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Abstract

The industry has relied on three primary measurements during fracturing operations: surface pressure, surface flow rate, and surface proppant concentration. The downhole environment is challenging for instrumentation.

What can the surface pressure indicate? What does the surface pressure not indicate? Often, the pressure measurement is miles away from the event. This long distance often obscures the transient pressure signature, the surface pressure measurement as a function of time. These obscurations are caused by the speed of sound in the carrier fluid, the compressibility of the pipe in the completion, and other factors.

A transient finite difference model of a horizontal multiple interval completion for an induced fracture shale reservoir was developed to investigate the pressure transient signatures of different events during the fracturing operation. Model results will be shown for different types of events, such as sleeve shifting, fracture initiation, opening a sleeve into a zone with an existing fracture, fracturing a zone with high leakoff, etc. Certain types of events have distinct pressure signatures. Other events can have similar pressure signatures. Some events can have no surface pressure signature at all. The model results will be compared with field data.

While the focus of the results is on horizontal multiple interval shale completions, the results can also provide insight into other types of fracturing operations and will aid in the interpretation of pressure signatures.

Introduction

Horizontal wells with transverse hydraulic fractures have gained industry acceptance as the preferred completion for low-permeability shale reservoirs. Achieving multiple transverse fractures can be accomplished with a completion, as shown in Fig. 1. Several annular compartments are created through the use of annular-isolation devices, such as swellable elastomeric packers or hydraulically set packers. Each of these compartments contains a stimulation sleeve (Fig. 2). The fracture stimulation treatments are pumped toe to heel. After the first fracturing stage is complete, it is immediately followed by a ball injected into the flow stream from the surface which is pumped through larger diameter ball seats until it reaches the ball seat that has a smaller diameter than the ball itself. As the ball lands on the seat, a differential pressure is created across the ball seat. When this differential pressure reaches a predetermined value, as determined by shear screws in the sliding sleeve tool, the shear screws fail and the sleeve shifts open. The next stage of fracturing pad volume and proppant slurry is then pumped into the compartment to create a transverse fracture(s) in the reservoir rock. Once this fracture pumping has completed, the cycle is repeated with a larger diameter ball.

Using pressure observations to ascertain fracture-growth characteristics (Nolte and Smith 1981) is not the focus of this work. The focus is on the interpretation of sudden changes in the pressure signature and specific downhole wellbore events, as listed in the paragraph above, and whether or not they result in a sudden change to the surface pressure signature.

System Wave Speed in Completions

The investigation of fluid-pressure transients requires the recognition that liquids are not truly incompressible and that pressures are transmitted through a fluid at a finite velocity. When considering piping systems, or in this case, well completions, the fastest rate at which pressure will be transmitted is known as the system wave speed. For the case of fluids in a pipe, system wave speed, a, is defined by the speed of sound in the fluid combined with the elasticity of the pipe in question (Equations 1 and 2). In Equation 1, the variables are: a is system wave speed (ft/sec), K is fluid bulk modulus (psi),is density (lbm/ft^³), C is a unit conversion factor (16.65 E-12), E is pipe modulus of elasticity (psi), D is pipe outside diameter (ft), and e is pipe wall thickness (ft). In Equation 2, a_fluid refers to the fluid speed of sound (ft/sec) through the fluid.