Drilling fluids utilized in horizontal wells must be as non-damaging as possible. In vertical wellbores, localized damage induced by invading drilling fluids and solids can often be bypassed through cementing and perforating. Due to the high cost associated with perforating horizontal wells, cleanup must be achieved with wellbore flow Or chemical stimulation techniques. With long horizontal sections, reservoir drawdown may not be sufficient to remove formation damage. To this end, stimulation attempts can be costly, and in some cases, ineffective. If damage is severe, productivity may be at an uneconomic level and a viable drilling play could subsequently be abandoned. By conducting core displacement tests with various drilling fluids on representative reservoir samples, the least damaging drilling fluid can be selected. Proper testing and core preparation procedures must be followed to ensure that results are representative. This paper will outline core displacement test methodology and present the results of several core displacement studies. The findings are substantiated with case histories of production data. These results illustrate that although damage due to drilling cannot be eliminated, it can be minimized. Introduction Formation damage is defined as any type of a process which results in a reduction of the flow capacity of an oil-. water- or gas-bearing formation. Formation damage has long been recognized as a source of serious productivity reductions in many oil anti gas reservoirs and as a cause of water injectivity problems in many waterflood projects (Bennion, 1991). Introduction Formation damage can occur whenever non-equilbrium or solid bearing fluids enter a reservoir, or when equilibrium fluids are displaced at extreme velocities. Thus, many processes used to drill, complete or stimulate reservoirs have the potential to cause formation damage. Some of these processes might include:DrillingCementingCompletions/Stimulation erforating acidizing fracturing Workovers kill fluids hot oil treatments Waterflooding or water disposal Ehanced oil recovery Enhanced oil recovery processes miscible flooding chemical flooding thermal flooding (in situ combustion/steamflooding) Excessive injection or production rates Horizontal wells are much more susceptible to damage than their vertical counterparts due to a number of reasons, these being:Substantially longer contact time with the drilling fluid. In a vertical well, drilling fluid may only be in contact with the pay zone a matter of hours while in a horizontal well, the time may be measured in weeks.Most horizontal wells are not cased and perforated and remain as open hole completions. Relatively shallow damage, which would be easily perforated through on a standard conventionally-cased vertical well, remains a major source of permeability reduction in many horizontal wells.Uniform drawdowns are difficult to obtain on horizontal wells due to the length of the well in the pay zone. This makes it much more difficult to clean up damage due to invaded fluid and/or solids except in selected zones.The physical mechanics of flow into horizontal wells are substantially different from vertical wells due to the fact that the vertical and horizontal permabilities in most formations differ.