A New Look at Fractures Tip Screenout Behavior

Abstract

Summary Increasing pressure during a fracture treatment has been attributed to theformation of a "tip screenout," which occurs as a result ofinsufficient pad fluid volume or excessive leakoff. Results from a numericalsimulator of fracture propagation that incorporates the effects of proppantconcentration on fluid rheology, fracture transmissibility, and proppanttransportability suggest that the observed pressure rise can proppanttransportability suggest that the observed pressure rise can result from theformation of a low-mobility proppant bank at or near the perforations. Theeffect is related to proppant scheduling and transport perforations. The effectis related to proppant scheduling and transport characteristics, not toexcessive leak-off or insufficient pad volume. Introduction During a hydraulic fracturing treatment, the observed wellbore pressurenormally behaves in a stable fashion throughout most of the job. While the padfluid is pumped, a stable pressure trend is often established. The trend mayshow nearly constant, slightly increasing, or steadily decreasing pressure, depending on the growth mode of the fracture and the degree of heightcontainment established by surrounding rock layers. Nolte and Smith discussedthese pressure trends and their relative fracture geometries. The treatingpressure observed during a fracture job frequently will begin to increasesharply after proppant is added to the fracture. This pressure proppant isadded to the fracture. This pressure rise usually occurs late in the job. Nolteand Smith have attributed the pressure rise to the occurrence of a tipscreenout in the fracture. A tip screenout generally is assumed to occurbecause of excessive fluid loss, which results in dehydration of the proppantslurry or consumption of the pad fluid ahead of the proppant bank. The proppantbank then bridges off at or near the tip of the fracture, thus stopping furtherlateral growth. Injected fluid is stored in the existing fracture volume whilethe fracture "inflates" and pressure rises. In some treatments, a sharppressure rise has been noted almost coincident with the introduction ofproppant to the fracture. This proppant-induced pressure increase (PIPI) earlyin the job is not pressure increase (PIPI) early in the job is not consistentwith the previous explanation of tip screenout behavior. Conway et al. and Palmer and Veatch hypothesized that early-time PIPI is caused by offsets in thecreated fracture, multiple fracture strands, or related tortuosity in thefracture channel. These effects combine to create severe flow restrictions orproppant bridges near the wellbore. Conway et al. and Palmer and Veatch statethat the observed phenomenon is related almost exclusively to fracturing incarbonate reservoirs where natural fracture frequency is high. The phenomenonhas also been observed in formations of other mineralogy, including friable andwell-consolidated sandstones, conglomerates, and carbonates. Because fracturefaces are by nature rough and tortuous, some restriction to fluid and proppanttransport must be expected above that proppant transport must be expected abovethat predicted by equations governing fluid flow predicted by equationsgoverning fluid flow between parallel plates or in elliptical channels. Concentrations of suspended particles also influence the apparent viscosity, particles also influence the apparent viscosity, or flow resistance, of thecarrier fluid. Furthermore, the high concentrations of proppant particles canbe expected to create a resistance particles can be expected to create aresistance to movement of the particles similar to that represented byhindered-proppant-settling correlations. To account for these effects, a morecomplete model of proppant transport during fracturing operations wasimplemented in a fully 3D fracture-growth simulator, the Grid-Oriented Hydraulic Fracture Extension Replicator (GOHFER). The model solves thediffusivity equation for particle concentration, including horizontal andvertical flow, hindered settling, and the effects of proppant concentration onflow resistance and proppant concentration on flow resistance and particletransport. These features are particle transport. These features are requiredto account for the complex interactions that occur during proppant transport. GOHFER Formulation GOHFER provides the capabilities to model the complex problem of proppanttransport. The mathematical formulation of the model is described in detail in Ref. 6. The validity of the model and its accuracy at representing fracturegeometry development were shown by comparing the model's results to analyticalsolutions for fracture width and published results for other 3D simulators.published results for other 3D simulators. The model uses a 2Dfinite-difference formulation to solve for fluid pressures and flow velocitieswithin the fracture. The fluidflow solution is based on the Navier-Stokesequation for viscous flow between parallel plates. The distribution of fluidpressures plates. The distribution of fluid pressures within the fracture thatresults from the fluid-flow solution is used to calculate the fracture-widthdistribution with the Boussinesq solution for the displacement of the surfaceof a semi-infinite half-space caused by a distributed load. A tensile-strength criterion is used to determine fracture extension by comparing thetensile-stress distribution around the fracture perimeter with an assumed maximum allowable perimeter with an assumed maximum allowable tensilestress. JPT P. 138