Degradable Frac Ball Holds Solution to Persistent Problem in Fracturing

32 JPT • NOVEMBER 2013 A highly efficient and popular technique for multistage fracturing involves placing a completion string in the open hole with a series of ball-actuated stages isolated by hydraulically set or swellable packers. This practice has been instrumental to the increase of activity in hydrocarbonbearing reservoirs, as it allows continuous operation while performing a large number of stimulation treatments. When using this technique, sections of the reservoir can be selectively accessed by pumping actuators, or frac balls, from surface that land on correspondingly sized seats that have progressively larger diameters as operations advance from toe to heel. When all stages have been treated, the well is allowed to flow back, flushing the balls back to surface where they are caught in a ball trap. Despite more than a quarter of a million stages that have been treated using the sliding sleeve technique, the results are not always up to expectation. Operators have experienced suboptimal production from wells in which logs and tests indicated high productivity. In searching for a root cause, it has been discovered that often not all balls are recovered. Further investigation has sometimes found that balls may have deformed and become jammed in their seats, plugging all production from beneath. The only solution is to trip into the well and mill out the seats. Aside from the added cost and risk, the challenge is that this undesirable situation is difficult to identify because other reservoir problems that milling will not solve can present a similar production profile. This adds uncertainty to any decision to mill out the system. Frac balls are typically made from phenolic or composite materials. The composite material is laminated and, depending upon the orientation of the laminar planes, it can fracture when it seats and is subjected to additional hydraulic pressure during fracturing operations. If the ball fractures before it has served its purpose, the entire job may be jeopardized. Both types of balls are subject to deformation, often referred to as egging, when under pressure (Fig. 1). Slight egging can cause the ball to stick in the upper seats when the well is put on production, or the ball can jam itself into the seat so tightly that the only way to remove it is by milling. A stuck or jammed frac ball acts as a permanent isolation point for all treated stages below it. Alloy Ball Technology Schlumberger has recently introduced a degradable alloy technology suitable for ball drop systems. The Elemental degradable alloy balls disintegrate after making contact with well fluid within a few hours of deployment. The ball does not dissolve, but rather goes through a controlled degradation (Fig. 2) in which an electrochemical reaction occurs over hours or days that slowly transforms the material into hydrate oxides and hydroxides while slowly releasing a small amount of hydrogen. All that remains is a fine powder (micron scale) that does not interfere with flowback or production. The mechanism of this material degradation includes intra-galvanic cells and depassivation. During the first process, different crystallographic phases electrochemically interact, leading to one dissolving, while another remains and slowly generates hydrogen. The first mechanism resembles the electrochemical process that takes place in a car battery and is well understood in the oil field. To understand the second degradation mechanism, take the analogy of stainless steel—a metal or alloy known for its self-passivating behavior in aerated conditions. Stainless steel, because it has at least 12 wt% chromium in iron, naturally forms a thin protective layer of chromium oxide, among possibly other more complex oxides. When one or several of the oxides are removed, for instance as a result of abrasive wear, they instantly re-form as long as sufficient aeration is present. In contrast to stainless steel, the degradable material is unable to form a self-protective passive Degradable Frac Ball Holds Solution to Persistent Problem in Fracturing