A New Flow Skin Factor Formulation for Hydraulically Fractured Wells in Gas Condensate Reservoirs

Abstract

Fracturing is one of the most common well stimulation techniques especially for tight gas-condensate reservoirs. Gas condensate flow around hydraulically fractured wells (HFWs) is different from conventional gas oil systems. This is mainly due to phase change, condensate drop out and coupling (i.e., the increase of relative permeability as velocity increases and/or interfacial tension decreases) and inertial (i.e., the reduction of relative permeability as velocity increases) effects. Description of HFWs in gas condensate reservoirs using the existing reservoir simulators requires the use of very fine grids to capture the significant changes of flow and rock properties occurring in and around the fracture. This tasks it very cumbersome, time consuming and impractical. In this work a two dimensional mathematical simulator has been developed, which is based on finite-difference methods and accounts for the combined effects of coupling and inertia using our recently developed generalized correlation. This single-well model, which simulates the steady-state flow of gas and condensate around HFWs, also allows for phase change in these low interfacial tension systems. A series of sensitivity studies were carried out using this simulator. The results were used to develop a general method for estimation of flow skin factor based on the definition of dimensionless effective fracture conductivity. In the proposed method the skin factor expressing the impact of pertinent parameters are then converted to an effective well bore radius, which can be used by a reservoir engineer to conduct a realistic open-hole simulation of flow around HFWs. Introduction Hydraulic fracturing is a well known and common practice to improve the well productivity especially in tight gas condensate reservoirs. A hydraulic farcture reduces the flow resistance around a well bore, decreasing the pressure drawdown hence, reducing the negative impact of the condensate banking. Because of the importance and wide applications of hydraulic fracturing, productivity calculations in such systems has been the subject of interest by many investigators [e.g., McGuire and Sikora (1960), Cinco-ley et al (1977, 1978), Valko et al (2003), Meyer and Jacot (2005), Mahdiyar et al. (2007)]. These studies were aimed at the determination of improvement in well productivity or optimum fracture design. In these studies the flow behavior and pressure distribution around the fracture at steady state or pseudo steady-state conditions have been investigated. Results of theses works are in the form of charts or correlations for calculating the well productivity, skin factor or effective well-bore radius. In a Hydraulically Fractured Well (HFW) system, the thickness of the fracture is usually less than 1 inch, requiring fine grids for the fracture cells to accurately simulate fluid flow in these systems. Furthermore, fine grids are required around the fracture to capture the abrupt changes in flow parameters in this region. Accurate skin factors, on the other hand, can help reservoir engineers to forecast the well productivity without using fine grid, which is time consuming and cumbersome. Skin factor, or effective well bore radius, is a useful tool for comparing the performance of a real system, which is often complex and difficult to replicate, with that of an open-hole system. Most of the available methods in the literature for estimation of fracture skin or effective well bore radius of a Hydraulically Fractured Well (HFW) were developed for single phase Darcy flow systems [McGuire and Sikora (1960), Prats (1961), Cinco-ley et al (1977, 1978), Valko et al. (2003), Meyer and Jacot (2005), Mahdiyar et al. (2007)].

The state-of-the-art in natural gas production

“…Several studies have been done on turbulence effects in hydraulically fractured gas wells (Wang and Economides, 2004;Gil et al, 2001;Lolon et al, 2004;Kakar et al, 2004;Miskimins et al, 2005;Ouyang, 2007;Huang and Ayoub, 2007;Friedel et al, 2007;Nashawi, 2007) and gas condensate wells Indriati et al, 2002;Ravari and Ibrahim, 2005;Ravari et al, 2007;Mahdiyar et al, 2008). Studies (Gil et al, 2001;Ubani and Evans, 1982) showed that significant turbulence flow can exist in a fracture at any time in the life of a vertically fractured gas well.…”