Do Viscoelastic-Surfactant Diverting Fluids for Acid Treatments Need Internal Breakers?
- Tianping Huang (Baker Oil Tools) | James B. Crews (Baker Oil Tools)
- Document ID
- Society of Petroleum Engineers
- SPE International Symposium and Exhibition on Formation Damage Control, 13-15 February, Lafayette, Louisiana, USA
- Publication Date
- Document Type
- Conference Paper
- 2008. Society of Petroleum Engineers
- 2.5.2 Fracturing Materials (Fluids, Proppant), 1.6.9 Coring, Fishing, 4.1.2 Separation and Treating, 4.1.5 Processing Equipment, 5.4.10 Microbial Methods, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.10 Drilling Equipment, 5.8.7 Carbonate Reservoir, 4.2.3 Materials and Corrosion, 1.8 Formation Damage, 2.4.6 Frac and Pack, 3.2.4 Acidising, 4.6 Natural Gas
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Matrix acidizing and acid fracturing treatments have been regularly applied for enhancement of oil and gas production. The success of the acid treatments requires uniform distribution of acid fluids across long treatment intervals or areas especially for vertical, horizontal, extended reach and multilateral wells with open hole completions. The unique properties of viscoelastic surfactant (VES) fluids provide good acid diversion fluids for blocking acid fluid flow into higher permeability sections in order to force the stimulation fluid into the lower permeability sections.
Recent SPE publications have shown that VES fluids used for acid diversion fluids have caused formation damage for matrix acidizing and acid fracturing, requiring a longer flow back periods, particularly for low permeability and depleted gas formations. Internal breaker technologies for VES fluids have been developed for hydraulic fracturing and frac-packing treatments. The internal breakers give a controllable break and go wherever the VES fluid goes, breaking the VES fluid into easily producible fluid.
This paper will introduce a VES organic acid fluid with internal breakers for treating carbonate reservoirs. The VES organic acid system is unique and will not generate noticeable viscosity until the acid starts to spend. The VES within the acid fluid reaches it highest viscosity as the acid completely spends and diverts fresh acid to new locations. After acid treatments, the internal breaker becomes activated to break the VES micelles, and the fluid loses its viscosity and flows back during production, minimizing any potential damage.
The paper will discuss lab results which demonstrate that the VES product generates very high viscosity at low shear rates in the spent acid fluid, and VES fluid will not broken only by contacting hydrocarbons. Further demonstrated is how internal breakers are available that reduce the viscosity of the spent acid diverter fluid over a controllable period of time. Carbonate wormholing core flow tests show non-broken VES fluids in wormholes need much higher pressure drop than than broken VES fluids for mineral oil cleanup.
To enhance treatment efficiency of matrix acidizing and acid fracturing in carbonate reservoirs, especially for long and heterogeneous intervals of horizontal wells, effective acid diversion is a key issue. Different technologies have been applied to solve this problem.1~3 Surfactant micellar-based fluids have been used as acid diverters for matrix acidizing and acid fracturing in the past several years because of their molecular weight surfactants. The surfacts form elongated micelle structures which exhibit visco-elastic behavior to increase fluid viscosity and maintain low formation damage characteristics. There are two types of application for acid diversion with surfactant micellar fluids. One is self diverting surfactant fluid. The other uses the surfactant micellar fluids as diversion pills.
Self-diverting surfactant fluid is composed of fresh acid and surfactant. The initial fresh acid system has water-like viscosity during mixing and pumping. When the fresh acid fluid system contacts and reacts with carbonate in the formation, the fresh acid system loses its acidity, the fluid pH increases, divalent ions like Ca2+ and/or Mg2+ forme, and the fluid starts to become viscous. As the local viscosity increases, the fluid system then effectively diverts the trailing acid fluid. The following fresh acid will be diverted to other areas to stimulate the formation. This is a continuous diverting process during acid injection.
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