New Pump Schedule Generator for Hydraulic Fracturing Treatment Design

Abstract

Abstract A typical hydraulic fracturing treatment executes a pump schedule that includes pad fluid volume and proppant stage concentrations. The automatic generation of such a schedule by a computer program is highly desirable. The generation of a pump schedule for hydraulic fracturing is an inverse problem. The fracture length and the proppant distribution inside the fracture are specified as input, together with formation and fluid properties, and a pump schedule generator (PSG) determines the amount of fluid and proppant needed in every stage of the schedule. In this paper, a lag-length concept is implemented in a new PSG to control the distance between the proppant flow front and the fracture tip. The lag-length is necessary to prevent premature bridging and screenout in many situations. In this new PSG, the lag-length is automatically determined to generate an optimum schedule that uses the minimum pad volume to avoid premature screenout. The PSG is based on a pseudo-3D fracture model, and schedules can be designed for conventional (non tip-screenout) or tip-screenout treatments. The schedule generator has been used to study the effects of proppant size and fluid types on the fluid and proppant stage volumes in a schedule. Lag-length is dependant on proppant size and other treatment conditions. The results of this study show that the application of the new PSG can benefit the fracture treatment design process. In particular, they highlight that changing the proppant size cannot be significantly changed independently from other pumping parameters. The schedule generator is also shown to be a valuable tool in the selection of fracturing fluids. By comparing pump schedules required for a particular fracture performance - as given by desired length and proppant pack characteristics - the selection of a particular fluid can be made based on the optimum pump schedule for each fluid rather than using a fixed, or "typical," schedule in a particular field for different fluids. Ultimately, the PSG is a primary tool to select the means to achieve a desired fracture conductivity within given operational constraints. It allows to compare all potential options: changing the proppant size, attempting a tip screen out design or changing the fluid system. Introduction The hydraulic fracturing design involves many engineering and economics considerations. The first one is to determine what kind of fracture is needed in order to achieve a desired production increase. Based on reservoir properties, production increase goal, and treatment cost, optimum fracture length and conductivity can be determined. Another consideration is to determine how such a fracture can be created. For a typical proppant fracture treatment, one needs to design a pump schedule that can create the specified fracture properties when executed in the treatment. A pump schedule can be designed using analytical formulae based only on fluid efficiency1. Fluid efficiency at the end of a calibration injection can be obtained from a pressure decline analysis. The main treatment fluid efficiency can then be estimated from the calibration fracture efficiency for simple fracture geometries (PKN, KGD, or Radial). To design a pump schedule for a fracture of more complex geometry, a P3D (pseudo-3D) or a planar 3D fracture simulator can be used. The design process often depends on experience and trial-and-error. Since fracture simulators are forward simulation tools, a pump schedule is required to run a simulation in the first place. By assuming an initial pump schedule, the simulator produces fracture geometry and conductivity. If the fracture geometry and conductivity do not meet the design criteria determined in the first step, the pump schedule can be manually adjusted and the simulation is carried out again. This process is repeated until the simulated fracture geometry and conductivity meet the design criteria, and is often time consuming.