Abstract: A process of recovering oil from an oil-bearing formation which process comprises injecting a surfactant containing enhanced oil recovery formulation into the formation via an injection well and recovering oil and optionally gas via a recovery well which process comprises determining the apparent viscosity of the enhanced oil recovery formulation at various ratios of enhanced oil recovery formulation to oil wherein the highest apparent viscosity is at most 4 times the lowest apparent viscosity measured.

Abstract: A method for determining the depth (DL) of a fluid contact level between a first fluid (F1), such as water, and a second fluid (F2), such as crude oil or natural gas, within the pores of an oil and/or gas bearing formation surrounding a borehole comprises measuring the phase pressure PF1 and PF2 of said pore fluids using a pressure probe assembly which is lowered to a depth (DP) above the depth of said contact level (DL) and determining the depth of said interface on the basis of the equation: D P - D L = P F1 - P F2 g ⁡ ( ρ F1 - ρ F2 )