No abstract

This paper presents an overview of the work carried out to date on the dynamic and static filtration characteristics of drilling muds. Factors controlling fluid loss to the formation that have been investigated include annular velocity, fluid temperature, hole angle, shear rate and the effect of the filter media. The effect of pressure on static fluid loss and filter cake permeability is also reported. Fluid loss during sequential filtration has also been evaluated. The fluids used comprised a calcium chloride brine with fluid loss additive and two water based muds. The filter media consisted of synthetic core material and a natural sandstone. Conventional photographic techniques have been used to identify filter cake response to backflushing. Introduction The occurrence of a change in permeability of the formation near the wellbore is most frequently related to the effects of the invasion associated with borehole filtration. As filtration proceeds the filtrate, with its accompanying fine particles, creates an invaded zone. Within that zone the productivity may be reduced by physical and/or chemical change to the formation rock or fluids and this impairment is commonly referred to as formation damage. Some appreciation of the complexity of this process can be obtained by considering the range of variables which could affect fluid loss in a borehole. Table 1 attempts to classify the more significant parameters into the following categories: parameters into the following categories:rock propertiesproperties of the wellbore fluidphysical conditions in the wellbore From the number of mechanisms shown it is clear that the control of formation damage is very difficult. It is likely that more that one mechanism may be operating at any one time and, since formations are not homogeneous, there will be variations in the extent to which these individual mechanisms occur. Limiting the extent of filtrate invasion is important and over the past 3 decades or so a great deal of time and effort has been spent on this past 3 decades or so a great deal of time and effort has been spent on this problem. Among the reasons for attempting to quantify and reduce the problem. Among the reasons for attempting to quantify and reduce the volume of mud filtrate are the following:invasion of filtrate with associated fine particles may create a zone of reduced permeability around the wellbore which may not respond to backflushing and results in lower production rates than anticipated.filtrate that penetrated shale sections may cause swelling and subsequent sloughing into the wellbore. Stuck pipe may then be a problem.the location of a zone of mud filtrate around the wellbore will affect the response of electrical logging tools. Correct interpretation of the logs requires accurate knowledge as to the extent of this zone.knowledge of the depth of invasion is required for the determination of perforation depths and possibly for the selection of stimulation treatment. Utilising muds with improved filtration control characteristics provides an efficient method of limiting the extent of the invaded zone. provides an efficient method of limiting the extent of the invaded zone. However it is impossible to eliminate fluid loss entirely since the formation of a low permeability filter cake necessarily involves invasion. Therefore attempts to minimise fluid loss must be accompanied by obtaining information as to the permanency of damage and possible techniques for its removal. p. 283