Tendeka: High Performance Water Swelling Elastomers Save Time and Money in the Bakken
The combination of shale and dolomite in the Bakken and Three Forks formations, and the presence of vertical to sub-vertical natural fractures make open-hole multi-zone completions an effective solution in this play. Swellable packers are used for stage isolation and the cost of a failed packer is one stage of production.
Swellable packers are manufactured using absorbable elastomers which increase in diameter automatically when exposed to well fluids. As the packer element increases in diameter, the elastomer will make contact with the borehole wall. After wall contact, the elastomer continues to swell producing an interface seal line pressure between the elastomer and borehole wall.
Oil swellable packers are commonly used for their proven performance but require diesel to be spotted into the open hole to enable these to swell at an addition cost of around $60,000 per well and increased HSE risk. The use of water swellables packers has the potential to save time and money but they must match or exceed the performance of oil swells.
Super Absorbent Polymer (SAP), is the most commonly available water swellable elastomer but is relatively weak. Osmotic water swellable elastomers offer improved strength and chemical resilience but can take too long to swell. Extensive elastomer development and testing has been performed to produce a water swellable elastomer which combines SAP with Osmotic swell mechanisms and a unique strengthening component to produce a water swellable packer specifically for Bakken conditions. The resultant elastomer has been successfully used to isolate over 10,000 frac stages to date.
Comparative test results for oil and water swellable elastomer types are presented along with case history result in the Bakken.
2. Water Swellable Elastomer Evaluated
2.1 Super Absorbent Polymer (SAP) Swells
SAP swell by the process of water absorbing into the SAP material and increasing in size. The rate of this process is dependent on both temperature and salinity. Although, these compounds are known to swell fast at low temperatures, their ability to swell in saline solutions can be limited. The fast swell performance can also lead to risks using SAP at high temperatures potentially causing issues during run-in. Tendeka typically recommended SAP water swellable packers for application in fresh water only at low temperatures.
2.2 Osmotic Swells
Tendeka’s proprietary osmotic swells provide an alternative to SAP swells. Swelling by the process of osmosis, this is also dependant on temperature and salinity. At lower temperatures the rate of swell for osmotic swells can be lower than that of SAP but it maintains a superior performance in saline solutions and has high internal osmotic pressures resulting in higher pressure holding capability. Covering a large temperature range these swellables are also resistant to acid, (at concentrations of 15% and 37% HCl acid).
3. Laboratory Testing in Bakken Conditions
Bakken conditions were replicated by laboratory testing at 20% NaCl at 121°C/250°F comparing the performance of water swell types and compared using Clairsol NS base oil also at 121°C/250°F for comparative oil swell tests.
Table 1: Testing Program
Swell testing is performed to determine not only swell performance, but also rubber compatibility and bonding agent compatibility. Testing samples are prepared in a similar fashion to a full scale product where the metal plate is prepared to the same anchor profile and the same bonding procedure is followed. Each sample has 13mm of rubber and measures 2” x 2”, figure 1. Each sample is then measured for weight, thickness and hardness and placed in an appropriate vessel for the temperature (figure 2) and placed in an oil bath (figure 3) with daily measurements.
Figures 1, 2, 3: Rubber Sample and Testing Equipment
A comparison of the testing results is shown in Table 2. This details the increase in swell seen as well as the decrease in hardness. From this it can be noted that TOC-01 shows faster swell, however TWC-06 maintains a higher Shore A measurement.
Table 2: Testing Program
It can be seen from Table 2 and Figure 4 that TWC-05 (SAP) showed faster initial swell, however this tailed off and the combination TWC-06 showed greater swell. It can also be seen that TWC-06 kept a larger hardness (indicating mechanical strength) when in comparison to all other compounds. The oil swell compound showed faster swell due to the high salinity of the 20% NaCl fluid tested in.
4. The Effect of Salinity on Swell Rates
Figure 5 shows the performance of the combined water swell compound compared to the performance of an oil swell. This shows water swell has a faster swell performance at 121°C/250°F in fluids with up to 10% salinity, than oil swelling in base oil at 121°C/250°F. Although oil swells are commonly used for fast performance, figure 5 shows that water swells show a superior swell when in low salinity fluids.
5. Translating Lab Results in Packer Performance
A typical Bakken swellable packer is a 4.5” x 5.625” element OD sealing in 6” ID, with a 5ft element holding 5000psi, at 121°C/250°F. The swell measurements taken in the lab are used to determine the contact point of the elastomer with the borehole wall. Time to full pressure capability is then determined by a fixed additional swell required to build internal contact forces.
Table 3: Time to Contact/Time to Pressure Oil Vs Water*
The data presented in the above table is verified using Tendeka’s swell engine created with empirical lab data, based on over 1500 laboratory tests and over 430 full scale pressure tests, which has been collated and fixed into the swell engine to give an accurate correlation between the simulation and swell testing performance curves. Lab testing has been conducted in a variety of fluids, including wellbore fluids, in extreme conditions and a variety of different pressures. This enables Tendeka to confidently simulate full scale products for customer’s requirements. The swell machine is available to download through the Apple Store.
6. Conclusion
Water swellable packers can achieve comparable swell rates to oil swellable packers at Bakken conditions in salinities up to 10% enabling time and cost saving to be achieved during the well completion operations by avoiding the need to circulate oil based fluids into the wellbore.
