Filtration Behavior of Circulating Drilling Fluids

Abstract

Abstract An investigation was carried out on the dynamic filtration behavior of drilling fluids. In a set-up consisting of a porous pipe through which the drilling mud was circulated, dynamic filtration rates and cake thicknesses were determined as a function of flow rate and filtration time. Eventually, a state of equilibrium was reached; i.e., both cake thickness and filtration rate attained constant values. Concurrently, dynamic filtration tests were carried out in a simple dynamic filter apparatus where mud flow was similar to that in a rotational viscometer. In both arrangements the relation between equilibrium filtration data and rate of shear at the cake surface was studied. Filter pipe and dynamic filter apparatus were found to give identical relations. It was therefore concluded that the latter apparatus, which is suitable for routine measurements on small amounts of mud, could be used to determine the dynamic filtration behavior of mud under borehole flow conditions. Some data are presented on the effect of filtration pressure and temperature on equilibrium filtration rate and cake thickness. INTRODUCTION In oil well drilling, the drilling fluid used generally forms a mud cake on the permeable sections of the borehole wall while filtrate is lost into the formation. As to the conditions under which this filtration process takes place, a distinction should be made between static mud filtration (mud is not circulating) and dynamic mud filtration (mud is circulating). In recent years several investigators1-5 have studied the process of dynamic mud filtration about which little had previously been known. This type of filtration is the subject of this paper which confirms several findings of previous investigators. It differs from already published works, however, in the following respects.It contains a description of a simple laboratory apparatus developed for dynamic filtration measurements on small amounts of mud.The effect of rate of shear at the cake surface on dynamic filtration has been studied. As a result of this study, dynamic filtration data pertaining to the borehole conditions of mud flow can be found from laboratory tests.Some data are presented on the effect of filtration pressure and temperature on dynamic filtration. The work was limited to a small number of muds. No attempt was made to compare data on API filtrate loss with dynamic filtration rates. Previous workers have clearly shown that no correlation exists between static and dynamic filtration rates. EXPERIMENTAL ARRANGEMENT To establish whether the effect of mud flow conditions on dynamic filtrations is governed by the rate of shear at the cake surface, two different types of mud flow were used. In this connection the experiments were partially conducted by means of a porous pipe through which mud was circulated, partly in a dynamic filter apparatus which somewhat resembles a rotational viscometer. In both cases the flow conditions of the mud were well defined, thus making it possible to calculate rates of shear at the cake surface. The filter pipe was 200 cm long with an internal diameter of 3 cm. Its perforated wall was covered on the inside with filter cloth on which the mud cake could form. Mud was pumped through the pipe at linear rates varying from 20 to 186 cm/sec. The required filtration pressure was obtained by means of a back-pressure valve downstream of the filter pipe. In this arrangement the type of flow existing in the borehole is reasonably well simulated. The dynamic filter apparatus (Figs. 1 and 2) consists mainly of two concentric cylinders with a clearance of about 0.5 cm. The outer cylinder (rotor) can be rotated at rates up to 1,000 rpm. The outer surface of the inner cylinder (radius about 2.5 cm) is covered with filter cloth or paper supported by copper gauze.