Spontaneous Imbibition of Water into Oil Saturated M_1 Bimodal Limestone
Authors: Clerke E.A. , Funk J.J. , Shtepani E. 
Published: March 2015

Abstract
The M_1 nested bimodal pore system is prevalent in many large limestone oil reservoirs in Saudi Arabia. Within this pore system is contained a large portion of these fields' oil in place. Very low initial water saturation in these large structural relief carbonate reservoirs results in oil emplaced into pores controlled by M macropore throats and also into pores controlled by much smaller Type 1 micropore throats. Approximately, seventy-five percent of the M_1 oil portion is stored in the macropore system and about 25% is stored in the Type 1 micropore system.
This prevalent M_1 petrophysical rock type (PRT) is an example of nested bimodal pore system consisting of an instance from the distribution of Macro possibilities (M porositon) and an instance from the Type 1 micro porositon distribution. The maximum pore-throat diameters of the Type 1 micro porositon are on the average 53 times smaller than the M macro porositon average maximums. M porosity average is 17% with a mean maximum pore-throat diameter of 58 microns. The Type 1 microporosity average is 5.6% with a mean maximum pore-throat diameter of 1.1 microns. Thus, common in Arab-D carbonate reservoir matrix is a bimodal pore network with a very large hydraulic contrast between a fine network of well-sorted tubular Type 1 micropore throats, connected and adjacent to a network of much larger diameter moderately-sorted M macropore throats.
In a previous publication by Clerke, it was shown that the very small micropore throats' contribution to the total permeability is commonly below the resolution and reproducibility of the permeability measuring device when in the presence of many much larger pore throats. The micropore network is permeable if only at a small value. For the two phase flow occurring in a waterflood for oil recovery, the M_1 PRT requires an understanding of the two phase recovery processes in each pore subsystem considering capillarity in the combined pore network.
This paper demonstrates that the Type 1 micropores are themselves a permeable network to water and to both oil and water when under waterflood. Hence for our carbonate reservoirs, "pores with throat diameters less than one micron when filled with oil in a bimodal M_1 pore system contribute to oil recovery through a time dependent spontaneous imbibition process and thereby contribute to oil recovery by waterflood." Further, it is demonstrated that the multimodality porositon classification proposed by Clerke are a form of dynamic rock type that classify the position and the type of internal pore level capillarity spatial gradients that affect ultimate oil recovery.
New high-precision laboratory data has been obtained at very low phase pressure: water imbibition into oil saturated M_1 pore systems at near zero phase pressures (spontaneous imbibition) and dispersion of D2O into water filled M_1 pore systems. These pore systems can now be analyzed to obtain the magnitude, direct time dependence and scaling behavior of this important and previously overlooked portion of the total carbonate oil recovery by waterflood.