Experimental Investigation of the Effect of Wettability and Drainage Rate on Tertiary Oil Recovery in Fractured Media

摘要

Vertical displacement of oil by gas is the most efficient known method for oil recovery from naturally fractured reservoirs. Unlike the homogeneous media, the ultimate oil recovery by gravity drainage in fractured media is very dependent on the production rate hence, finding the optimized production rate with respect to the properties of the media is essential. This paper addresses the conditions of optimal oil recovery through controlled gravity drainage of a single matrix block, in a fractured experimental model. The economical aspects of oil production rate and ultimate oil recovery were considered to assess the optimal drainage rate. During the experimental work, the contribution of different parameters such as the extent of the wettability of the matrix on the optimal oil production rate is studied for strongly water-wet, strongly oil-wet, fractional wet and mixed wet cases. Moreover, the effect of oil production rate on the vertical displacement of gas oil contact in the matrix and fracture, and the height of oil capillary rise is visually determined. The results showed that in very low oil production rate, gravity forces is strong enough to overcome all other contributing forces, to reach the maximum tertiary oil recovery by gas injection at gas breakthrough. It is also shown that maximum tertiary recovery at gas breakthrough occurs in fractional wet models at low and medium oil recovery rates; however, when the production rate approaches free fall gravity drainage the trend is reversed. Observation of oil bank size in the fracture part showed that it has the most influence on the oil recovery, which is also affected by the oil capillary rise in the matrix part. It is also shown that during the high production rates, the oil bank in the fracture separated from the matrix oil bank. Monitoring oil recovery rate by gravity drainage continued after gas breakthrough. The results showed that both the rate and ultimate oil recovery are strongly dependent on the residual oil saturation, the wettability of the matrix and the production rate.