B.Bazin Reservoir Engineering Unit - OAPEC June 2008,Productivity of tight gas reservoirs,B. Bazin (IFP),IFP Reservoir Engineering unit OAPEC - June 2008,2,Contents,Presentation of tight gas reservoirs Description of fracturing technology Fracturing fluids Reasons for fracture productivity impairment fracture conductivity damage effect of fluid filtration on fracture efficiency Understanding the fracture damage mechanisms from a reservoir point of view depth of fracturing fluid invasion absolute permeability damage (fracture stress and water sensitivity) relative permeability effect and flow initiation pressure Consequences Conclusions,IFP Reservoir Engineering unit OAPEC - June 2008,3,Unconventional Energy Potential,Unconventional reservoirs include:Tight gas sandsCoal bed methaneGas hydrate depositsHeavy oil,IFP Reservoir Engineering unit OAPEC - June 2008,4,From Holditch, JPT, January 2003,Unconventional reservoirs include:Tight gas sandsCoal bed methaneGas hydrate depositsHeavy oil,IFP Reservoir Engineering unit OAPEC - June 2008,5,Gas recoverable resources in the USA (from DOE/AEO 2004),Unconventional gas resources represent around 1.5 times proved and associated recoverable gas,38%,IFP Reservoir Engineering unit OAPEC - June 2008,6,Conventional vs Unconventional Accumulations,Characteristics of Conventional Accumulations Relatively high matrix permeability Obvious seals and traps High recovery factors,Characteristics of Unconventional Accumulations Regional in extent Diffuse boundaries Low matrix permeabilities Low recovery factors Includes tight sandstones, coalbed gas, oil and gas in fractured shale and chalk. Productivity of a tight gas reservoir is only 25% of gas in place in the best case (average 10-15%). Gas connectivity only in 4% of porosity.,From John Lee (Texas A&M University) Advances in Unconventional Resources Technology: Assessment Methodology,IFP Reservoir Engineering unit OAPEC - June 2008,7,What makes a reservoir tight ?,Effective porosity, viscosity, fluid saturation, capillary pressure are some of the input parameter controlling the effective permeability of a reservoir,Gas reservoirs (dry or condensate)1 microD K 1 mD ; 5% porosity 15-20 %Difficult to produceGenerally produced after hydraulic fracturing,IFP Reservoir Engineering unit OAPEC - June 2008,8,IFP Reservoir Engineering unit OAPEC - June 2008,9,Performance of fractured wells in tight gas reservoirs.,Frac technique: horizontal well with multiple fractures Test rate: 14,000 m3/h vs 4,000 m3/h without technology,From Gaz de France,Seven Fracture system ready to open sliding sleeves,3.5 increase in well productivity,IFP Reservoir Engineering unit OAPEC - June 2008,10,Causes of productivity loss of fractured wells in tight gas formation,Rheological fluid properties (affect fracture length) Polymer residus in the fracture (affect fracture conductivity) with cross-linked fluids up to 90% of conductivity loss optimization of cleaning fluids like “breakers“ Filtration (affects reservoir properties) fracturing fluid invasion into the reservoir (decrease of the fracture length, reservoir damage around the fracture) Water blocks (affect gas back flow) decrease the gas permeability,Magnified by very low permeability,IFP Reservoir Engineering unit OAPEC - June 2008,11,Impact of fluid filtration on fractured well efficiency,from Cinco-Ley in Economides “Petroleum Production Systems“.,IFP Reservoir Engineering unit OAPEC - June 2008,12,Main fracturing fluids,Aqueous solutions of polymer HPG, CMC, HEC. Cross linked fluids Guar + metal cation (borate, zirconate) Foams Nitrogen or CO2 60 to 80 % of gas Non conventional fluids: CO2, methanol, VES (Visco Elastic Surfactant) Surfactants cationic or anionic WaterFrac Water + DRA (Drug Reducing Agent),IFP Reservoir Engineering unit OAPEC - June 2008,13,Example of Fracturing Fluids,Structure of Hydroxy Propyl Guar,Cross linked fluids: reticulation between polymer molecules with metal cation,Voronoi structure of a fluid in emulsion from Penny,VES fluids,IFP Reservoir Engineering unit OAPEC - June 2008,14,Use of breakers as cleaning fluids - Oxydants (peroxydisulfate) - Enzymes Limitations with temperature, pH.,Consequence of polymer residus on fracture conductivity damage,Up to 90% of conductivity loss,IFP Reservoir Engineering unit OAPEC - June 2008,15,Understanding damage in the reservoir. Depth of fracturing fluid invasion.,Specificity for tight: Polymer molecules too large to enter the poresAbsolute permeability damage very low,Filtration rate is measured in tight gas samples to calculate depth of invasion for real fracturing operation,IFP Reservoir Engineering unit OAPEC - June 2008,16,Absolute permeability damage Water sensitivity,. Huge irreversible permeability damage from initial 4 microD to 1 microD after water contact.,Water sensitivity is due to clay swelling,Needs special additives in the fracturing fluid,