In Situ Water Blocking Measurements and Interpretation Related to Fracturing in tight gas reservoirs
- Brigitte Bazin (Institut Francais du Petrole) | Yannick Peysson (Institut Francais du Petrole) | Francois Lamy (IFP) | Frederic Martin (Institut Francais du Petrole) | Eric Aubry (Total) | Christian Chapuis (Total S.A.)
- Document ID
- Society of Petroleum Engineers
- 8th European Formation Damage Conference, 27-29 May, Scheveningen, The Netherlands
- Publication Date
- Document Type
- Conference Paper
- 2009. Society of Petroleum Engineers
- 5.8.1 Tight Gas, 5.1 Reservoir Characterisation, 5.6.2 Core Analysis, 5.4.2 Gas Injection Methods, 1.6.9 Coring, Fishing, 5.5.2 Core Analysis, 1.8 Formation Damage, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.2.1 Phase Behavior and PVT Measurements, 2.2.2 Perforating, 1.6 Drilling Operations
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Invasion of aqueous drilling, completion or fracturing fluids can reduce the relative permeability to gas and thereby causes a waterblock. In the case of low permeability formations, the capillary pressure tends to be high because of the small pore size. Clean up of water blocks requires high draw down unless water vaporization by the flowing gas is improved by using specific additives like alcohols.
The purpose of this work is to investigate fracture face damage by measuring relevant petrophysical parameters: absolute permeability damage and gas return permeabilities. Measurements are performed in representative conditions of a fracturing operation in a tight gas formation: cores with an absolute permeability of 10 microDarcy set at Swi, experimental pressure of 200 bars for the fracturing fluid invasion. Water and gas saturations during the invasion of the fracturing fluid as well as during the gas back flow are monitored thanks to a XRay equipment. Adding alcohol in the fracturing fluid has a striking effect on resolving the water block. Cake formation on the simulated fracture face is also discussed.
Numerical simulations are performed to assess relative permeabilities from the experimental results. It is shown that the hysteresis of gas and water relative permeabilities has a strong impact on the rate of water removal. Sufficiently high pressure draw down is crucial to overcome capillary forces and initiate alcohol assisted vaporization process. Water removal by water vaporization is assessed and compared to the experimental results.
Tight reservoirs contain large resources. These reservoirs have usually a very low permeability (less than 0.1 mD), and most of the time do not yield any flow after initial perforations. Stimulation via hydraulic fracturing is used commonly in the industry to increase productivity or deliver commercial gas rates 1,2.
During the process of a fracturing job, large quantities of fluid are injected into the formation. These fluids contain several chemicals, of which high concentrations of gelling agent, which may cause formation damage3,4. Water blocks are commonly mentioned as the main mechanism responsible of permeability damage. In the case of low permeability formation, the capillary pressure is high because of very small pores. Hence the ratio of drawdown to capillary pressure has a significant effect on return
permeability5. Addition of alcohols or surfactants decreases the capillary pressures by a surface tension, interfacial tension and contact angle effect. With a water wet rock sample alcohol is mainly decreasing the interfacial tension. In addition the volatility of alcohols may contribute to a better flow of gas and water.
The clean up occurs in two regimes: displacement of the fluids from the formation followed by vaporization by the flowing gas6-8 . It can take a very long time to clean up. In a previous paper the two phase flow mecanism was investigated. It is shown that the water displacement is hindered by a relative permeability hysteresis induced by the imbibition of the fracturing fluid. Gas return permeability is strongly affected by the amount of gas trapped during the imbibition. Even at high draw down, water removal is difficult and gas return permeability reach a plateau because of the very low relative permeability to water when the gas saturation increases. Water
cannot be expelled even by maximizing the draw down.
The improvment in gas permeabilities with the addition of alcohol can occur due to two possible reasons: (1) the decrease of the interfacial tension and (2) higher volatility of the alcohol. The first mechanism is dominant in the two phase flow regime. The second mechanism acts mainly after the two phase flow displacement by evaporation of the remaining water phase saturation.
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