Patent GB-1256537-A (GB)
IPC: C09K8/36
Filed: 1968-12-06
Abstract: 1,256,537. Boring earth; wells. ESSO PRODUCTION RESEARCH CO. 6 Dec., 1968, No. 58110/68. Heading E1F. [Also in Division G1] A method of determining the suitability of a fluid containing water for use in a well passing through a water sensitive formation comprises determining the direction of water migration between the fluid and a sample of the formation. Damage to shale or similar argillaceous formations is prevented by using a treatment or drilling fluid chosen so that little or no water passes from the fluid to the formation. A sample of the formation in its in situ state of hydration and temperature is immersed in the fluid. Changes in the physical properties of the sample which are caused by water absorption may be measured to give an indication of water content of the sample. Such properties include weight, sonic velocity, resistivity, compressive strength, and dimensions. The change in length of a sample 34 immersed in fluid retained by a cylinder 29 bends a beam 22 mounting strain gauges 25..28 which form resistors in the bridge shown in Fig. 3. Initially a variable resistor 32' is adjusted to zero a voltmeter 44 so that subsequent readings of the voltmeter give an indication of the amount the sample has lengthened or shrunk and whether water flows from the fluid to the sample or from the sample to the fluid. In one method of selecting a drilling fluid, samples of the formation are respectively placed in oil based drilling fluids having different vapour pressures, and the sample immersed in fluid having no or little absorption is the most suitable for drilling the formation. The vapour pressure of the fluids depend on the concentration of inorganic salts in the fluid. In a second method of selecting a drilling fluid, formation samples are suspended above different saturated salt solutions having known vapour pressures until equilibrium is reached. The vapour pressure of the solution above which is suspended the sample which does not change in weight is the vapour pressure of the formation. An oil based drilling fluid having this vapour pressure is suitable for drilling the formation. The drilling fluids comprise diesel fuels, kerosenes and crude oil; and 10-70% of common salt water and an emulsifier. Na Cl, Ca Cl, Zn Br 2 , Zn Cl 2 , Li Br, Li Cl may be added to commercially available emulsion drilling fluids to depress the vapour pressures. The drilling fluid is monitored during forming the hole by taking samples of the successive formations penetrated and of the fluid in contact with these formations. During monitoring synthetic samples may be used or samples may be dispensed with and the structure of cuttings observed. Water damaged formations are treated with oil-in-water emulsions having sufficiently low vapour pressure to attract water from the formation. Displacing fluids comprising emulsions or micro-emulsions formed from soluble oil have the vapour pressured of their aqueous phases controlled to be below the formation vapour pressure to prevent damage to the formation. The invention is applied to water-in-oil emulsions used as fracturing fluids, and where the fluid is lubricated down the well surrounded by an annulus of water it is desirable to add the vapour pressure depressant to both the emulsion and the water since these form a further emulsion during passage through the casing perforations.