while the invention will be described in connection with a preferred embodiment , it will be understood that it is not intended to limit the invention to that embodiment . on the contrary , it is intended to cover all alternatives , modifications and equivalents as may be included within the spirit and scope of the invention defined by the appended claims . the liquid phase of the annular fluid is a non - aqueous , non - corrosive , environmentally acceptable fluid . as utilized herein , an environmentally acceptable fluid is a liquid which meets u . s . environmental protection agency requirements ( ex . sheen and bioassay ) for discharge into bodies of water and tributaries . the liquid is preferably selected from the group consisting of esters , polyalpha olefins , ethers , food grade paraffins linear alpha - olefins and mixtures thereof . in a preferred embodiment , the liquid is an ester oil of vegetable or animal origin . u . s . pat . nos . 5 , 106 , 516 and 5 , 232 , 910 , herein incorporated by reference , provide a detailed description of a particularly suitable ester . specific examples of commercially available suitable liquids are baroid corporation &# 39 ; s petrofree , m - i drilling fluid co .&# 39 ; s novasol liquid , baker - hughes inteq &# 39 ; s synteq and anchor drilling fluids aquamul liquid . the clay gellant of the invention is preferably a hydrophilic clay . more preferably , the clay is an organophillic clay . clays which are particularly preferred in this invention are the water - swelling smectite clays such as montmorillonite , hectorite , saponite and particularly bentonite . smectite - type clays prepared synthetically or by either a pneumatolytic or hydrothermal synthesis process can also be used . preferably , the clay has an average particle size between about 0 . 1 and 6 . 0 microns . among the particularly preferred organophillic clays vg - 69 from m - i drilling fluids co ., geltone ii from baroid corporation , and carbogel from baker - hughes inteq are particularly suitable . preferably the clay comprises between about 1 and about 7 percent by volume of the fluid . more preferably , the clay comprises between about 1 . 5 and 3 . 5 percent by volume of the fluid . in a commercial operation in which barrels are the typical units of measure , the clay preferably comprises between about 10 pounds and about 40 pounds per barrel of liquid . more preferably , the clay comprises between about 15 pounds and about 30 pounds per barrel of liquid . the clay dispersant of the invention can be any polar organic material well known to those skilled in the art . such dispersants are generally low molecular weight , polar organic compounds , such as alcohols , ketones , diketones , nitryl alkines , alkine carbonates , ether alcohols , ether esters , and the like . representative dispersants of the preferred invention are methanol , ethanol , acetone , methyl ethyl ketone , acetenyl acetone , acetophenone , diethyl ketone , methyl - n - propyl ketone , 2 , 3 - butane dione , nitro methane , nitro ethane , 1 - nitro propane , propylene carbonate , ethylene carbonate , ethylene glycol monomethyl ether , diethylene glycol monomethyl ether , ethylene glycol monoacetate , diethylene glycol monoacetate , and similar molecular weight polar organic compounds . commercially available methanol , due to its low cost , is a particularly preferred polar dispersant . preferably , the dispersant comprises between about 2 and about 6 volume percent of the fluid . more preferably , the polar dispersant comprises about 4 volume percent of the fluid . in a commercial operation in which barrels are the typical unit of measure , the dispersant comprises between about 1 / 2 gallon and about 2 1 / 2 gallons per barrel of liquid . preferably , the apparent viscosity of the inventive fluid ranges from 40 to about 150 centipoise . this is primarily controlled by the concentration of organophillic clay and dispersant . as is known to those skilled in the art , a weighting agent , such as barite may be added to the fluid to increase the fluid &# 39 ; s density . in the event barite is added , emulsifiers , surfactants and brine may also be added to the fluid . an exemplary manner of preparing a preferred composition of the annular fluid on a per barrel basis is to mix about 20 pounds of the organophillic clay into one barrel of the environmentally acceptable fluid . preferably , mixing is continued for about 30 minutes . thereafter , about 1 . 68 gallons of the polar dispersant is slowly added to the composition and mixing is continued for an additional 30 minutes . the mixed fluid can then be placed in the wellbore annulus according to any method currently used in the art . in order to more completely describe the present invention , the following examples are given . the examples are offered to more fully illustrate the practice of this invention but are not intended to limit the invention . 350 ml . samples of baroid petrofree , clay , and methanol ( according to the quantities depicted in table # 1 ) were mixed together at a moderate speed on a hamilton beach mixer . a variety of rheological properties ( table # 1 ) were evaluated for each sample at room temperature ( 75 ° f .) using a fann 35 vg meter . table 1______________________________________initial rheologies at 75 ° f . sample # 1 2 3 4 5 6 7 8 9______________________________________geltone ii , grams 20 20 20 -- -- -- -- -- -- m - i vg - 69 , grams -- -- -- 20 20 20 -- -- -- carbogel , grams -- -- -- -- -- -- 20 20 20methanol , % 2 4 6 2 4 6 2 4 6a . v ., cps 31 35 33 . 5 14 65 48 60 * * p . v ., cps 13 17 19 12 21 17 26 * * y . p . 36 36 29 4 88 62 70 * * gels 27 27 23 3 54 37 48 * * ______________________________________ * too high to measure the samples were then hot rolled using an ofi hot - rolling oven for sixteen hours at 150 ° f . thereafter , the samples were cooled to room temperature , re - mixed for five minutes at a moderate shear on a hamilton beach mixer and their rheological properties re - evaluated ( table # 2 ). table 2______________________________________rheologies after hot rolling at 150 ° f . sample # 1 2 3 4 5 6 7 8 9______________________________________a . v ., cps 50 51 43 19 85 78 46 146 1295p . v ., cps 21 23 23 14 20 22 28 68 43y . p . 58 56 40 10 130 111 36 156 173gels 44 39 24 9 80 67 24 124 118______________________________________ table # 3 demonstrates the results of additional fluid compositions prepared as described above with rheological measurements taken at 120 ° f . table 3______________________________________initial rheologies at 120 ° f . sample # 10 11 12 13 14 15______________________________________m - i vg - 69 grams 20 -- -- -- -- -- carbogel , grams -- 15 15 17 . 5 20 20methenol , % 4 4 6 4 4 6a . v ., cps 67 . 5 33 . 5 46 . 5 124 137 . 5 106 . 5p . v ., cps 15 14 15 23 22 21y . p . 105 29 63 202 231 171gels 70 22 36 129 156 121______________________________________ samples 10 , 13 , and 14 , which demonstrated excellent apparent viscosities of greater than 50 centipoise , were static - aged for sixteen hours at 190 ° f . and their rheological properties re - evaluated at 120 ° f . ( table # 4 ). thereafter , the samples were cooled to room temperature and the presence of free oil ( separation %) was determined by visual inspection . table 4______________________________________rheologies after static - aging for 16 hrs at 190 ° f . sample # 10 13 14______________________________________a . v ., cps 45 52 51p . v ., cps 14 14 12y . p . 62 76 78gels 40 53 52separation , % 0 0 0______________________________________ in order to determine the amount of long - term separation that occurs , five samples (# 16 -# 20 ) prepared in a manner consistent with the prior samples were evaluated at various time intervals of heating at 190 ° f . each formulation was placed in a test tube ( 15 cm . tall with a 1 . 75 cm . diameter ) and one test tube of each of the five formulations placed in three separate 500 ml . baroid static - aging cells . after pressurizing to 100 psi with nitrogen , the static - aging cells were placed in an oven at 190 ° f . a first static - aging cell was removed after 16 hours , a second cell after 40 hours , and a third after 112 hours . after each cell was cooled and opened , visual observations were made to determine if free oil had separated from the sample ( table # 5 ). table 5______________________________________fluid / solids separation testssample # 16 17 18 19 20______________________________________m - i vg - 69 , grams 20 -- -- -- -- carbogel , grams -- 15 15 20 20methanol , % 4 4 6 4 6free oil visible after : 16 hrs @ 190 ° f . no no no no no40 hrs @ 190 ° f . no no no no no112 hrs @ 190 ° f . no yes yes yes yes______________________________________ a wellbore model was designed and constructed to evaluate the thermal insulating properties of fluids in an annular space . the model was constructed of a five foot section of 2 7 / 8 &# 34 ; tubing inserted in a 9 5 / 8 &# 34 ; fiberglass casing to create a 3 . 25 &# 34 ; annular section between tubing and casing . two resistance temperature detectors were placed one foot from each end of the model , two at the outside surface of the tubing , and two at the interior wall of the casing . for the test , approximately 20 gallons of insulating fluid having the relative composition of sample # 10 were prepared in a 50 gallon mixing tank . this insulating fluid was then pumped into the annular space of the wellbore model . with this configuration , the temperature differential across the annulus was measured . water was heated to 200 ° f . and circulated at a flow rate of 5 . 0 - 5 . 5 gallons per minute through the tubing . with the heated water circulating through the tubing , the temperature on the outside of the tubing ( top and bottom ) and the inside of the 9 5 / 8 &# 34 ; casing ( top and bottom ) was recorded at five minute intervals . using these recorded temperatures , the temperature differential across the annular space was calculated . fig1 demonstrates a 24 hour test cycle with fluid heated and circulated for eight hours and additional circulation of the water for 16 hours without heating . a preferred packer fluid formulation was then evaluated for toxicity by performing a bioassay test . the tested fluid was the composition of sample # 10 . test results indicated that the packer fluid formulation was environmentally acceptable for offshore use , having a 48 hour rangefinder lc 50 of greater than 300 , 000 parts per million . the environmental protection agency ( epa ) mandates that any fluids being discharged in u . s . off - shore waters must be tested for toxicity by running a mysid shrimp bioassay test for 96 hours . any fluid having a lc 50 ( lethal concentration to 50 % of the test shrimp ) of less than 30 , 000 ppm fails the test and cannot be discharged into u . s . waters . it is shown by the above results , that the subject invention demonstrates the viscosity , thermal insulation properties , stability , and environmental compatibility necessary to function as a superior packer fluid . thus , it is apparent that there has been provided , in accordance with the invention , a packer fluid that fully satisfies the objects , aims , and advantages set forth above . while the invention has been described in conjunction with specific embodiments thereof , it is evident that many alternatives , modifications and variations will be apparent to those skilled in the art in light of the foregoing description . accordingly , it is intended to embrace all such alternatives , modifications and variations that fall within the spirit and broad scope of the appended claims .