Productivity impairment in tight-gas formations is a typical phenomenon for fractured wells. Processes responsible for this behavior are related to the characteristics of the porous media, and are induced as a consequence of the fracturing process. Fracture damage has been discussed in the literature for decades. In almost all cases, effects were considered in isolation. This is often not appropriate since natural effects such as stress dependency, fracture closure, and non-Darcy flow are interdependent. The same applies for the cleanup process where the back production of the load-water from the leakoff zones can be influenced by mechanical damage caused during the fracturing process, or by capillary forces and the gel residues of unbroken fracturing fluids within the fracture plane. This study analyzed the most common damage mechanisms by means of both generic and real field data. The latter was taken from German Rotliegend gas wells, which were history-matched and used for the evaluation of their cleanup and long-term production behavior. The results obtained were used to rank the individual processes for their damage potential. In addition to a commercial model, a customized in-house simulator was required in order to capture the specific physics. Results suggest that the consequences of processes independent of the reservoir conditions are not negligible when compared to the damage induced by the fracturing itself. In particular, in tight-gas, the stress dependency of the reservoir rock and fracture closure both tend to have a significant impact on the long-term productivity. Furthermore, inertial non-Darcy flow can cause much higher production impairment than, for example, hydraulic damage. It also shows that low permeability reservoirs are more affected by non-Darcy flow effects in both the fractures and the reservoir than is generally assumed. Introduction A wide variety of studies have been published in the literature dealing with damage mechanisms in fractured wells. A realistic cleanup scenario can imply, among others,complex three phase flow,the formation of a load-water invasion zone accompanied by hydraulic and mechanical damage in the fracture vicinity,filtercake buildup and erosion,gel residue damaging the proppant pack, incorporating complex non-Newtonian rheology,viscous fingering through the proppant pack, andunbroken fracturing fluids within the proppant pack.1,2,3,4,5,6,7,8,9 In the course of the subsequent production, inertial non-Darcy flow and geomechanical effects, e.g., stress dependency of reservoir permeability and fracture closure, additionally impact the behavior of the fractured well. These mechanisms are often neglected in analytical or numerical studies for the sake of simplicity. Unlike mechanical or hydraulic formation damage they are not "artificially induced", but rather a natural process. Many publications deal with these effects in isolation. This is often not appropriate since mechanisms such as stress dependency, fracture closure, and non-Darcy flow are interdependent. The same applies for the cleanup process where the back production of the loadwater from the leakoff zones can be influenced, for example, by mechanical damage caused during the fracturing process, or by capillary forces and the gel residues of unbroken fracturing fluids within the fracture plane. This study analyzed the most common damage mechanisms by means of both generic and real field data. The latter was taken from German Rotliegend gas wells, which were history-matched and used for the evaluation of their cleanup and long-term production behavior. The results obtained were then used to rank the individual processes for their damage potential under typical conditions. In addition to a commercial model, the use of a customized in-house reservoir simulator was necessary in order to capture the specific physics.
Summary This paper describes the development and capabilities of a novel and unique tool that interfaces a hydraulic fracture model and a reservoir simulator. This new tool is another step in improving both the efficiency and consistency of connecting hydraulic fracture engineering and reservoir engineering. The typical way to model hydraulically fractured wells in 3D reservoir simulators is to approximate the fracture behavior with a modified skin or productivity index (PI). Neither method captures all the important physics of flow into and through the fracture. This becomes even more critical in cases of multiphase flow and multilayered reservoirs. Modeling the cleanup phase following hydraulic fracture treatments can be very important in tight gas reservoirs, and this also requires a more detailed simulation of the fracture. Realistic modeling of horizontal wells with multiple hydraulic fractures is another capability that is needed in the industry. This capability requires more than an approximate description of the fracture(s) in the reservoir-simulation model. To achieve all the capabilities mentioned above, a new tool was developed within a commercial lumped 3D fracture-simulation model. This new tool enables significantly more accurate prediction of post-fracture performance with a commercial reservoir simulator. The automatically generated reservoir simulator input files represent the geometry and hydraulic properties of the reservoir, the fracture, the damaged zone around the fracture, and the initial pressure and filtrate fluid distribution in the reservoir. Consistency with the fracture-simulation inputs and outputs is assured because the software automatically transfers the information. High-permeability gridblocks that capture the 2D variation of the fracture conductivity within the reservoir simulator input files represent the fracture. If the fracture width used in the reservoir model is larger than the actual fracture width, the permeability and porosity of the fracture blocks are reduced to maintain the transmissibility and porous volume of the actual fracture. Both proppant and acid fracturing are handled with this approach. To capture the changes in fracture conductivity over time as the bottomhole flowing pressure (BHFP) changes, the pressure-dependent behavior of the fracture is passed to the reservoir simulator. Local grid refinement (LGR) is used in the region of the wellbore and the fracture tip, as well as in the blocks adjacent to the fracture plane. Using small gridblocks adjacent to the fracture plane is needed for an adequate representation of the filtrate-invaded zone using the leakoff depth distribution provided by the fracture simulator. The reservoir simulator input can be created for multiphase fluid systems with multiple layers and different permeabilities. In addition, different capillary pressure and relative permeability saturation functions for each layer are allowed. Introduction Historically, there have been three basic approaches commonly used for predicting the production from hydraulically fractured wells. First, analytic solutions were most commonly used, based on an infinite-conductivity or, later, a finite-conductivity fracture with a given half-length. This approach also was extended to cover horizontal multiple fractured wells (Basquet et al. 1999). With the development of reservoir simulators, two other approaches were developed. For complicated multiwell, multilayer, multiphase simulations (i.e., full-field models), the fracture stimulation was usually approximated as a negative skin. This is the same as increasing the effective wellbore radius in the simulation model. An alternate approach, developed initially for tight gas applications, was to develop a special-purpose numeric reservoir simulator that could explicitly model the flow in the fracture and take into account the special properties of the proppant, such as the stress-dependent permeability or the possibility of non-Darcy flow. Such models typically were limited to a single-layer, single-phase (oil or gas) situation.
Summary This paper provides a detailed description of conditions in the hydraulically damaged fracture environment after closure and how to integrate them into a reservoir-simulation model. A special model-initialization algorithm was developed and realized in a support tool to make possible the computing of a post-fracture performance in tight gas formations by a reservoir simulator. The input represents the treatment schedule of the fracturing process and some results produced by commercial fracturing packages or geophysical measurements. To represent the fracture geometry and properties, the information about the distribution of the proppant concentration in the fracture as well as the fracture-width variation is translated into the permeabilities and porosities of the fracture gridblocks. To determine the fracturing-fluid saturation in the invaded zone, a new approach was derived to imitate the fracture propagation at a fracturing period under consideration of the leakoff processes. The penetration of the fracturing fluid into the matrix was modeled by the Buckley-Leverett equations for two-phase nonmiscible displacement, with boundary conditions provided by a classical leakoff theory. The approach is illustrated with a simulation model prepared for the analysis of the cleanup process in a damaged fractured well within a Rotliegende tight gas formation in north Germany. Introduction The fluid that leaked off into the tight gas formation during the fracturing treatment may significantly suppress gas production because of the two-phase flow effects and the capillary end effect between the reservoir and fracture (Holditch 1979). Therefore, for more plausible evaluation of hydraulic fracture stimulation, an accurate representation of the flow in the immediate fracture environment becomes a necessity. In terms of numerical simulation of post-fracture well performance, the problem can be addressed by (1) an adequate representation of the fracture in a reservoir simulator and (2) a reasonably accurate picture of the initial fluid distribution around the fracture.
The paper shows a way to provide a detailed description of conditions in the hydraulically damaged fracture environment after closure and how to integrate it into a reservoir simulation model. A special model initialization algorithm was developed and realized in a support tool to make possible the computing a post-fracture performance in tight gas formations by a reservoir simulator. The input represents the treatment schedule of the fracturing process and some results produced by commercial fracturing packages or geophysical measurements. To represent the fracture geometry and properties, the information about the distribution of the proppant concentration in the fracture as well as fracture width variation is translated into the permeabilities and porosities of the fracture gridblocks. To determine the fracturing fluid saturation in the invaded zone, a new approach was derived to imitate the fracture propagation at a fracturing period under consideration of the leakoff processes. The penetration of the fracturing fluid into the matrix was modeled by Buckley-Leverett equations for two-phase non-miscible displacement with boundary conditions provided by a classical leakoff theory. The approach is illustrated with a simulation model prepared to the analysis of the cleanup process in a damaged fractured well within a Rotliegende tight gas formation in North Germany. Introduction The fluid leaked off into the tight gas formation during the fracturing treatment may significantly suppress gas production due to the two-phase flow effects and the capillary end effect between the reservoir and fracture.1 Therefore, for more plausible evaluation of hydraulic fracture stimulation, an accurate representation considering the flow in the immediate fracture environment becomes a necessity. In terms of numerical simulation of post-fracture well performance, the problem can be addressed byan adequate representation of the fracture in a reservoir simulator anda reasonable accurate picture of the initial fluid distribution around the fracture. The present work is focused on these two main points which are explained bellow. 1. In our previous paper2 we gave a comprehensive comparison analysis of different approaches to numerical modeling of the fractured wells in tight reservoirs. Among them are:Fracture is considered by a series of high permeability grid blocks of a reservoir model;Separated reservoir and fracture models are coupled through the source/sink term;Fracture is presented by modification of the transmissibilities of the reservoir gridblocks containing the fracture. It was shown that the first, more traditional technique with a reasonable local grid refinement can be a preferable method when the flow under study does not contain any features making difficult to simulate within the framework of a reservoir model. So, the present work was oriented to the use of a conventional reservoir simulation program. The integration of the fracture into the reservoir model is not a problem of great concern. A short discussion of this may be found, e.g., in Settari et al.3, Banerjee et al.4. Reasoning of the time step limitation and because of Peaceman's condition for structured orthogonal grids, the fracture in the model can not take the actual width. Correspondingly, the permeability and porosity of the fracture blocks are reduced in order to maintain the transmissibility and porous volume of the fracture. The starting values of the hydraulic parameters within the fracture are calculated from the proppant distribution after the closure. These data can be obtained by the fracturing simulation with consideration of proppant transport processes or indirect, geophysical measurements.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.