SPE 58738
A Laboratory Drilling Mud Overbalance Formation Damage Study Utilising Cryogenic SEM Techniques
A.J. Twynam, SPE, BP Amoco, and M.T. Byrne, SPE, I.S. Spark, SPE, and I.T.M. Patey, Corex (UK) Ltd.
Abstract
Laboratory reservoir conditions flood tests were used to determine the efficiency of clean up of a drilling mud system which was applied at two different overbalance pressures. The test was designed to simulate the drilling operation downhole. Relevant core material was used and the pore/overburden pressures, mud overbalance pressures, temperature and production drawdown pressures/rates applicable to field conditions were applied in the laboratory simulation. Drilling mud filtrate loss versus time measurements were made at reservoir conditions using sophisticated pumps. All wetted parts of the rig were constructed of hastelloy C276 alloy to avoid corrosion artifacts interfering with the test. Permeability measurements were made prior to mud application and again at the end of the flood test analysis to determine the level of any formation damage. In order to determine the nature of the formation damage mechanisms, geological techniques which included dry and cryogenic SEM and thin section analyses were undertaken. These techniques were applied to the plug trims prior to testing (untreated samples) to determine the natural state of the rock, paying particular attention to the clay mineral and cement types, morphologies and distributions. The plug samples were examined at the end of testing (treated samples) and any variations noted which would highlight the damage mechanisms. The dry SEM and thin section analyses were used to determine solid damage mechanisms such as clay fines migration, scale precipitation and drilling mud solids invasion. The cryogenic SEM analyses were used to examine any fluid damage mechanisms such as wettablity alteration, microemulsion formation, water/oil blocks or asphaltene deposits. A new technique of cryogenic SEM using EDX (Energy Dispersive X-Ray) analysis was used for X-ray mapping of the remnant mud cakes to determine solid and fluid distributions.
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