Drilling fluid

#The present invention relates to drilling fluids of the type employed in oil and gas wells. More particularly, the invention relates to weighting materials for water and oil based drilling fluids for horizontal wells.

#In drilling geopressured horizontal wells, a drilling fluid with density between 12-19 ppg is often required. Traditionally, barite (barium sulfate) is used as weighting material for drilling fluid. Barite is not suitable for horizontal wells because a barite-laden filtercake cannot be dissolved. Undissolved barite particles can invade the production screen in the horizontal well and plug it causing cessation or reduction in flow of hydrocarbon.

#The present invention in response to this need in the art provides a solution as described hereinafter.

#Zinc oxide is a particularly suitable material for weighting because, it has a high density (5.6 g/ml versus 4.5 for barite); It is soluble in acids (e.g. HCl); and its particle size can be designed so that it does not invade the formation. Acid solubility is particularly desirable because dissolved ZnO can be produced through a production screen without plugging it. A high density means less weighting material is needed per unit mud volume to achieve a desired density. Tests were performed with commercially available ZnO particles. Surprisingly, it was found that the particle size (of the order of 10 microns) is such that the ZnO particles do not invade the formation core with the filtrate. Also it is not large enough to settle out of suspension. Other weighting material such as manganese tetraoxide is so fine that it invades the formation with the filtrate. Zirconium oxide possesses similar properties as ZnO. It has a density of 5.7 and is soluble in nitric acid and hot concentrated hydrochloric, hydrofluoric and sulfuric acids, making it useful in the invention.

#As above observed, these materials are acid soluble. Hence filtercake formed from them can be dissolved and produced through the production screen without plugging it. Also they are fine enough to be easily suspended in the oil or water base mud, but not too fine to invade the formation with the filtrate. The very high acid solubility of ZnO makes it particularly suitable as weighting material. This problem is unique to horizontal wells because they are often completed as open holes without a casing. A production screen is placed in the open hole to retain sand and yet allow hydrocarbon to be produced.

#Solubility of ZnO in HCl at elevated temperatures was measured. At 150.degree. F., solubility of ZnO in 15% w/v HCl is 60 lb/bbl and 98 lb/bbl in 25% w/v HCl. Return permeability experiments with ZnO weighted waterbase mud showed that the ZnO mud did not damage the formation and that the ZnO filtercake can be dissolved by HCl. Experiments also showed that an aqueous NaBr mud weighted with ZnO has acceptable rheology for drilling.