BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
throat) and thus reduce the permeability of the rock. Mobilization, migration, and retention of clays
and other fine particles have been recognized as the major formation damaging factors in sandstone
formations. If these fines are trapped near the wellbore, very significant production losses occur
because of the reduction in the number of alternative flow paths into the borehole. The prevention
of formation damage by clay swelling and fines migration requires detailed information about the
composition of the producing formation and expert planning based on the analysis of the
information gathered.
A secondary source of solids that can cause formation damage is the precipitation from solution of
insoluble salts or colloidally suspended particulates created when completion or workover fluids
contact incompatible connate water. For example, when calcium brines come in contact with
connate water containing CO 2 , insoluble calcium carbonate (CaCO 3 ) may precipitate and plug
near-well-bore pore spaces. Whenever high-density calcium and/or zinc brines are used, formation
of salt precipitates is possible when the connate water contains significant amounts of the anions
2− 2− 2−
3−
−
CO
3
, SO
4 , S , PO
4
, F , or dissolved NaCl. NaCl precipitates because it is less soluble that
the corresponding calcium or zinc chlorides or bromides; this precipitation is a problem if it occurs
in tight formations where it may be difficult to contact with fresh water to cause dissolution. Also,
because of their physical and chemical properties, heavy brines or oil-based drilling fluids filtrates
may cause the precipitation of asphaltenes and paraffins from the in-situ hydrocarbons. Dilution of
calcium and zinc completion brines with formation connate water may also cause precipitation of
the corresponding hydroxide because of the rise in the pH of the diluted solution. Finally, the
subject brine may interact with the rock matrix to loosen fines and/or cause precipitation of oxides
or silicates.
Formation damage may also be the result of a change in the nature of the fluid wetting the surface
of the pore space, i.e., a wettability change. Because of the chemical makeup of the silica (sand)
and clays that compose the majority of the world’s producing formations, water has a greater
affinity for the exposed solid surfaces than does oil. Both the water and the exposed solid surfaces
in these formations are polar. Water is attracted to the surface, where it forms a film over and
around each surface and particle. Unless the oil in place consists of components other than large
molecules of hydrogen and carbon, the hydrocarbon is non-polar and thus is only weakly attracted
to the solid surface. Given these conditions, the formation is water-wet. If, however, the
hydrocarbon has been oxidized or otherwise contains polar groups, such as carboxyls, amines,
amides, sulfonates, and thiols, the partial polarity of the hydrocarbon may actually be such that it is
attracted to the rock surface. Water is displaced, and the formation rock is oil-wet.
When the wettability changes, it does so non-uniformly; i.e., some of the pore surfaces remain
water-wet and some become oil-wet. Because the relative permeabilities of water and oil are
different, and depend on the nature of the fluid wetting the pore surface, a zoning effect results and
hydrocarbon productivity is impaired. The formation is more permeable to oil when the formation
is water-wet because the oil flows through the center of the pores that compose the larger flow
channels while water stays in the narrower channels and along the rock surfaces. Conversely, the
formation is more permeable to water when the formation is oil-wet because the water is not
restrained by its affinity for the polar surfaces of the rock and is less viscous that the oil.
Sometimes the change from water-wet to oil-wet causes a well that is producing almost no water to
begin producing almost all water. Wettability changes can be caused by the presence of surface
active agents in the drilling, completion, or workover fluids used to penetrate the productive zone
of the formation. Agents such as detergents and lubricants in water based drilling fluids and oilwetting
agents in oil based fluids are the most likely causes of wettability changes. Excessive use
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-18