Pressure Transient Analysis of Multifractured Horizontal Wells

Abstract

Summary This paper presents results from a comprehensive investigation of the pressure transient behavior of horizontal wells with single or multiple vertical fractures. The approach used can handle fractures with radial and linear inflow to the well, with radial inflow in transverse fractures and linear inflow in longitudinal fractures. Fractures following the wellbore with communication to the well only at the center, i.e., with radial inflow, can also be handled, with these and the tranverse fractures having a circular outer boundary (truncated to the formation if necessary). The longitudinal (rectangular) fractures are assumed to have the full length perforated. Intervals between fractures are assumed not perforated. Most of the paper is devoted to discussions of flow periods exhibited by single- and multifractured horizontal wells under various ideal conditions. Identifying these are important since the special flow periods provide data that can be analyzed by conventional methods. At least four fundamental flow periods can be exhibited by horizontal wells with a tranverse or longitudinal fracture. For a tranverse fracture, or a radial fracture following the well, the periods are fracture radial, radial-linear, formation linear and pseudoradial for fractures fully or nearly confined to the formation. For longitudinal fractures the early flow periods are fracture linear and bilinear. For longer transverse fractures one can also have fracture linear and bilinear flow periods preceded by fracture radial flow. Generally, the fracture radial or fracture linear flow period occurs at a time too early to be of practical use. In any case, due to the transition periods, observing more than two of the fundamental periods in real data is not likely. For highly conductive fractures the early transient behavior is likely to be dominated by formation linear flow, while for poorly conductive fractures this flow period can be absent with the early transient behavior instead dominated by radial-linear or bilinear flow during several log cycles, if not masked by wellbore storage effects. Assuming no boundaries, the pseudoradial flow period will be reached eventually independent of fracture and formation parameters. For all the four fundamental flow periods, conventional straight-line analyses based on semilog, t or t4 plots can be used to determine fracture and formation properties,. During the early flow phase, horizontal wells with single or multiple fractures will experience the same flow periods. However, unless the individual fractures are far shorter than the distance between neighboring fractures, interference will occur before pseudoradial flow is exhibited around each fracture. Before interference occurs the individual fractures drain independent parts of the reservoir and are appropriately analyzed by assuming a single fracture and dividing the total flow rate by the number of fractures. For a multifractured horizontal well, a compound-formation linear flow period may also occur, with linear flow towards the well (as seen from above) dominating the pressure transient behavior. Eventually, provided no flow barriers are encountered, pseudoradial flow will also develop for a multifractured horizontal well. During both the compound-formation linear flow period and the pseudoradial flow period, the full flow rate must be used in the analysis of transient pressure data.