the types of reservoirs in which the vertical flooding techniques are usually carried out are either thick reservoirs or steeply dipping reservoirs where the vertical thickness is fairly large . the miscible fluid to be injected into the top of this reservoir may be any fluid which is partially or totally miscible with the reservoir oil and less dense than the reservoir oil . for example , propane , butane and naphtha or mixtures of these are suitable . the temperature of the fluid must be such that when the fluid reaches the reservoir its temperature is equal to or greater than the temperature of the reservoir fluids . therefore , it follows that since heat will be lost as the fluid is being injected , the temperature of the fluid at the surface will always be required to be greater than that needed at reservoir depth . how much greater depends on the depth of the reservoir and other factors that will cause the fluid to lose thermal energy as it is being injected . it is within the knowledge of one skilled in the art to determine the proper surface temperature of the miscible fluid to achieve a desired temperature at reservoir depth . when applied to an actual production situation the slug of miscible fluid must be followed by gas or other fluid miscible with the slug . this is necessary because most miscible fluids are too expensive to be used except as a slug . it is apparent that to minimize the convective currents on the trailing edge of the slug or miscible fluid , it will be necessary to have the following gas at a temperature equal to or greater than the slug or miscible fluid . this will maintain the integrity of the slug at the trailing edge . fig1 illustrates convective mixing of reservoir oil and a slug of miscible fluid colder than the reservoir oil . the cold fluid 1 is pumped into a well 2 which penetrates and is in communication with the oil reservoir 5 . the cold fluid is pumped into the formation through openings 4 , in the well . a slug of miscible fluid 3 is built up at the top of the oil reservoir . however , at the interface 6 between the miscible fluid and the oil , convective currents , as depicted by the arrows , mix the miscible fluid and the oil . if not checked these convective cuurents will destroy the interface between the miscible fluid and the oil and the miscible slug will be absorbed into the oil and lose its displacing properties . fig2 illustrates the process of my invention where the corresponding elements are numbered as in fig1 except that there the miscible fluid 1 is at a temperature equal to or greater than the reservoir temperature when it reaches the reservoir ; so that the convective currents in fig1 are absent and the miscible fluid will not be prone to mix with the oil . this example will demonstrate the effect of temperature gradients in the reservoir on the injection of a typical miscible slug followed by natural gas to displace the slug through the reservoir . ______________________________________assumed fluid properties solvent reservoir gas slug liquid______________________________________density at 2145 psia - 167 ° f ( lbs / ft . sup . 3 ) 8 . 67 23 . 56 43 . 00molecular weight 21 . 9 37 . 3 105 . 6______________________________________ the following is a table of enthalpy for methane through butane . ______________________________________ molecular enthalpy ( btu / lb ) weight 170 ° f 70 ° f δh______________________________________methane 16 358 280 78ethane 30 248 185 63propane 44 225 150 75butane 52 205 140 65 average 70______________________________________ both the slug and gas usually have average molecular weights in the range between methane and propane . the change in heat content does not vary widely between the above hydrocarbons . aδh of 70 btu / lb was used in the following calculations of heat removed from the formation by the injection of fluids at 70 ° f . assume a hypothetical reservoir containing about 400 × 10 6 bbls . of stock tank oil and a solvent slug of 30 × 10 6 reservoir barrels will be injected followed by 300 × 10 9 scf of gas . the solvent is injected into six wells and the gas into seven wells . solvent slug______________________________________injection volume = 30 × 10 . sup . 6 reservoir barrels h bbl ft . sup . 3 / bbl lbs / ft . sup . 3 btu / lbtotal heat = ( 30 × 10 . sup . 6 ) ( 5 . 61 ) ( 2 . 36 × 10 ) ( 7 × 10 ) = 279 × 10 . sup . 9 btu______________________________________ ## equ1 ## heat removed from formation______________________________________ heat ( 10 . sup . 9 btu ) fluid total no . wells per well______________________________________slug 279 6 46 . 5residue gas 1 , 210 7 173 . 0slug plus residue gas 1 , 489 7 212 . 5______________________________________ in order to obtain an estimate of the volume of the reservoir that would be affected by the injection of colder fluids , the following calculations were made . it was assumed that ( 1 ) the overall specific heat of the formation was 0 . 2 btu / lb /° f , ( 2 ) the overall density of the formation was 2 . 93 g / cc or 183 lbs / ft 3 , and ( 3 ) the volume of the formation that is affected is cooled from 170 ° to 70 ° f , the remainder of the formation remaining at 170 ° f . ## equ2 ## sphere diameter ( ft ) volumes per well ( each well ) ______________________________________slug 12 . 6 × 10 . sup . 6 ft . sup . 3 286gas 47 . 1 × 10 . sup . 6 ft . sup . 3 447slug plus gas 58 . 0 × 10 . sup . 6 ft . sup . 3 480______________________________________ sphere diameters were calculated for the above volumes which would be for the hypothetical case of uniform flow of injected fluids into the formation in all directions . these diameters are listed above . the radius from each well ( 6 ) that would be affected by the slug only , having a uniform thickness of 30 feet , based on the above assumptions , is 365 feet or a diameter of 730 feet . this establishes a high temperature gradient across 440 , 000 square feet of contact between the slug and the reservoir oil , resulting in convective mixing of the two fluids and loss of slug identity . injection of the fluids heated to at least reservoir temperature would remove this deleterious effect .