harmed appreciably by drilling fluids undoubtedly accounts for the large 

 amount of oil and gas production obtained in the past. 



It has been recognized during the past decade that a great amount of oil 

 and gas production may have been overlooked or condemned in the past because 

 of the injury to the pay zones by the drilling fluid used to pentrate them. For- 

 mations that are damaged easily by drilling fluids are usually termed sensitive 

 formations. These formations usually have relatively low permeabilities and 

 contain much shale, clay, and silt. 



Permeability impairment may result from one or more of several causes: 

 (1) if a drilling fluid is used that permits much water to filter into the pay 

 formation, clays present in the formation may swell and cause reduced perme- 

 ability; (2) water filtrates may weaken adhesion bonds between small particles 

 in the interstices within the formation and permit these particles to migrate 

 and plug flow channels at constrictive points; (3) water filtrates may collect as 

 globules in the interstices to plug flow channels at constrictive points because of 

 interfacial effects; (4) water filtrates may contain soluble salts that will react 

 with soluble salts in the interstitial water to precipitate solids or form gels that 

 will plug flow channels at constrictive points and injure the permeability of the 

 formation; and (5) solids from the drilling fluid may penetrate the formation 

 and cause some permanent reduction of permeability to oil. 



Drilling fluids that filter oil into producing formations will overcome many 

 of the difficulties caused by water filtrates but sometimes may not completely 

 prevent damage to the producing formation. Oil filtrates will not permit the 

 swelling of interstitial clays, but they may weaken adhesion bonds between small 

 particles in the interstices within the formation and permit them to migrate and 

 plug flow channels at constrictive points. Oil filtrates will eliminate the pos- 

 sibility of water blocking oil-wet formations, but constituents of some oil filtrates 

 may react with soluble materials in the formation oil to form precipitates that re- 

 duce permeability. Solid particles from oil-base muds, especially when weighted, 

 may penetrate the formation during the initial stages of filtration before a filter 

 cake is deposited, plug the flow channels in the producing formation at con- 

 strictive points, and reduce permeability. 



No single drilling fluid will protect all pay formations from every possible 

 cause of sensitiveness. Several preventive measures, however, can be used until 

 the specific causes are discovered and the correct drilling fluid is designed. 

 Because solid particles from the drilling fluid must penetrate the pay formation 

 before they can cause plugging, such penetration by these solid particles can be 

 kept at a minimum by maintaining in the drilling fluid a uniform distribution 

 of particle sizes capable of plugging the extreme range of expected pore sizes 

 in the pay formations. The differential pressure between the circulating drilling 

 fluid and the fluid in the pay formations should be maintained as low as possible 

 to minimize the tendency of the drilling fluid to spurt into the pay formation 



721 



