From both the theoretical and experimental standpoints, the writer studied the mixing of solvent and oil at the solvent injection into oil reservoirs or its models. As the results, the following are chiefly indicated by this study: 1. The length and shape of mixing zone are controlled by the following factors, namely, elapsed time, distance, flow velocity, porosity, permeability, pressure gradient, water saturation, diffusion coefficients, viscosity, properties of crude oil and solvent, and temperature etc. 2. The occurrence of mixing zone between solvent and oil will be caused by the action of solvent flooding and/or solvent extraction. The former is piston-like displacement, the latter is dragging-like displacement. The dragging-like displacements would occur, to a great extent, in some cases such as in the low-viscosity solvent into high-viscosity oil.
Decline curves of oil production can be analysed by a method based upon the study which has been explained in the papers.1, 2) The following features characterize exploitation of old oil fields in Japan, and these facts enable the precise analysis of decline curves: (1) generally, constant producing method was adopted for all of the wells in one oil field, (2) production restriction was not considered, (3) secondary recovery method was not introduced, (4) oil production of all wells was recorded comparatively acculately from the beginning of exploitation, and (5) too narrow well spacing was applied in some oil fields. Results of the analyses are summarized as follows: (1) It is assumed that in most oil fields in Japan the solution gas is main energy of oil production. (2) Caused by oil production, the reservoir pressure decreases indicating approximately similar value at all points in the reservoir. In most oil fields there is no fault which interrupts flow of fluid. (3) There is a definite relationship between reservoir structure and value of (effective permeability of oil)×(thickness of reservoir). Generally, there is more oil at central part of a reservoir than at marginal part. (4) Caused by greater solubility of gas to oil, some wells, during their initial stages, indicate greater production rate and greater coefficient of production decline curve than average values. (5) Unusual increase of coefficient of decline curve occurs frequently at wells located marginal place of a reservoir or at wells of a depleted reservoir. (6) When distance between wells is greater than at least 60 metres, there is no relationship between well spacing and production rate of oil and between well spacing and coefficient of production declirle curve. (7) As mentioned above, results obtained from the study on production decline curve provide knowledge relating to physical properties of oil reservoirs which can not be obtained otherwise: Therefore, this study provide valuable data for economical exploitation of oil field, especially for problem of well spacing and for secondary recovery.