Fracture Design For Suspended Sand Packs
Authors: Kaspereit Dennis H. 
Published: December 2007

This paper presents a simple method of designing a suspended fracture of a desired conductivity. The design balances volume, pump rate, and fluid loss to get the desired length. A sand schedule then is calculated to give the optimum flow capacity. The fluid and sand used to build the excess flow capacity in an equilibrium pack design are used to obtain better vertical coverage and deeper penetration.

The method is most applicable to low permeability reservoirs where the conductivity of an equilibrium pack is not needed. In this type of reservoir, the pack is not needed. In this type of reservoir, the surface area of a fracture face will give up only a finite amount of fluid. It is not necessary for the fracture to have a greater flow capacity to the wellbore than the formation has into the fracture. The design also can be applicable in some massive zones where the permeability is higher, but the cost to build an equilibrium pack is prohibitive. With a suspended pack design using the same amount of fluid, a deeper fracture with 100% vertical coverage can be obtained. In many cases, this produces a higher productivity increase with a lower cost. productivity increase with a lower cost. The design centers on the sand concentration of the sand slurry in a finite number of segments along the fracture length. Using a compound-interest formula, the approximate amount of fluid that has leaked off in each segment is calculated. Then, with the final desired slurry concentration known, the sand concentration needed in each segment when pumped can be calculated. pumped can be calculated. The design is done in 10 easy steps with a program written for a TI-59 that will do almost all program written for a TI-59 that will do almost all of the calculations included. The design is not applicable to every well, but where it does apply it can (1) give better vertical coverage, (2) get deeper penetration, (3) reduce costs for the came penetration, (3) reduce costs for the came productivity increase, and (4) reduce the amount of productivity increase, and (4) reduce the amount of load to recover.

Introduction

Most of the work on fracture design has been based on guar gels or other "sand-banking" fluids. In these designs, the sand falls to the bottom of the fracture and fills the created fracture width. The controlling factor of the final fracture conductivity is the created fracture width and not the concentration of the sand in the fluid. Sand is looked at only in terms of total volume, or how far it will fill a fracture of a given width. Sand scheduling is done by experience only to prevent sand-outs. Recently, with development of "perfect support" fluids, a suspended sand slurry can be placed in the fracture. This makes the sand placed in the fracture. This makes the sand concentration of the fluid the controlling factor of the final fracture conductivity. Therefore, the design of a suspended pack centers around sand scheduling and leakoff to obtain a fracture of a desired conductivity. When a fracture closes on a sand slurry, the resultant "suspended pack" has less conductivity than an equilibrium pack, but has 100% vertical coverage and a deeper penetration. This makes the design of a suspended pack most applicable to low permeability reservoirs where the conductivity of an equilibrium pack is not needed. in this type of reservoir, the surface area of a fracture face will give up only a finite amount of fluid. It is not necessary for the fracture to have a greater flow capacity to the wellbore than the formation has into the fracture. In many low permeability reservoirs, the flow capacity of the formation can be matched with only a suspended pack, which is a monolayer to a few layers thick when closed. By using a suspended-pack design, costs can be reduced and the same production increase maintained.

This design also can be applicable in some massive zones where the cost to build an equilibrium pack is prohibitive. In some cases, the formation pack is prohibitive. In some cases, the formation may have a greater conductivity into the fracture than the fracture has to the wellbore. However, the 100% vertical coverage and deeper penetration of the suspended pack can overcompensate, and a higher productivity increase with a lower cost can result. Generally, when the optimum fracture capacity cannot be obtained, the suspended-pack design is more applicable the thicker the pay zone.