an extensive library of solutions is documented to obtain pressure distributions and well responses for a wide variety of wellbore configurations: partially penetrating vertical wells, horizontal wells, and fractured wells (complete or limited entry). Wells may be located in infinite or bounded systems (rectangular or circular reservoirs). Several combinations of closed and/or constant-pressure boundary conditions are considered at the vertical and lateral reservoir boundaries. These solutions may be used to examine homogeneous or naturally fractured reservoirs.

New Pressure Transient Analysis Methods For Naturally Fractured Reservoirs

This paper presents new methods for analyzing pressure drawdown and buildup data obtained at wells producing naturally fractured reservoirs. The model used in this study assumes unsteady-state fluid transfer from the matrix system to the fracture system. A new flow regime is identified. The discovery of this flow regime explains field behavior that has been considered unusual. The probability of obtaining data reflecting this flow regime in a field test is higher than that of obtaining the classical responses given in the literature. The identification of this new flow regime provides methods for preparing a complete analysis of pressure data obtained from naturally fractured reservoirs. Applications to field data are discussed.

New Solutions for Well-Test-Analysis Problems: Part 2 Computational Considerations and Applications

In this paper computational considerations in obtaining well responses and pressure distributions for several problems presented in Part 1 are discussed. In addition, new asymptotic expressions for pressure distributions in closed drainage volumes applicable during the boundary-dominated flow period are derived. Interestingly, these expressions, which are much simpler than those available in the literature, can be used to derive shape factors for a variety of completion conditions (vertical, horizontal, and vertically fractured wells). Application of constant-rate solutions to more complex conditions is also presented.

Horizontal Well Pressure Analysis

This paper presents an analysis of the pressure-transient behavior of a horizontal well or a drainhole. The performances of horizontal wells and fully penetrating vertical fractures are compared. Dimensionless wellbore pressures are computed for two classic boundary conditions: infinite conductivity and uniform flux. Results are presented as pseudoskin factors and as type curves. In addition to conventional pressure-vs.-time type curves, derivative type curves from pressure/time predictions are presented. The derivative approach the authors discuss is applicable to a broader range of problems than considered here.

Well Test Analysis of Data Dominated by Storage and Skin: Non-Newtonian Power-Law Fluids

The authors examine pressure falloff behavior dominated by storage and skin subsequent to the injection of a non-Newtonian power-law fluid. New solutions to analyze falloff tests are presented in a form suitable for analyzing field data. The effective wellbore concept is used to combine the wellbore storage constant and skin factor. This phase of their work required extensions to the effective wellbore radius concept (for power-law fluids) given in the literature. To improve analysis procedures, they examine the use of the pressure-derivative technique. The authors discuss the advantages of this method for the problem under consideration.

Productivity of Multiple Drainholes or Fractured Horizontal Wells

This paper provides guidelines to assess the productivity of horizontal-well completions by drilling multiple drainholes or by stimulation with vertical fractures. The paper also offers reliable methods to compute productivity gains for realistic conditions

Practical Considerations in the Analysis of Gas-Condensate Well Tests

Several pressure buildup tests are analyzed with a view to evaluate the potential of the ideas given in the literature. A broad range of tests is examined to demonstrate the characteristics of responses in wells producing below the dew point. Methods to obtain quantitative information that is consistent for different tests are outlined.

Performance of Finite-Conductivity, Vertically Fractured Wells in Single-Layer Reservoirs

Although even a perfunctory survey of the literature suggests that considerable information is available on the response of finite-conductivity fractures in single-layer systems, the influence of the settling of propping agents and the effect of fracture height on the well response need to be examined. These topics are examined in this paper. The authors suggest methods to analyze well performance when the fracture conductivity is a function of fracture height and fracture length. The performance of wells with fracture height greater than the formation thickness is documented. The consequences of being unable to contain the fracture within the pay zone are also examined. Although incidental to this study, the authors found that solutions presented by various authors are not in agreement for all time ranges. In this paper, they discuss a systematic procedure to obtain a grid (mesh) so that accurate results are obtained by a finite-difference model. This procedure can be used for both two-dimensional (2D) and three-dimensional (3D) problems.

Pressure transient analysis methods for bounded naturally fractured reservoirs

New methods for analyzing drawdown and buildup pressure data obtained at a well located in an infinite, naturally fractured reservoir were presented recently. In this work, the analysis of both drawdown and buildup data in a bounded, naturally fractured reservoir is considered. For the bounded case, the authors show that five possible flow regimes may be exhibited by drawdown data. They delineate the conditions under which each of these five flow regimes exists and the information that can be obtained from each possible combination of flow regimes. Conditions under which semilog methods can be used to analyze buildup data are discussed for the bounded fractured reservoir case. New Matthews-Brons-Hazebroek (MBH) functions for computing the average reservoir pressure from buildup data are presented.

Fully coupled analysis of well responses in stress-sensitive reservoirs

A fully-coupled geomechanics and single-phase, fluid-flow model is developed to evaluate the combined effects of stress, fluid flow and reservoir property changes on well performance. In particular, we pay particular attention to the interpretation of pressure buildup tests and to changes in the production characteristics of wells. In general, for weak hydrocarbon reservoirs that exhibit nonlinear, elastic and plastic constitutive behaviors and stress-dependent properties such as permeability and porosity, the coupling effect may not be ignored in well test analysis. The coupled interaction between geomechanics and reservoir fluid production markedly affects the stress state and reservoir properties. Because we are using a coupled, numerical model, we evaluate the consequences of using simplified relationships (e.g., permeability as a function of pressure). Numerical analyses are performed to quantitatively assess the impact of reservoir stress-sensitivity on practical well test problems. The key variables investigated in the study, that are important in evaluating stress-sensitive reservoirs, include permeability, porosity, and constitutive behaviors of reservoir rock including hysteresis and loading conditions. The development of high-stress regions around wellbores and its consequences on well performance are considered. The numerical results from the study indicate that for analyzing highly stress-sensitive reservoirs, a fully-coupled geomechanics and fluid-flow modeling approach is necessary and the developed model employed in this study provides such a tool.