Pressure-Transient-Analysis of Horizontal Wells with Transverse, Finite-Conductivity Fractures

Abstract This paper discusses the analysis of pressure-transient responses of horizontal wells intercepting finite-conductivity transverse fractures. We use a hybrid, numerical-analytical model to simulate the impact of the fracture properties on the early-time flow regimes and pressure transient characteristics of fractured horizontal wells. It is shown that the fracture geometry and well location strongly influence the flow convergence in a horizontal-well fracture and lead to early-time flow regimes different from vertical-well fractures. Accordingly, appropriate pressure-transient models and analysis procedures should be used to determine fracture properties. We present straight-line analysis equations for radial-linear and pseudo-bilinear flow regimes for circular and rectangular fractures, respectively. We also bring the conventional fracture half-length and conductivity concepts into perspective and question the assumption that the properties estimated from pressure-transient tests of horizontal-well may be taken as effective properties when fractures are not rectangular. We show that flow convergence toward the wellbore may increase non-Darcy flow within the fracture. If the additional pressure drop because of flow choking and non-Darcy flow is not taken into account, pressure-transient tests indicate smaller effective conductivity or fracture size. Because this additional pressure-drop is flow rate dependent, the estimated effective fracture properties are not useful for performance prediction purposes. Introduction Since the early 1980s, horizontal wells have proven to be a vital technology in the enhancement of hydrocarbon recovery. Along with the increased tendency of completing wells horizontally came an increased interest in hydraulically fracturing these wells as means of improving their productivity especially in tight formations.(1,2) Hydraulic fractures intercepting horizontal wells fall into two main categories: transverse fractures and longitudinal fractures. (2) In the case of transverse fractures, the fracture intercepts the wellbore through a small piercing in the fracture plane that causes flow choking around the wellbore-fracture intersection. When flow converges toward the wellbore within the fracture, the reduced flow area (and volume expansion for gas) may also increase the flow velocities and cause non-Darcy flow. These aspects of flow in transverse hydraulic fractures emanating from horizontal wells are different from vertical-well fractures and should be taken into account in the estimation of fracture properties from the analysis of early-time pressure-transient responses. Pressure-transient characteristics and analysis of fractured horizontal wells have been discussed in several studies. (2–8) These studies can be divided into two groups. In the first group, (2–5) horizontal-well fractures have been modeled by considering radial-flow convergence within the fracture for all times. The radial flow assumption within the fracture leads to the conclusion that the first two of the early-time flow regimes (fracture-storage-induced flow regimes) are fracture-radial and radial-linear flow. These studies explicitly or implicitly assume radial (or square) fracture geometry and may not be applicable for cases that are more general. (9,10)