Abstract This paper discusses the results obtained from high temperature, highpressure filter loss studies in which field samples of clay-water emulsion andoil base fluids were used. High temperature-high pressure tests of some premiumpriced emulsion and oil base drilling fluids show filter loss peculiaritiesthat are not predicted by standard API tests. It is recommended that hightemperature-high pressure filter loss tests be used to evaluate the performanceof such fluids. Apparatus is described which proved to be satisfactory for evaluating filterloss behavior over a wide range of temperatures and pressures. Introduction The petroleum industry spends large sums of money each year on chemicaltreating agents for lowering filter loss and on premium-priced low filter lossdrilling fluids. While it is an accepted fact that low filter loss is advantageous duringdrilling operations, it is questionable whether the present standard method ofdetermining filter loss gives a reliable indication of the loss to be expectedunder bottom hole conditions. The purpose of this paper is to show that hightemperature, high pressure filter loss tests should be used to evaluate filterloss behavior of fluids for deep drilling. Concern over possible effects of filter loss on oil well drilling and wellproductivity dates back to the early 1920's. During the years 1922 to 1924, filtration studies were reported by Knapp, Anderson and Kirwan. These studieswere the first to be reported in the literature on this subject. No furtherinformation was published on the subject until 1932 when Rubel presented apaper in which he discussed the effect of drilling fluids on oil wellproductivity. In 1935, Jones and Babson constructed the first laboratory tester designedto study the effects of temperature and pressure on the filter loss behavior ofclay-water drilling fluids. In a discussion of their investigations, Jones andBabson stated. "Performance characteristics of a mud can be evaluated withconsiderable reliability by a single test at 2,000 psi and 200'F. Exactcorrelation between the results of performance tests made under theseconditions and the behavior of muds in actual drilling operations is of courseimpossible." Jones and Babson apparently were well aware that at bestlaboratory tests can give only qualitative answers to the question of what isthe actual behavior of a drilling fluid when subjected to deep drillingconditions. Jones presented a paper in 1937 in which he described a staticfilter loss tester to be used for routine filter loss tests. This instrumentsubsequently was adopted as the standard API filter loss tester. T.P. 3323