Dual Hydraulic Fracturing Technique Minimizes Proppant Convection and Increases Hydrocarbon Production
Authors: Nitters G. , van Gijtenbeek K. , van Domelen M. 
Published: December 2007

Abstract
A dual hydraulic fracturing technique has successfully increased hydrocarbon production in oil and gas wells on the European and African continents. This technique reduces the proppant convection that would otherwise adversely affect fracturing treatment results.
In some fracturing operations, a radial fracture growth pattern occurs when the initial or subsequent net fracturing pressure during the job exceeds the stress contrast between formations. This condition may result in unwanted downward fracture growth out of the zone of interest. In addition, the combination of a radial growth pattern and density contrasts in treatment fluid may cause proppant convection to the bottom of the fracture, which could impair production results.
Dual fracturing consists of an initial "settle" fracturing treatment followed by a main treatment. The settle-frac treatment features a low-viscosity fluid with high breaker loading and a proppant to create enough length and settled height. This treatment creates an artificial barrier that minimizes downward fracture growth and proppant convection.
The main fracturing treatment can then be modelled with an artificial barrier below the pay zone. This model allows adequate propped fracture length in the pay zone and a good conductivity contrast. When these procedures were followed, proppant convection was reduced during the main treatments, and the wells showed as much as four-fold production improvement. This paper presents case histories and production data from wells in Western Europe and West Africa.

Introduction
Uncontrolled height growth adversely affects many fracturing treatments. The greater this vertical fracture growth, the lesser the lateral fracture growth, which often reduces the production improvement factor. In addition, downward growth can often increase the risk of water influx. Over the years, many authors have proposed limiting vertical fracture growth by placing artificial barriers to fracture extension. In the 1960s, both Prater and Braunlich filed patents on the subject.
More recently, Barree, Mukherjee, and Conway have provided design guidelines for artificial barrier placement. While most published work has emphasized the creation of fracture length, few have focused on minimizing downward growth to prevent water influx. Uncontrolled fracture height growth, which causes poor near-wellbore proppant bed conductivity, has been reported only as a side effect.
With great fracture heights, proppant convection or density-driven flow may result in an insufficient fill of proppant opposite the perforation interval as described by Cleary and Fonseca. Their work suggests that under adverse conditions, the highest concentration of proppant may be found well away from the perforations. In laboratory experiments and computer simulations, Barree and Conway recently showed that proppant convection - and thus placement - can be influenced by injection rate and the rate of proppant ramping. Immediate flowback after the treatment can also improve the proppant distribution, especially in wellbores having uncontrolled vertical fracture growth.
Rate changes during pumping, however, pose major disadvantages. Many operators prefer a constant rate to prevent the treatment from terminating prematurely. In addition, immediate backflow also poses risks, and is not always possible. To improve the fill of the fracture opposite the perforations, operators can use an artificial barrier (referred to as a settle frac) and force a screenout at the end of the treatment at a constant rate. Ideally, this barrier is created with the same proppant used for the main fracturing treatment. As a result, the settled proppant bank also contributes to the post-frac productivity.
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