FACTORS INFLUENCING Many factors influence the relative amounts 



AMOUNTS OF OIL AND of oil and gas in the drilling mud and cuttings 



GAS IN CUTTINGS when an oil-bearing formation is penetrated. 



The following are the most important: (1) 

 flushing or water-flooding action of the mud filtering at or ahead of the bit; 

 (2) pressure balance between the mud column and the formation; (3) factors 

 related to the type of porosity, shape of pore space, and permeability of the 

 formation; (4) rate of drilling; (5) rate of mud circulation ; and (6) character- 

 istics of the drilling mud such as density or weight, filtration properties, and 

 viscosity and gel properties. 



The following brief discussion illustrates how these factors influence the 

 amount of oil and gas in the mud and what happens to the formation fluids 

 near the drilling bit as an oil-bearing formation is penetrated. 



In the first place, the fluid pressure asserted by the drilling mud is normally 

 greater than the fluid pressure in the formation. There is a tendency, therefore, 

 for the drilling mud to be forced back into the exposed surfaces of the forma- 

 tion. Solid particles of the mud are filtered out and deposited on the surface 

 of the formation as a filter cake while water from the drilling mud penetrates 

 the pores of the permeable formation. 



The laws governing the deposition of filter cakes and rate of filtration of 

 drilling mud are believed to be the same as the general laws of filtration, which 

 have been well established in engineering practice. The rate of filtration at 

 any time after the start of the filtration process is inversely proportional to the 

 square root of time, or: 



filtration rate = a constant -=- square root of time 



When the bit rotates on the bottom of the hole, it either chips off the 

 formation on which the previous mud cake has been deposited or scrapes off 

 the mud cake and re-exposes the formation so that the conditions prevalent at 

 the bottom of the hole are such that the mud filtration rate remains relatively 

 high because the cake is being removed or rapidly disturbed. Even with muds 

 having the best wall-building properties, there are ample possibilities for 

 filtration to occur ahead of the bit. 



Just below the bit, formation water flooding occurs and reduces the oil 

 and gas content of the formation but does not remove the oil and gas completely, 

 in much the same manner as secondary recovery. Some of the oil and gas is 

 held by the capillary forces active in the formation pore space. The bit chips off 

 particles of the formation; the cuttings join the mud stream and are normally 

 carried to the surface. At the same time, the gradual reduction of pressure on 

 the fluids in the formation pores allows expansion of the fluids in the pores. 

 The gas expands greatly and pushes some of the oil and water out of the 



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