Evaluation of Drilling Fluids Damage using a Closed Loop Dynamic Slurry Circulation, System
Authors: Lynn Jack D. 
Published: December 2007

Jack D. Lynn*
The effects of drilling fluid type and composition on the reservoir rock have traditionally been difficult to analyze since the dynamics of the drilling system often cannot be suitability duplicated in the laboratory environment. This paper will present a test apparatus and procedure used to evaluate permeability damage caused by drilling fluids in the near-well bore region of the reservoir. The closed loop design allows for evaluation of the drilling fluid in a dynamic system, rather than as a static test. Flow velocity across the core face can be controlled, resulting in close simulation of shear effects. The system can be modified to allow for the examination of the combined effects of various drilling. completion, and stimulation fluids used during drilling and production operations. The pumps used for slurry circulation are progressing cavity type pumps, which allow the circulation of slurries with large particle sizes (up to 2000 microns), without damaging the particles. The system can be operated at temperatures of 250 F, hydrostatic confining pressures of 10,000 psig, and overbalance pressures of up to 500 psig. Reservoir crude oil was used for the evaluation of return permeabilities, but brines could be substituted to simulate drilling of non-oil producing reservoirs.
The system was evaluated using test data obtained during the design of drilling and completion fluids for Saudi Arabian fields. The preparation of these cores for testing provided special problems due to the poorly consolidated nature of some of the cores. Lucite encapsulation was employed to prevent sample failure. A series of five formulations was investigated, ranging from a standard bentonite based fresh water mud, to an oil based drilling fluid. The test apparatus was found to be well suited for the acquisition of the data. The results of the study proved the following:The apparatus was effective at comparative testing and evaluation of drilling and completion fluids.The circulation system was able to circulate slurries containing large micron size bridging solids without allowing settling of the particles.Stable drilling fluid rheology was maintained by the circulation system.The device proved to be effective for the evaluation of drilling, clean-up. and completion fluids in a laboratory environment.Reproducibility of the test procedure proved to be excellent, as shown by duplicate testing of selected fluids.It was found that the device could reproduce several damage effects common to field operations, including solids invasion and fines mobilization through excessive production rates.

Introduction
The apparatus was developed in response to operational requirements to evaluate drilling fluids under near reservoir conditions, before going on to field trials with un-proven fluids. Five formulations were investigated. The material ranged from a standard bentonite based fresh water mud to an oil based fluid. The drilling fluid formulations, and their respective properties are presented in Table 1.
The samples were selected from two Saudi Arabian wells, CA-1 and CA-2. The sample selection was based on the porosity/permeability data from the routine core analysis. The core material had not been preserved after drilling of the routine core plugs, and was damaged by drying. In addition, one core (CA-1) had been drilled using a non-inhibited drilling fluid, again providing the potential for damage to the core material before testing. The core from the second well had been drilled with a 15 pound/barrel KCl water based mud. This mud should have provided some protection to the clay materials, but questions concerning the wettability of the two sample sets were considered during interpretation of the results.
The samples were drilled from the core material using a 30,000 ppm KCl brine to prevent clay degradation and further damage to the core material. Samples were drilled in a horizontal orientation to simulate well flow. Samples with diameters of 1.0 and 1.25 inches were drilled for the testing.
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