STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

682 Drilling and Well Completions
to control formation pressures, check caving, facilitate pulling dry drill pipe
on round trips, and aid in combatting some types of circulation loss.
The most common, commercially available drilling mud additives are published
annually by World Oil. The listing includes names and description of over
2,000 mud additives.
Environmental Aspects of Drilling Fluids
Much attention has been given in recent years to the environmental aspects
of both the drilling operation and the drilling fluid components. Well-deserved
concern with the possibility of polluting underground water supplies and of
damaging marine organisms, as well as with the more readily observed effects
on soil productivity and surface water quality, has stimulated widespread studies
on this subject.
Drilling Fluid Toxicity
Sources of Toxicity. There are three contributing mechanisms of toxicity in
drilling fluids, chemistry of mud mixing and treatment, storage/disposal practices,
and drilled rock. The first group conventionally has been known the best
because it includes products deliberately added to the system to build and
maintain the rheology and stability of drilling fluids.
Petroleum, whether crude or refined products, need no longer be added to
water-based muds. Adequate substitutes exist and are, for most situations,
economically viable. Levels of 1% or more of crude oil may be present in drilled
rock cuttings, some of which will be in the mud.
Common salt, or sodium chloride, is also present in dissolved form in drilling
fluids. Levels up to 3,000 mg/L chloride and sometimes higher are naturally
present in freshwater muds as a consequence of the salinity of subterranean
brines in drilled formations. Seawater is the natural source of water for offshore
drilling muds. Saturated brine drilling fluids become a necessity when drilling
with water-based muds through salt zones to get to oil and gas reservoirs below
the salt.
In onshore drilling there is no need for chlorides above these “background”
levels. Potassium chloride has been added to some drilling fluids as an aid to
controlling problem shale formations drilled. Potassium acetate or potassium
carbonate are acceptable substitutes in most of these situations.
Heavy metals are present in drilled formation solids and in naturally occurring
materials used as mud additives. The latter include barite, bentonite, lignite, and
mica (sometimes used to stop mud losses downhole). There are background levels
of heavy metals in trees that carry through into lignosulfonate made from them.
Recently attention has focused on the heavy metal impurities in barite.
Proposed U.S. regulations would exclude many sources of barite ore. European
and other countries are contemplating regulations of their own.
Chromium lignosulfonates are the biggest contributions to heavy metals in
drilling fluids. Although studies have shown minimal environmental impact,
substitutes exist that can result in lower chromium levels in muds. The less used
chromium lignites (trivalent chromium complexes) are similar in character and
performance with less chromium. Nonchromium substitutes are effective in many
situations. Typical total chromium levels in muds are 100-1000 mg/l.
Zinc compounds such as zinc oxide and basic zinc carbonate are used in some
drilling fluids. Their function is to react out swiftly sulfide and bisulfide ions