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Abstract Severe changes in rock permeability among adjacent oil bearing horizons presents an enhanced oil recovery problem. Fluids used to displace indigenous oil tend to follow the high permeability avenues, bypassing large reserves of oil. The use of a modified biopolymer, starch, as a method to enhance oil recovery was evaluated by analyzing:changes in rock permeability caused by the macromolecular gel matrix of the polymer solution,mobility reduction due to the higher viscosity of the polymer solution compared to that of water,the chemical and mechanical stability of the biopolymer at reservoir conditions,the effect of various polymer injection strategies, andsweep efficiencies by Nuclear Magnetic Resonance Imaging (NMR/MRI). Artificially as well naturally consolidated porous media with initial permeabilities ranging from 1,500 to 2,500 md were used. The average permeability reduction was from 1,500 to 500 md when a 1% by weight biopolymer was used. NMR/MRI proved to be a very useful tool to evaluate the effectiveness of a starch biopolymer for rock permeability control. Vertical sweep efficiencies measured by NMR means improved from 25% to 95% with the introduction of the biopolymer. Oil recovery was dramatically enhanced from 2% of the Original Oil in Place (OOIP) using brine as driving fluid to 98% when the in-house biopolymer was used during the displacement tests. Differential pressure (Ap) across 3-inch long samples constantly increased from 10 to 130 psi. A xanthan-gum based commercial biopolymer yielded a recovery of only 58% of the OOIP when used under similar conditions. Displacement experiments were conducted at 1,000 psi and 160F. The polymer solution exhibited moderate shear thinning viscoelastic behavior and pronounced shear thickening viscosity with increasing brine and polymer concentration. Introduction Most reservoirs are abandoned after about one third of the OOIP has been recovered because of technical and/or economical constraints. Severe changes in rock permeability usually lead to inefficient oil recoveries. Conventional secondary recovery methods sweep oil from high permeability zones and bypass substantial amounts of oil trapped in the lower permeability zones. The main disadvantage of water flooding is that low water Viscosity compared to that of oil allows bypassing of oil in place. This detrimental effect results in low recovery efficiencies and high water to oil production ratios. However, waterflooding costs are minimal when compared to other oil displacement processes. Therefore, its use is widespread. Polymer flooding has been identified as an effective procedure to redistribute flow paths to lower the residual oil saturation and to improve sweep efficiencies. P. 445^ |
