Solvent enhanced waterflooding in fractured reservoirs

Oil recovery in fractured reservoirs by water flooding critically depends on the wetting properties of the matrix blocks between the fractures. The recovery from oil-wet reservoirs is almost negligible. In incompletely oil-wet systems, the presence of initial water may change the wettability characteristics so that very slow imbibition and some oil recovery can occur. The hypothesis in this thesis is that water soluble solvents enhance the wettability change and lead to enhanced recovery. Another mechanisms is that the water soluble solvent dissolves in the oil and oil is recovered due to swelling. The solvent may also decrease the oil viscosity and the interfacial tension between the oleic and aqueous phases. This thesis comprises of an experimental study into the recovery enhancement by water soluble solvents (diethyl ether). We used an Amott imbibition cell studying oil saturated samples of various wettabilities, permeabilities using oil of different viscosities and two different solvent concentration in the aqueous phase. In the first stage of the experiment the water-wet core was exposed to brine without solvent. In a second stage the core was put in a new Amott cell, which was filled with solvent/ brine mixture. The additional recovery was small. We found that the oil recovery is faster when the oil viscosity is lower, but the ultimate recovery is about the same. The recovery with higher permeable samples is faster, but ultimate recoveries are close. Low permeable samples benefit from the presence of solvent, but the effect is rather small. For the oil-wet samples we also started with exposing the core into pure brine without solvent. In spite of being oil-wet, oil was recovered from these samples. In view of the large inverse Bond number it would appear unlikely that this a gravity drainage effect; however the produced oil droplets are coming from the top. Contrary to the water samples there was a significant increase in recovery rate when the sample is transferred to another Amott cell where it is exposed to a mixture of solvent and brine. Now oil drops come from all sides. Therefore it is concluded that wetting alteration is the main mechanism of water soluble solvent enhancement in partially oil-wet cores. The effect of solvent on completely water-wet cores is less but significant.

Subject

solvent
spontaneous imbibition
fracture

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master thesis

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