The recent success of coiled tubing fracturing in shallow wells has increased interest in using coiled tubing for fracturing deeper and hotter wells. Industry efforts need to now focus on understanding what properties of a fracturing fluid are required to successfully carry proppant at high rates through the coiled tubing, fracports, perforations and into these deeper formations. The key performance requirements of a coiled tubing fracturing fluid for deeper wells are low friction pressure and adequate proppant-carrying capability after exposure to high shear zones and higher temperatures.
This paper summarizes the results of pilot and field scale testing that led to the development of an optimized coiled tubing fracturing fluid. Results show that polymer-based fracturing fluids can be controllably delayed to have low friction pressure through the curved coiled tubing unit and straight tubing. However, results also show that fluid stability can be significantly reduced when pumped through small diameter tubing followed by high shear zones such as fracports and then perforations. Results demonstrate that correct fluid choice and fluid optimization are required to meet proppant transport requirements. For coiled tubing fracturing to be successful, the fluid and treatment design recommendations should balance the friction pressure limitations with the fluid stability limitations.
A recent review of the literature revealed that minimal information is available on how a fracturing fluid is affected by pumping at high rates through long lengths of small diameter tubing followed by the high shear zone in the fracports and the perforations. Several papers have been written describing the adverse effect that pumping fracturing fluids at high shear rates has on fluid stability.1,2 Papers have also presented data on the adverse effects on fluids stability caused by pumping fracturing fluids at low rates through coiled tubing,3 but little information is available on the adverse effects caused by pumping at high rates. Papers have also described the erosional effects of sand when pumped through small diameter coiled tubing and through isolation tools, but the effects of these high shear environments on fluids has been largely neglected.4,5 Because of the lack of information on fluids used in coiled tubing applications and particularly deep well coiled tubing applications, it was necessary to conduct pilot and field scale testing to determine how a fracturing fluid is affected and how its fluid properties can be optimized specifically for deep well coiled tubing fracturing applications. Crosslinked fracturing fluids may show ideal properties in the laboratory, but may exhibit completely different behavior under actual field conditions. Therefore, the properties of crosslinked fluids must be determined and optimized in laboratory testing that simulates as closely as possible actual field and downhole conditions.1
The first step in the development of a fluid optimized for deep well coiled tubing fracturing applications was to set product performance specifications based on targeted job requirements. Targeted job requirements were to pump through 1,000 ft of 2-in. coiled tubing and then down 7,500 ft of straight tubing at pump rates of 8–10 bbl/min. Jobs would treat small zones with 25,000 lbs proppant with proppant loadings up to 4 ppa. Product specifications required the product to have minimal friction pressure to allow pumping through the coiled tubing and then long lengths of straight tubing at high pump rates. The fluid crosslink time would be delayed for at least three-quarters of the tubing length. The fluid would also have at least one-hour stability at 250°F. The fluid must also have adequate viscosity to carry proppant at high flow rates through the fracports and then immediately through the perforations.
The development of the improved coiled tubing fracturing fluid for deep well application was done in two testing phases. Test phase 1 involved screening of potential fluids in a pilot scale friction test loop. Test phase 2 involved field scale testing of the best candidate fluid from test phase 1 through a coiled tubing unit and straight tubing test loop.
In the pilot scale screening tests, the candidate fluids were recirculated through small diameter tubing, and the differential pressure was continuously measured across 10 ft of the tubing.